CHAPTER 13 WET DIGESTION METHODS Henryk Matusiewicz

CHAPTER 13 WET DIGESTION METHODS

Henryk Matusiewicz Politechnika Poznańska, Department of Analytical Chemistry, 60-965 Poznań, Poland

ABSTRACT Sample preparation is the critical step of any analytical protocol, and involves steps from simple dilution to partial or total digestion. The present review focuses on wet digestion methods used for solid and liquid sample pre-treatment. The methods include wet decomposition and dissolution of the organic and inorganic samples, in open and closed systems, using thermal and radiant (ultraviolet and microwave) energy. The present and future tendencies for sample preparation also involve on-line decomposition and vapor-phase acid digestion. The intent is not to present the procedural details for the various samples, but rather to highlight the methods which are unique to each instrumental technique exist for the elemental analysis. The advantages and disadvantages of the various wet digestion methods have been emphasized. The bibliography accompanying this review should aid the analytical chemist in his/her search for the detailed preparation protocols.

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1 INTRODUCTION AND BRIEF HISTORY Sample (matrix) digestion plays a central role in almost all analytical processes, but is not often recognized as an important step in analytical chemistry, with primary attention being directed to the determination step. This sense of priorities is reflected all too conspicuously in the equipment and investment planning of many analytical laboratories. However, a welcome trend in recent years points toward fuller recognition of the true importance of sample digestion (decomposition, dissolution) in the quest for high-quality analytical results and valid conclusions. Wet digestion with oxidizing acids is the most common sample preparation procedure. Many of the sample preparation methods currently in use were actually developed during the 19th century. In the early 1800s, Berzelius developed test tubes, separatory funnels, and platinum crucibles; in 1831 he first made use of the conversion of SiO2 to SiF4 by reaction with HF for analytical purposes. In 1834, Henry and Zeise developed methods for the gravimetric determination of sulfur as sulfate in organic samples. Their method called for the sample to be digested with fuming nitric acid or aqua regia and fused with potassium hydroxide or potassium nitrate. The first published wet digestion reagent was chloric acid from HCl + KClO3, as described in 1838 by Duflos [1] as well as by Remigius Fresenius and Babo [2] in 1844. The classical wet digestion reagent HNO3 + H2SO4 (the most important and most versatile of the so-called wet –oxidation mixtures) was investigated by Danger and Flandin in 1841 [3], for the destruction of organic matter. The use of pure concentrated HNO3 in a closed system under elevated temperatures and pressure is well known since 1860 from Carius [4]. Kjeldahl [5] digested organic biological material in 1883 with boiling concentrated H2SO4 in an open system. Hydrogen peroxide was introduced by Classen and Bauer [6] in 1884, and HClO4 was used at elevated temperatures by Stcherbak [7] in 1893. Relatively new is Van Slyk’s [8] mixture of H2SO4 + H3PO4 + KIO3 + K2Cr2O7 (1954). In 1955, Polley and Miller [9] introduced a mixture of 50% H2O2 + conc. H2SO4 as a most powerful oxidizing reagent. Rediscovery of the high oxidizing power of OH• 2+ radicals (Fenton’s reagent H2O2/Fe ) [10] for biological materials in 1961 and 1968, by Sansoni et al. [11,12] led to a technique for wet digestion at temperatures below 110 oC. Since the beginning of the seventies, a large increase of general interest in different digestion techniques and in publications dedicated especially to wet digestion methods, has been evident. This Chapter gives an overview of wet digestion methods and recent developments and applications of the digestion of different materials. Other sample preparation methods, such as chemical extraction and leaching, solubilization with bases, enzymatic decomposition, thermal decomposition and anodic oxidation, are beyond the scope of this contribution and will not be discussed here.

2 NOMENCLATURE For some methods of analysis, it may be required that the analytical sample be in a liquid form – the sample solution. Thus, standard procedures to convert solid (or solid containing) samples to solutions prior to detection is required. However, this conventional designation is often imprecise or even misleading with respect to the actual mechanism of the process. Several very different names are sometimes applied to a single technique, which presents a considerable obstacle for anyone (particularly a non-specialist) interested in acquiring a quick overview of systems applicable to a specific task. The terms decomposition (of organic materials), dissolution (of inorganic materials), destruction, digestion, ashing, mineralization, acid-digestion, wet-ashing and even oxidative acid digestion all refer to this process. In this Chapter, the general

225 Chapter 13 expression will be digestion, which is specified as wet digestion; therefore wet digestion will be the term used for obtaining the resulting acid solution. It should be mentioned that guidelines for terms used in sample digestion are provided by the International Union of Pure and Applied Chemistry (IUPAC) Analytical Chemistry Division [13, 14].

3 BIBLIOGRAPHY There are numerous publications giving useful information on the digestion (dissolution and/or decomposition) of any conceivable combination of matrix and analyte. Some comprehensive books and general review articles (and references cited therein) contain material pertinent to either organic [15-18] or inorganic [19-22] matrices; others, to both [23-29]. Within the scope of this Chapter, a comprehensive discussion on digestion techniques is not feasible. For more comprehensive information, the following reviews and books are available: books by Šulcek and Povondra [20], by Bock [23] and by Krakovská and H-M. Kuss [29] are dedicated solely to digestion methods. Other books deal exclusively with a single technique: microwave-assisted sample preparation [30, 31], this topic has also been reviewed elsewhere [32-39]; even the literature on the use of microwave-assisted digestion procedures for subsequent sample analysis by means of electrothermal atomic absorption spectrometry (ET-AAS) is reviewed [40]. In 1997 the establishment of a site on the World Wide Web (WWW) for information transfer and education in the areas of sample preparation and microwave chemistry (http://www.sampleprep.duq.edu/sampleprep) was announced. Recommended guidelines for sample preparation (methods of digestion) of different matrices are also available from the Encyclopedia of Analytical Chemistry [41]. Although it is very difficult to refer to every paper published in this area, the enlisted bibliography of this Chapter gives comprehensive coverage of advance of the topic made to date. To follow the latest development and new applications in this field, the reader may consult the annual reviews in the Journal of Analytical Chemistry and Journal of Analytical Atomic Spectrometry. Relevant material is to be found under the headings “Sample preparation”, “Sample digestion” and “Sample dissolution”, for the appropriate topics. Literature cited herein is not intended to be comprehensive, but has been selected with a view to relevance, a pertinent review or seminal topic paper, or for potential application, novel developments, and progress in wet digestion techniques.

4 REAGENTS AND VESSEL MATERIALS FOR WET DIGESTION PROCEDURES Sample wet digestion is a method of converting the components of a matrix into simple chemical forms. This digestion is produced by supplying energy, such as heat; by using a chemical reagent, such as an acid; or by a combination of the two methods. Where a reagent is used, its nature will depend on that of the matrix. The amount of reagent used is dictated by the sample size which, in turn, depends on the sensitivity of the method of determination. However, the process of putting a material into solution is often the most critical step of the analytical process, since there are many sources of potential errors, i.e., partial digestion of the analytes present, or some type of contamination from the vessels of chemical products used. It is beyond the scope of this contribution to discuss all possible systematic errors, therefore, further details on how to avoid systematic errors during sample digestion cannot be referred to in detail here.

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The majority of wet digestion methods (total decomposition and strong attack) involve the use of some combination of oxidizing acids (HNO3, hot conc. HClO4, hot conc. H2SO4) and non-oxidizing acids (HCl, HF, H3PO4, dilute H2SO4, dilute HClO4) and hydrogen peroxide. All of these acids are corrosive in nature, especially when hot and concentrated, and should be handled with caution to avert injury and accidents. Concentrated acids with the requisite high degree of purity are available commercially, but they can be purified further by subboiling distillation [42]. Detailed discussion of the properties and applications of these reagents may be found elsewhere [20, 22-25]. Wet digestion has the advantage of being effective on both inorganic and organic materials. It often destroys or removes the sample matrix, thus helping to reduce or eliminate some types of interference. The physical properties of the common mineral acids used in sample preparation are summarized in Table 1.

TABLE 1 Physical properties of common mineral acids and oxidizing agents used for wet digestion

Compound Formula Molecular Concentration Density Boiling Comments weight (kg l-1) point (oC) w/w Molarity (%)

Nitric acid HNO3 63.01 68 16 1.42 122 68% HNO3,

azeotrope

Hydrochloric acid HCl 36.46 36 12 1.19 110 20.4% HCl,

azeotrope

Hydrofluoric acid HF 20.01 48 29 1.16 112 38.3% HF,

azeotrope

Perchloric acid HClO4 100.46 70 12 1.67 203 72.4% HClO4,

azeotrope

Sulfuric acid H2SO4 98.08 98 18 1.84 338 98.3% H2SO4

Phosphoric acid H3PO4 98.00 85 15 1.71 213 Decomposes to

HPO3

Hydrogen peroxide H2O2 34.01 30 10 1.12 106

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Most wet digestion procedures are conducted under conditions that, in terms of temperature or reagents used, must be considered as extreme. Thus the material of which the flasks, crucibles, etc. are made must be chosen carefully according to the particular procedure to be employed. The material from which the digestion device is fabricated is also a frequent source of elevated blanks. Elements can be either dissolved from the material or they can be desorbed from the surface. Very important in this respect is the nature of the material. The suitability of materials may be estimated according to the following criteria: heat resistance and conductance, mechanical strength, resistance to acids and alkalis, surface properties, reactivity and contamination, whereby the specific characteristics of the organic and inorganic material must also be given special consideration. Table 2 shows preferred materials for digestion vessels. The apparatus and containers that are used for the wet digestion procedures must be scrupulously cleaned and tested for any possible contamination. Usually it is sufficient to boil the flasks in concentrated nitric acid followed by rinsing several times with ultrapure water before use. In cases where this procedure is not adequate, one of the most powerful cleaning procedures is steaming the vessels with nitric or hydrochloric acid with assembly in a microwave-heated sealed Teflon vessel [43]. This procedure is particularly recommended for quartz, borosilicate glass and polytetrafluorethylene (PTFE) vessels.

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TABLE 2 Preferred materials for wet digestion vessels

Material Chemical name Working Heat Water Comments Temperature deflection Absorption, o , C temperature, % oC # * Borosilicate glass SiO2 <800 Ordinary laboratory glass is not suitable for use in wet digestion procedures & Fused quartz SiO2 <1200 For all procedures involving wet digestion of organic material, quartz is the most suitable material for vessels Glassy carbon graphite <500 Glassy carbon is used in the form of

229 crucibles and dishes for alkaline melts and as receptacles for wet digestion procedures PE polyethylene <60 PP polypropylene <130 107 <0.02 PTFE polytetrafluorethylene <250 150 <0.03 PTFE is generally used only for digestion vessels in pressure digestion systems PFA perfluoralkoxy <240 166 <0.03 FEP tetrafluorperethylene <200 158 <0.01 TFM tetrafluormetoxil <0.01 # SiO2 content between 81 and 96%. * Softens at a temp. of 800 oC. & >SiO2 99.8%.

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To generalize this section, nitric acid is an almost universal digestion reagent and the most widely used primary oxidant for the decomposition of organic matter, since it does not interfere with most determinations and is available commercially in sufficient purity. Hydrogen peroxide and hydrochloric acid can usefully be employed in conjunction with nitric acid as a means of improving the quality of a digestion. Hydrochloric acid and sulfuric acid may interfere with the determination of stable compounds. Mixtures with hydrochloric acid are generally used for samples containing principally inorganic matrices, and combinations with hydrofluoric acid are used to decompose silicate insoluble in the other acids. Safety considerations are particularly important when using perchloric acid.

5 WET ACID DIGESTION (DECOMPOSITION AND DISSOLUTION) PROCEDURES The task of preparing samples with acid treatment to release the elements of interest from the sample matrix and transfer them to a liquid matrix for subsequent analysis is commonplace in many laboratories. A variety of techniques is employed – from ambient-pressure wet digestion in a beaker on a hot plate (or hot block) to specialized high pressure microwave heating. Dissolution is usually defined as the simple process of dissolving a substance in a suitable liquid at relatively low temperature, with or without a chemical reaction. The term decomposition denotes a more complex process that is usually performed at higher temperature and/or at increased pressure, with the aid of reagents and special apparatus. A clear distinction between these terms cannot, however, be made. Table 3 gives an overview of the wet digestion methods, one of the oldest and still most frequently used techniques, for organic and inorganic samples. The intent is not to present the procedural details for the various sample matrices, but rather to highlight those methods which are unique to each technique and sample.

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TABLE 3 Schemes for wet digestion methods

Digestion technique Required reagents Application

Open systems

Conventional heating HNO3, HCl, HF, H2SO4, inorganic/organic

HClO4, H2O2

Microwave heating HNO3, HCl, HF, H2SO4, inorganic/organic

HClO4, H2O2

Ultraviolet digestion H2O2, K2S2O8 waters, slurries Closed systems

Conventional heating HNO3, HCl, HF, H2O2 inorganic/organic

Microwave heating HNO3, HCl, HF, H2O2 inorganic/organic Flow systems

Conventional heating HNO3, H2SO4, H2O2 inorganic/organic

UV on-line decomposition H2O2, K2S2O8 waters, slurries ?

Microwave heating HNO3, H2SO4, H2O2 inorganic/organic

Vapor-phase acid digestion HNO3, HCl, HF, H2O2 inorganic/organic

5.1 Open systems Open vessel acid digestions, one of the oldest techniques, are undoubtedly the most common method of sample decomposition or dissolution of organic and inorganic sample materials used in chemical laboratories. This very inexpensive technique is of inestimable value for routine analysis because it can easily be automated; all the relevant parameters (time, temperature, introduction of digestion reagents) lend themselves to straightforward control. Thousands of different methods and minor variations on these methods have been described in the literature. The main advantage of wet digestion (ashing) over dry ashing is its speed. However, systems of this type are limited by a low maximum digestion temperature, which cannot exceed the ambient-pressure boiling point of the corresponding acid or acid mixture. For instance, the oxidizing power of nitric acid with respect to many matrices is insufficient at such low temperatures (boiling point 122 oC). One possible remedy is the addition of sulfuric acid, which significantly increases the temperature of a digestion solution. Whether or not this expedient is practical depends on the matrix and the determination method. High-fat and high-protein samples are generally not subject to complete digestion at atmospheric pressure. Other disadvantages relate to the risk of contamination through laboratory air, the necessarily rather large amounts of required reagents (very often employing expensive reagents), and the danger of losses of trace elements. Losses can be kept low by using an excess of acid (mainly nitric)

231 Chapter 13 combined with a reflux condenser and by optimization of temperature and duration. Nevertheless, systems operated at atmospheric pressure are preferred from the standpoint of workplace safety.

5.1.1 Conventional heating (thermally convective wet digestion) The conventional approach to wet digestion, which has proven its worth over many years, entails a system equipped with heated conventional source (Bunsen, heating plate, send bath, etc.) operating either at a fixed temperature or in response to a temperature program. Acid digestions are often accomplished in any vessel, usually in glass or PTFE (beaker, conical flask, etc.) with or without a refluxing condenser. However, when a sample is decomposed by open wet digestion, refluxing is compulsory. The necessary apparatus has been described by Bethge [44]. Open block digestion systems have been popular in sample analysis over the past decades, but have consistently suffered from the major drawback of their sensitivity against corrosion and subsequent risk of contamination. Therefore, block digestion systems (hotplate techniques) have not been considered state-of-the-art technology in trace and ultratrace sample preparation. Graphite block digestion systems are becoming more frequently considered. These systems overcome the deficiencies of the traditional systems, made from stainless steel or aluminum, because the block is manufactured from graphite and typically coated with a fluoro-polymer to prevent the possibility of metallic contamination from the surface of the system during the handling of the samples. Graphite block systems present an alternative to the current mainstream technology of open and closed vessel (“classical” or microwave-assisted) digestion systems, as they allow large numbers of samples to be digested simultaneously, thus overcoming one of the major weaknesses of closed vessel systems. Commonly employed digestion agents include nitric acid, sulfuric acid, hydrofluoric acid, perchloric acid and hydrogen peroxide, as well as various combinations of these. Most applications of wet digestion involve aqueous or organic matrices, such as surface waters, waste water, biological and clinical samples, food samples, as well as soil, sediment and sewage sludge, coal, high- purity materials, and various technical materials. More recently, open systems have progressed: the usual digestion ramps consist of several vessels equipped with reflux condensers to limit possible volatilization losses of some analytes and to avoid the evaporation of the reactive mixture. Such assembling is entirely satisfactory for ensuring concurrent digestions of large series of samples. Modern commercially available Hach Digesdahl Digestion Apparatus (Hach Comp., USA), is designed to digest organic and mineral samples for subsequent analysis.

5.1.2 Microwave heating (microwave-assisted wet digestion) The most innovative source of energy for wet digestion procedures is microwaves. Because the heating takes place inside the digestion mixture, microwave digestion is more efficient than with conventional means of heating. Using microwaves, both the speed and the efficiency of digestion for some types of samples considered difficult to solubilize are often improved. Additionally, automation becomes possible with some instrumentation. This technique will be briefly summarized here. Several different names are applied to this technique. An example of incorrect terminology involves uncritical use of the expression “microwave digestion” for acid digestion with microwave excitation. Although this technique makes use of microwave radiation, the direct effects of this radiation are of minor importance, at most. Microwaves cannot rupture molecular bonds directly because the corresponding energy is too low to excite electronic or vibrational

232 Chapter 13 states. Rotational excitation of dipoles and molecular motion associated with the migration of ions are the only processes that are observed in this microwave field [45]. For this reason, an expression such as “microwave-assisted digestion” is preferable and recommended. Since Abu-Samra et al. [46] reported on the application of microwave techniques to wet digestion of biological samples in 1975 (the first paper published on microwave-assisted digestion), there has been a rapid development in microwave- assisted digestion for elemental analysis. Recent reviews [26-41] detail the application of microwave-assisted digestion to a wide variety of sample types, such as geological, biological, clinical, botanical, food, environmental, sludge, coal and ash, metallic, and synthetic materials and mixed samples and present specific experimental conditions as a function of the matrix to be digested. The earliest attempts at microwave-assisted digestion were performed using home appliance microwave ovens. This was necessary because commercial devices were not available at the time. The use of domestic microwave ovens in laboratory experiments should be discouraged because of safety and performance. Microwave-assisted digestion in open systems at atmospheric pressure (focused microwaves using open vessels fitted with refluxing facilities made of borosilicate glass, quartz or PTFE) is generally applicable only with simple matrices or for strictly defined objectives, and the results are reproducible only if the specified digestion parameters are strictly observed. The performance of the focused-microwave- assisted systems and a wealth of applications have been reviewed by White and Mermet [30, 31] and very recently by Nóbrega et al. [47]. Focused microwave-assisted sample preparation is a suitable strategy for dealing with high masses of organic samples (up to 10 g). Losses may be encountered with mercury and possibly also with organometallic compounds (e.g., those containing arsenic, antimony or tin). Addition of sulfuric acid is essential in order to achieve a sufficiently high digestion temperature using atmospheric pressure equipment, where the boiling point of the acid establishes the maximum digestion temperature, although it is important to remember that the presence of sulfate interferes with many procedures for metal determination (e.g., graphite furnace atomic absorption spectrometry or electrochemical techniques). New equipment, the non- pressurized digestion systems (STARPlus Systems, CEM Corp., USA; QLAB6000 System, Questron Technologies Corporation, Canada) are at the frontier of this field. They are designed for routine use and can easily be automated. In addition, automated evaporation-to-dryness can be accomplished in an open vessel by attaching a pump to evacuate the fumes while the container is heated and multiple methods for different samples can be simultaneously applied owing to the possibility of operating each reaction vessel independently. All relevant parameters, such as reagent volume, digestion time, applied power, temperature and addition of reagent composition lend themselves to straightforward control. Although non-pressurized microwave systems are limited by a low maximum digestion temperature, which cannot exceed the ambient pressure boiling point of the acid (or the acid mixture), they provide the best option with regard to the safety of personnel, because no overpressure can occur. Moreover, non- pressurized microwave-assisted digestion is suitable for on-line digestions in continuous-flow systems. A compact apparatus in which a specific position can be irradiated by microwaves (MW) and ultrasound (US) simultaneously has been developed [48]. The combination of these two types of irradiation, electromagnetic (2.45 GHz) and mechanical (20 KHz), and their application to physical processes like digestion appears interesting. The MW-US reactor has been designed for atmospheric pressure decomposition and dissolution of biological (olive oil) and chemical products

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(refractory mineral material Co3O4) in nitric acid and hydrogen peroxide. Simultaneous microwave and ultrasound irradiation is shown as a new technique for atmospheric pressure digestion of solid and liquid samples suitable for chemical and biological analysis.

5.1.3 Ultraviolet digestion (photolysis) Ultraviolet (UV) digestion is utilized mainly in conjunction with uncontaminated or slightly contaminated natural water matrices (aqueous solutions), such as sea, surface, fresh, river, lake, ground, estuarine and coastal water. Liquids or slurries of solids are decomposed by UV radiation (light) in the presence of small amounts of hydrogen peroxide, acids (mainly HNO3) or peroxodisulphate (i.e., beverages, special industrial waste water, water of sewage treatment plants, soil extracts) [49]. Dissolved organic matter (DOM) and complexes of the analyte elements are decomposed to yield free metal ions. The corresponding digestion vessel should be placed in the closest possible proximity to the UV lamp (low- or high-pressure) to ensure a high photon flux. In photolysis, the digestion mechanism can be characterized by the formation of OH• radicals from both water and hydrogen peroxide that is initialized by the aid of the UV radiation [49]. These reactive radicals are able to oxidize, to carbon dioxide and water, the organic matter present in simple matrices containing up to about 100 mg/l of carbon. Complete elimination of the matrix is, of course, possible only with simple matrices or by combining photolysis with other digestion techniques [50]. The method does not oxidize all organic components possibly present in water; chlorinated phenols, nitrophenols, hexachlorobenzene and similar compounds are only partly oxidized. Effective cooling of the sample is essential, since losses might otherwise be incurred with highly volatile elements. Hydrogen peroxide addition may need to be repeated several times to produce a clear sample solution. Modern UV-digestion systems are commercially available [see ref. 49, Table 1].

5.2 Closed systems During the last few decades, methods of wet sample preparations using closed vessels has become widely applied. Closed systems offer the advantage that the operation is essentially isolated from the laboratory atmosphere, thereby minimizing contamination. Digestion of the sample is essentially ensured by a common wet digestion procedure, which is performed under the synergistic effects of elevated temperature and pressure; digestion occurs at relatively high temperature due to boiling-point elevation. The pressure itself is, in fact, nothing more than an undesirable – but unavoidable – side effect. These techniques are generally much more efficient than conventional wet digestion in open systems, the loss of volatile elements is avoided, any contribution to blank values may be reduced and the digestion of more difficult samples is possible. The principal argument in favor of this form of digestion is the vast amount of relevant experience acquired in recent decades. The literature is a treasure trove of practical information with respect to virtually every important matrix and a great number of elements. Closed system digestion is particularly suitable for trace and ultratrace analysis, especially when the supply of sample is limited. Since the oxidizing power of a digestion reagent shows a marked dependence on temperature, an arbitrary distinction should be made between low (simple) pressure digestion and high-pressure digestion. Low-pressure digestions (< 20 bar) are limited to a temperature of ca. 180 oC, whereas with high-pressure apparatus (> 70 bar) the digestion temperature may exceed 300 oC.

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5.2.1 Conventional heating (thermally convective wet pressure digestion) The expression “pressure digestion” is another – very timely – example of incorrect terminology, giving the impression that pressure is critical for the digestion process. Indeed, it is a relatively high boiling temperature that ensures more effective digestion, not the associated high pressure. However, it is important to note that the pressure buildup, amongst other things, does introduce some dangers to the application of these methods. These methods are better described if they are grouped under the title, “wet digestion methods in closed systems”. However, the conventional designation should, in the author’s opinion, nevertheless be retained in this case, if only because of its wide acceptance; attempting to rename the procedure now would introduce more confusion than clarity. The digestion of inorganic and organic substances in sealed tubes was the method first proposed for pressure digestion at the end of 19th century, and some of these applications are still difficult to replace by other digestion methods. The use of sealed glass tubes goes back to Mitscherlich [51] and Carius [4, 52], often referred to as the Carius’ technique, first described in 1860. Carius undertook digestion of organic materials with concentrated nitric acid at 250 – 300 oC. The sample and the acid were mixed in a strong (thick) – walled quartz ampoule and sealed. The ampoule was transferred to a “bomb canister” and heated in what was called a “bomb oven” for several hours, after which it was cooled, opened, and the contents analyzed. Carius tube digestion involves the generation of internal pressure in excess of 100 bar (atm) at 240 oC. The modern redesign and employment of the Carius combustion tube for the digestion of some refractory materials was accomplished at the U.S. National Bureau of Standards during the 1940s [53-55]. Extreme care should be used in the handling and venting of pressurized tubes. A discussion of Carius tube design for minimizing losses and hazards by explosion has been provided by Gordon [54]. For safety, any stainless- steel sleeve jacket (along with solid CO2 pellets, to maintain equal pressure across the tube wall when heated) which is large enough to contain the Carius tube will suffice as an external pressure vessel [56]. With the development of the so-called Carius tube, the field of closed vessel digestion was born. Digestion in autoclaves with metal inner reaction vessels was originally proposed in 1894 by Jannasch [57], but was not widely employed because of a number of drawbacks (such as strong corrosion of the platinum vessel). Seventy years later, May and Rowe [58] designed a new type of autoclave with an inner lining made of Pt-Ir alloy (platinum-lined crucible and bomb), which is more resistant than platinum alone, for digestion with hydrofluoric acid. It is difficult to construct an autoclave with an inner metal reaction vessel and it is thus very expensive. Therefore, this technique has not been used extensively in analytical practice and thus the apparatus is not manufactured commercially. Extensive use of pressure digestion in analytical procedures began in 1960, as a result of the considerable technological progress in the manufacture of organic polymers. Convectively heated pressure vessel systems have proved to be the most valuable systems for guaranteeing complete, or almost complete digestion of solid samples because they provide elevated digestion temperatures (about 200 – 230 oC [59]). Most sample vessels (containers) for use in thermally convective pressure digestion are constructed from PTFE [60-62], PFA [63] or PVDF [64], although special quartz vessels with PTFE holders [65] or glassy carbon vessels [66] are available for trace analysis purposes. The sample vessel is mounted in a stainless-steel pressure autoclave and then heated, usually in a laboratory drying oven, furnace or heating block, to the desired temperature. Because of the necessity to examine numerous samples,

235 Chapter 13 mechanized multi-sample pressure digestion systems able to process rather large sample numbers of the same matrix type were developed [67]. A cooling circuit can be fitted into the metal casing (jacket) to permit rapid manipulation of the solution formed immediately after removing the “digestion bomb” from the oven or heating block [68]. Dissolution can be also accelerated by mixing the reactants, preferably by using a stirring bar (covered with PTFE) [69]. An alternative design has been proposed by Uchida et al. [70], wherein a small screw cap vial for sample digestion is placed inside the Teflon digestion vessel. This digestion system (Teflon double vessel) can reduce the risk of sample leakage and contamination with extraneous materials. To improve dealing with pressure-temperature evaluation and the carbon balance for some materials, a system with a Teflon lined membrane pressure meter and a thermocouple was designed [71]. Recently, a digestion vessel for use with a convection oven was proposed [72] which has an unusual design in which the vessel consists of three nested structures: an innermost PTFE container of 30 ml capacity, an intermediate PTFE container of 100 ml capacity, and an outer stainless-steel shell. It should be stressed here that digestion in a Teflon-lined pressure vessel using one or a mixture of acids does not result in complete decomposition (see section 6.5.5), because of the limited temperature. Pressure digestion systems are all feasible below ca. 200 oC, but above this temperature PTFE begins to “flow”, rendering it unsuitable for use in high-pressure applications, and consequently at higher temperatures. All thermally initiated digestions have the disadvantage that a considerable amount of time is consumed in preheating and cooling the digestion solutions and sample vessel [73], the limited sample size, and the inability to visually check the progress of the digestion. The contributions of Langmyhr, Bernas, Tölg and co-workers are worth mentioning with regard to the commercialization of the digestion vessels or “digestion bombs”, as they are often called. Today there are a number of digestion bombs covering the whole market range, including: the popular Parr acid digestion bombs (Parr Instrument Company, USA), Uniseal decomposition vessels (Uniseal Decomposition Vessels Ltd., Israel), stainless-steel pressure vessels with Teflon inserts (Berghof Laborprodukte GmbH, Germany), the pressure decomposition system CAL 130FEP (CAL Laborgeräte GmbH, Germany) and the pressure digestion system (PRAWOL, Germany). To avoid the problem of loss of mechanical stability at high temperatures, vessels made of quartz are now being used in a new pressure digestion system [74, 75]. The introduction of a high-pressure ashing (HPA) technique by Knapp [76] has not only reduced the effective digestion time but also opened the way to digestion of extremely resistant materials, such as carbon, carbon fibres, mineral oils, etc. In academic terms an HPA currently represents the highest standard in pressurized wet digestion techniques, combining the advantages of the Carius technique with easy and safe handling. Essentially complete digestion can be accomplished with the vast majority of samples so far investigated. Nitric acid alone is a sufficiently powerful reagent in many cases. High-pressure digestion is conducted in quartz vessels, with a maximum digestion temperature as high as 320 oC at a pressure of ca. 130 bar. For dissolution requiring HF, glassy carbon vessels are used instead of quartz. The quartz vessel (or glassy carbon) is stabilized during the digestion process by subjecting it to an external pressure roughly equivalent to or higher than that developed internally. Since the pressure within the vessel is lower than the pressure applied, the vessel is protected from explosion. A perfected system of wet digestion under high temperature and pressure developed by Knapp is commercially available, the HPA-S High Pressure Asher system (Anton Paar GmbH, Austria).

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Very recently, again in respect to complete digestion of organic waste materials, a potent digestion technique was developed [77] based on a prototype of a high pressure ashing device using infrared heating (IR-HP-asher). High-pressure digestion is conducted in six quartz vessels inside a steel autoclave, with a maximum digestion temperature as high as 300 oC at a pressure of 130 bar. The novelty of this approach lies in the design of an HPA system with IR heating. In comparison to open vessel digestion, closed vessel digestion methods have many advantages, but there is one disadvantage – complex and expensive vessel designs. A new technique – pressurized wet digestion in open vessels – combines the advantages of closed vessel sample digestion with the application of simple and inexpensive open vessels made of quartz or PFA [78]. The vessels are placed in a High Pressure Asher HPA, which is adapted with a Teflon liner and partially filled with water. The vessels (in principle any shape of vessel can be used) are loaded with sample material and digestion reagent and are simply covered with PTFE stoppers and not sealed. The vessels are transferred to a specially adapted HPA and digested at temperatures up to 270 oC. The digestion time is ca. 90 min, and cooling to room temperature requires a further 30 min. As metal autoclaves are expensive, a pressure vessel without an outer metal casing has been designed. The technique of digestion at slightly increased pressure has been found to be very useful for routine laboratory analysis, primarily because of its simplicity. Almost an unlimited number of samples can be digested simultaneously. The vessel can be sufficiently well sealed by using a screw cap [79]. Volatile components are not lost during heating and the laboratory atmosphere is thus not contaminated by acid vapors. All-Teflon thick-walled PTFE vessels (bombs) have been used in the dissolution of refractory oceanic suspended matter using HCl, HNO3 and HF [80]. Translucent Nalgene sealed bottles have been proposed for the “wet pressure digestion” of biological materials (fish, bird, plant tissue) using a combination of HClO4 and HNO3 [81]. A method utilizing a pressure digestion technique for real sample matrices using linear polyethylene bottles has been proposed [82]. Vessels of polyethylene are transparent, permitting observation of the whole digestion process and reduction of the reaction time to a minimum. A complete digestion of fatty material with slight overpressure (< 4 bar) was possible in a closed system completely made from quartz [83]. A closed PTFE bomb (30 ml capacity, screw-cap vessel machined from molded, stress-relieved Teflon-TFE rod) was designed for the digestion of materials using a conventional heating (drying) oven [84].

5.2.2 Microwave heating (microwave-assisted pressurized wet digestion) Closed vessel microwave-assisted digestion technology has been acknowledged as one of the best solutions for clean chemistry applications and has a unique advantage over other closed vessel technologies. The vessels used for microwave acid digestion are either low-pressure or high-pressure bombs. The current generation of microwavable- closed vessels consists of a two-piece design, liners and caps composed of high-purity Teflon or PFA with casings (outer jacket) made of polyetherimide and polyetherethereketone or other strong microwave transparent composite material. Their practical working temperature is 260 oC (softening point of Teflon), and their pressure limit is 60 – 100 bar. Closed vessel digestion is ideal for those samples which are being dissolved in HNO3 and/or HCl. However, for those digestions where H2SO4 is required, such as digestions of petroleum products, there is little advantage in using the regular o closed vessel approach because the boiling point of H2SO4 (330 C) exceeds the temperature available in a Teflon-lined vessel. The use of closed vessel microwave-

237 Chapter 13 assisted digestion techniques minimizes the analytical blank by minimizing the amount of reagents used and by controlling the digestion environment, as well as through augmentation of the operator’s skills. Microwaves only heat the liquid phase, while vapors do not absorb microwave energy. The temperature of the vapor phase is therefore lower than the temperature of the liquid phase and vapor condensation on cool vessel walls takes place. As a result, the actual vapor pressure is lower than the predicted vapor pressure. This sort of sustained dynamic, thermal non-equilibrium is a key advantage of microwave technology, as very high temperatures (and, in turn, short digestion times) can be reached at relatively low pressures. The inspiration for pressure digestion studies came from a US Bureau of Mines report [85], which described how rapid dissolution of some mineral samples had been achieved using a microwave oven to heat samples and an acid mixture contained in polycarbonate bottles. To overcome the presence of “hotspots” in the oven, which result in uneven heating, they designed a polypropylene rack to fit on top of the standard microwave carousel. Although sealed polycarbonate bottles were used as pressure vessels, the plastic quickly became opaque and brittle (the melting point of polycarbonate is 135 oC). Smith et al. [86] substituted Teflon PFA fluorocarbon resin (a tetrafluoroethylene with a fully fluorinated alkoxy side-chain) vessels for polycarbonate because of its superior chemical and mechanical properties. Buresch et al. [87] used low pressure-relief type containers made of PTFE or quartz. Alvarado et al. [88] exploited modified thick-walled Pyrex glass test-tubes fitted with polypropylene screw caps as pressurizable vessels. Kojima et al. [89] modified a Teflon digestion bomb by using a double Teflon vessel with a polypropylene jacket to permit leak-free and safe digestion of samples. A closed vessel microwave digestion system was described [90]. In situ measurement of elevated temperatures and pressures in closed Teflon PFA vessels during acid decomposition of organic samples was demonstrated. Temperature and pressure monitoring permitted controlled digestions, studies of digestion mechanisms, and the development of transferable standard microwave sample preparation methods. Laboratory-made all Teflon bombs, used for low- or medium-pressure work, also are appropriate for microwave-heated digestion purposes [91], some fitted with pressure relief holes, valves or membranes (rupture discs). Low-volume microwave-assisted digestion methods have found applications for studies involving small sample sizes where loss of sample in large digestion equipment is inevitable. Small quantities of tissue (5 – 100 mg dry weight) are digested in high- purity nitric acid by use of a modified Parr microwave acid digestion bomb with modified Teflon liner [92]. The use of low-volume (7 ml) Teflon-PFA closed vessels designed for the preparation of small-sized (< 100 mg dry mass) biological tissues has been described [93]. In order to prevent excessive pressure rises during closed microwave acid digestion of fairly large (1 g) samples having high organic content, an open-vessel pre- digestion technique under reflux was designed to allow the escape of oxidation products, such as carbon dioxide, without incurring evaporation losses of acid or analytes. Following pre-digestion, the vessels were capped and subjected to microwaves to complete the digestion under pressure [94]. In an attempt to minimize the delay in opening Teflon pressure vessels following microwave acid digestion, and thus significantly reduce sample preparation time, digestions with the pressure vessels immersed in liquid nitrogen and the use of liquid nitrogen as a pre- and post digestion coolant were applied [95]. In other developments, a special type of Teflon bomb was constructed in which the vapor pressure can be

238 Chapter 13 maintained at a moderate level (up to 5 bar) by means of an internal quartz or Teflon cooling spiral (a water cooling spiral is inserted into a closed space through the cover). During operation, reflux of the condensed acid and water vapors continuously renews the liquid phase over the sample [96]. Several microwave-heating configurations were presented by Pougnet et al. [97,98] based on 500 W or 1200 W, 2.45 GHz fundamental-mode microwave waveguide cavities, which heat pressure vessels currently used in laboratories for sample digestion and other applications. The capsule concept was reviewed in detail by Légère and Salin [99,100]. The sample is handled in an encapsulated form until it is in the digestion solvent. The operation of the capsule-based microwave-assisted digestion system proceeds in several steps, during which temperature and pressure are monitored. The heating in such a system, as in all microwave bomb systems, is from the solution outward, and the system performance is dictated by the same chemical and physical laws governing other microwave-assisted systems. From the previous discussion, it is clear that microwave acid digestion can be easily adapted for closed-vessel digestions; hence, its application has been limited to digestions in closed Teflon-lined vessels made of nonmetallic microwave-transparent materials operating with a maximum upper safe pressure of around 60 – 100 bar. In response to these limitations, and focusing on the fact that rapid heating of solvents and samples within a polymer vessel can lead to significant advantages over high-pressure steel-jacketed Teflon bombs (which are thermally heated), Matusiewicz [101] developed a focused-microwave—heated bomb that would exceed the operational capabilities of existing microwave digestion systems and permit the construction of an integrated microwave source/bomb combination. The combined advantage of having a high-pressure Teflon bomb [59] incorporating microwave heating [31] has produced a focused high-pressure high-temperature microwave-heated digestion system [101] capable of being water or fluid cooled in situ. Another vessel configuration integrates the microwave chamber around the vessel. These systems consist of one or several microwave transparent vessels (Teflon, quartz), which can either be opened [102] or sealed [103], enclosed in an acid-resistant stainless-steel chamber. The steel chamber acts as both the pressure vessel and microwave chamber. Modern systems can handle acid decompositions at temperatures up to 320 oC and pressures of 130 – 200 bar. Very recently, a novel microwave-assisted high temperature UV-digestion for accelerated decomposition of dissolved organic compounds or slurries was developed [104]. The technique is based on a closed, pressurized, microwave digestion device wherein UV irradiation is generated by immersed electrodeless Cd discharge lamps (228 nm) operated by the microwave field in the oven cavity. The immersion system enables maximum reaction temperatures up to 250 – 280 oC, resulting in a tremendous increase of mineralization efficiency. Today, there are a number of microwave-digestion bombs available: Parr bombs (Parr Instrument Company, USA), Berghof all-PTFE digestion vessels (Berghof GmbH, Germany) and TFM digestion vessels (Bohlender, Germany).

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5.3 Flow systems Discrete vessel systems, whether at elevated or atmospheric pressure, require a large amount of handling. Processes such as assembling, closing, opening, and positioning the vessel in the ordinary oven or microwave filed are laborious and time-consuming. Continuous flow-through thermal digestion, UV digestion and microwave digestion systems were designed to overcome some of the limitations by replacing the vessels with flow-through tubing (coil). Samples are digested by pumping them through a coil containing a digestion matrix while being heated (by thermal, UV or microwave). The continuous flow of a carrier stream through these systems washes the system, removing the need for tedious vessel clean-up procedures. These systems can handle reactions that produce sudden increases in temperature and pressure, or unstable samples. Many different designs of flow digestion systems have been published, but very few meet the prerequisites for high performance sample decomposition.

5.3.1 Conventional heating (thermal) Many of the disadvantages of sample digestion can be overcome by automating sample preparation in an enclosed system through the use of flow technology. A well- established digestion system, based on the flow-stream principle, was developed by Technicon [105]. A slowly rotating, very wide glass tube with a spiral-type cavity is heated externally. Liquid sample, together with the decomposition reagent, is continuously pumped in one direction. By rotation of the tube, the decomposition mixture moves through the heated tube within a few minutes. At the other end of the tube, the finished sample is continuously pumped away and is ready for further analytical steps. A disadvantage of this system is that only liquid samples can be used, and the “memory effect” is large. The sample throughput is very high, however, for comparatively small apparatus cost. Another system was presented by Gluodenis and Tyson [106]. Here, the PTFE tubing is loosely embedded in a resistively heated oven. By using PTFE tubing, the maximum digestion temperature is restricted to ca. 210 oC. The limited mechanical strength of the material merely allows maximum working pressures of up to 35 bar. Therefore, the usual working pressure is about 10 – 20 bar. The potential of the system was illustrated by the digestion of cocoa powder slurried in 10% HNO3 injected into the manifold and digested under stopped-flow, medium-pressure conditions. In a series of papers [107-109], Berndt described development of a new high temperature/high pressure flow system for the continuous digestion of biological and environmental samples. It was shown [107] that temperatures up to 260 oC and pressures up to 300 bar can be reached in a flow system when an electrically heated Teflon lined HPLC tube is used as the digestion capillary. The required back-pressure was obtained by using a restrictor capillary with an inner diameter of 50 µm and a length of about 10 cm. Digested biological samples (blood, liver, leaves) were collected at the outlet of the flow system. In subsequent papers [108,109], an electrically heated Pt/Ir capillary served as a digestion tube at temperatures of 320 – 360 oC and pressures of about 300 bar, and withstands concentrated acids. Due to the totally glass-free environment, samples having high silicate content can be digested by the addition of hydrofluoric acid.

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5.3.2 UV on-line decomposition UV digestion is a clean sample preparation method, as it does not require the use of large amounts of oxidants. Furthermore, UV digestion is effective and can be readily incorporated into flow injection manifolds. The sample flows, in the presence of i.e., H2O2, H2S2O8 or HNO3, through a tube (PTFE, quartz) coiled around a fixed UV lamp(s). A short review of such flow systems has appeared recently [49]. Analyzers of this kind are produced by SKALAR Analytical, Holland, for example. Fernandes et al. [110] developed a manifold based on a two-stage on-line UV/thermal induced digestion procedure for oxidation purposes. The UV digestion apparatus consisted of a 4 m long PTFE tube tightly wounded directly around the UV source (15 W) to form a spiral reactor. The thermal digestion apparatus consisted of a 2 m long PTFE tube coiled in a helix and submerged in a thermostatic bath at 90 oC. Flow systems are becoming more popular in analysis, because of their ease of automation, speed, small volume of sample and elegance, and thus provide for a promising future.

5.3.3 Microwave heating (microwave-assisted pressurized flow-through digestion) Many different designs of microwave-assisted flow digestion systems have been published [31, 39, 111], which open up new possibilities, primarily in fully automated sample preparation for elemental analysis. The earliest work reported in this field was by Burguera et al. [112] who applied a flow injection system for on-line decomposition of samples and determined metals (Cu, Fe, Zn) by flame AAS. The methodology involved the synchronous merging of reagent and sample followed by decomposition of serum, blood or plasma in a Pyrex coil located inside the microwave oven. This approach permits essentially continuous sample digestion and drastically reduces sample processing time, and is suitable for those samples that require mild digestion conditions (especially liquids). According to the location of the digestion unit in the system, there are two types of manifolds described in the literature to date: before and after the injection unit. In the former arrangement, the sample is introduced into the microwave oven in a continuous flow [113] or a stopped flow mode [114]; after digestion, a discrete aliquot is delivered to the detector. In the second arrangement, the injected sample flows to the microwave oven unit together with the reagent(s) to be digested, and is then cooled and degassed prior to its delivery to the detector [115]. In both cases, the measurements can be performed partially or totally off-line or on-line. Solid samples call for more sophisticated flow systems because they need to digested in the presence of highly concentrated acids, which rapidly destroy organic matrices. A first attempt aimed at simplifying manipulation of the digest was reported in 1988 [116]. Lyophilized, finely ground and weighed samples of liver and kidney were placed in test tubes together with mineral acids and the contents shaken before exposing them to microwave radiation to avoid violent reaction with abundant foam formation. The tubes were loaded into a covered Pyrex jar inside a domestic microwave oven operated for a specified time at a given power. Carbonell et al. [113] initiated the determination of metallic elements in solid samples using the slurry approach coupled with microwave oven digestion in a flow injection system for F-AAS determination of lead. Various natural samples (artichoke, chocolate, sewage sludge, tomato leaves), real and certified, were slurried in a mixture of HNO3 and H2O2 using magnetic stirring, followed by continuous pumping around an open recirculating system, part of which (120 cm PTFE tubing) was located in a domestic microwave oven.

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A microwave-heated, flow-through digestion container (coiled Teflon tubing) was designed for a commercial (Prolabo A300) focused microwave system (instead of microwave oven) and applied to the on-line preparation of biological samples, including milk, blood and urine [117]. For an extensive oxidation of organic sample constituents with nitric acid, temperatures of more than 200 oC are necessary. The PTFE tubes used, however, cannot withstand the vapor pressure of the digestion mixture at 200 oC or more. Thus, new alternatives had to be found to overcome this limitation. One way to increase the pressure resistance of the tubes is to wrap them with a plastic tape of high mechanical strength. Results from a digestion system (CEM SpectroPrep system) equipped with such tubes have been published [118]. A CEM SpectroPrep system was used at moderate powers to perform on-line digestion of slurried samples of biological tissues (0.5% m/v) and marine sediment (1 % m/v). The pressure thresholds of this system are near 25 bar. To achieve the desired temperatures of approximately 250 oC, however, it is necessary to be able to increase the pressure in the system up to 35 bar or so. A recently developed device enables the application of such high temperatures (250 oC) by means of a new pressure equilibrium system (with a pressure of 40 bar) [119]. The pressure equilibrium system keeps the pressure inside and outside the digestion tube (PTFE or PFA) equal, even for extremely fast oxidation reactions. Advantages of this high-performance flow digestion device are high sample throughput (up to 60 samples per hour), fast and complete digestion of liquids, emulsions and slurries, and computer- controlled fully automated sample decomposition. The systems ability to handle only up to 1% m/v slurries and lower slurry concentrations for biological materials restricts the type of sample that can be analyzed, unless the most sensitive elemental detection devices are used, such as ICP-MS. Therefore, Mason et al. [120] modified the SpectroPrep oven and developed a wide bore continuous flow microwave digestion system for the determination of trace metals (Cd, Cr, Mn, Ni, Pb) following aqua regia extraction. This device differs from existing commercially available devices as it uses a double pumping action to replace the back pressure regulator traditionally used to achieve internal pressurization. The described system demonstrated an ability to cope with real soil samples ground to a larger particle size (250 µm) and slurried without the use of surfactants. Perhaps the current fascination for using microwave heating for on-line digestion has led to the introduction of commercial instruments based on this hybrid technique. CEM developed the SpectroPrep continuous-flow automated microwave- digestion system. Similarly, Questron Technologies Corporation is marketing the QLAB AUTOPREP DISCRETE FLOW SYSTEM. Perkin-Elmer offers an on-line flow injection microwave-digestion system, as does SGT Middelburg BV (The Netherlands, FLOWAVE), a fully automatic continuous flow sample preparation and digestion system based on microwave technology. The advantages of microwave-enhanced flow systems basically include a significant reduction in sample preparation time, the ability to accomplish reactions that would normally be too dangerous in a closed vessel because of sudden increases in temperature and pressure, and the capability to handle transient or readily decomposed samples or intermediates. However, flow-through systems are a problem area because all samples must be homogeneous and small enough to pass through the tube, and the majority of samples require some form of processing before they can be put into the tube.

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5.4 Vapor-phase acid digestion (gas phase reactions) An alternative approach to acid digestion of the sample matrix which prevents introduction of impurities exploits gas-phase reactions. In the past four decades, several novel approaches to sample digestion procedures have been considered using inorganic acid vapor produced in one vessel to attack and dissolve material in another. A review by Matusiewicz [121] summarized analytical methods based on vapor-phase attack in promoting the dissolution and decomposition of inorganic and organic materials prior to determination of their trace element content. This approach is currently used (in open, semi-closed and closed systems) whenever applicable because digestion using gas phase reagents is preferred to the solution. The combination of hydrofluoric acid and nitric acid vapor as a digestion agent has proven effective in the preparation of samples for spectrographic determination of trace impurities in open system. Zilbershtein et al. [122] used this approach to dissolve silicon and to concentrate impurities on a PTFE sheet. The residue and PTFE sheet were transferred to a graphite electrode that subsequently served as one electrode of the dc arc for spectrographic trace analysis. However, dissolution with acids in open systems is unsuitable for attaining a sufficiently low determination when the analyte impurities in the reaction mixture are often far greater than those in the test component. With respect to semi-closed systems, a PTFE apparatus generating HF vapor has been specifically designed to minimize contamination during trace-element determination of ultrapure silicon, quartz and glass [123]. The sample is placed in a PTFE beaker mounted on a perforated PTFE plate that is kept above the level of liquid HF in the chamber. The apparatus has the advantage of a closed system by preventing air-borne particulates from entering the vessel but is continuously purged by a positive pressure of HF vapor during operation. Thomas and Smythe [124] describe a simple all- glass apparatus for vapor-phase oxidation of up to 90% of plant material with nitric acid. Addition of perchloric acid ensured fast and complete oxidation, and the presence of HNO3 during the final HClO4 oxidation step eliminated any danger of explosion. Klitenick et al. [125] used the same technique, with a simplified pressurized PTFE digestion vessel, for determination of zinc in brain tissue. Some materials may not be fully dissolved by acid digestion at atmospheric pressure. A more vigorous treatment involves bomb digestion in pressure vessels designed to incorporate the techniques of a closed pressure vessel and vapor-phase digestion in a single unit. This has the advantage of being easier to construct than the apparatus described in previous papers [122-125], and it requires considerably smaller volumes of acids. Heating can be accomplished in an ordinary oven (with conductive heating) or using a microwave field. A predecessor of this concept of closed-vessel vapor-phase sample digestion was introduced by Woolley [126]. He described a low-temperature (up to 110 oC) and high- temperature (up to 250 oC) version of the apparatus. Each device consists of an airtight PTFE vessel containing two concentric chambers: an inner chamber that holds the sample cup, and an outer chamber. Both vessels were designed for the digestion of high- purity glass using relatively impure solvent acids: a 50:50 mixture of concentrated HNO3 and HF. A completely closed PTFE bomb or autoclave [127] has been developed with a temperature gradient for digestion of more difficult compounds, such as siliceous material. Marinescu [128] presented an interesting development in which the conventional single-sample pressure digestion bomb was converted for multi-sample vapor-phase digestion. A multiplace holder for field sampling was developed to fit directly into the digestion bomb. This technique has been used for organic and inorganic solid, semisolid, and liquid samples. Kojima et al. [129] modified a sealed PTFE bomb

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in which the dissolution of highly pure silica with HNO3, HCl and HF acid vapor was possible using a PTFE vial placed in a PTFE outer vessel. A possible disadvantage of this system is that the vial has to be replaced regularly when used under pressure. This could make the method very costly. A laboratory-made high-pressure digestion bomb with a PTFE microsampling device was developed by Matusiewicz [130]. This simple and inexpensive apparatus was found to be convenient for treating a small number of samples and can be easily made by modifying available PTFE bombs [131]. It should be noted that PTFE microsampling devices can be used for both vapor-phase digestion and discrete nebulization techniques in atomic spectrometry. Vapor-phase digestion in a closed system (bomb) of high-purity materials for spectrographic determination of trace elements is a convenient and useful technique [132]. The method uses graphite electrodes with an enlarged cavity and excludes the use of a collector. Contact of the probes and the analytical impurities with laboratory glassware and the atmosphere during the pre-concentration process is also avoided. The blank signal is determined only by the purity of the electrode used for spectral analysis. A technique [133] has been developed that employs the vapor-phase acid generated in the quartz vessel of a commercial high-pressure, high-temperature digestion apparatus (High Pressure Asher HPA, Anton Paar, Graz, Austria). Small biological samples (50 – 165 mg) were digested in a mini-quartz sample holder (3.1 ml volume). When biological standard reference materials were digested at 230 oC and 122 bar, the residual carbon content in the digested samples was less than 1.8%. Despite methodologies previously proposed for closed systems with conventional heating being successful, very few attempts to employ microwave power for vapor-phase digestions have been described. An early trial with a low-pressure microwave arrangement was unsatisfactory [134], although recently an interesting variant of the digestion vessel design has been proposed for dissolution and decomposition of samples [135]. The method developed was an extension of the acid vapor-phase thermal pressure decomposition of biological materials previously reported by Matusiewicz [136]. Microwave-assisted vapor-phase acid digestion employing a special PTFE microsampling cup, suitable for 250 mg subsamples of marine biological and sediment reference materials were digested with HNO3 and HNO3-HF, respectively, at a maximum pressure of ca. 14 bar [135]. Very recently, several papers [137-141] discussed the further application and evaluation of this pioneering concept of Matusiewicz et al. [135], employing either commercial pressurized microwave digestion systems and quartz sample containers [137], quartz inserts [138,139], TFM inner vessels [140] or focused microwave ovens operating at atmospheric pressure and PTFE microsampling cups [141]. To summarize this section, use of acid vapor-phase digestion and attack of some organic and inorganic matrices as a sample preparation method is a convenient and useful technique. Closed pressure systems are the technique of choice to avoid losses of elements by volatilization while still maintaining extremely low values for the blank (by application of isopiestic distillation of the reagents and technical grade acids).

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5.5 Efficiency of wet digestion (decomposition and dissolution) procedures Quality control is becoming increasingly more significant in analytical chemistry. However, it is presently applied primarily to measurement techniques and not to sample preparation. For quality control in sample digestion, it is necessary to measure and record certain parameters exactly to be able to subsequently trace the course of the digestion process. In spite of that, complete decomposition of the sample is required to achieve reproducible and accurate elemental results by instrumental analytical methods. This is particularly the case for all voltammetric and polarographic determinations [142-145]. Interferences caused by incomplete decomposed organic compounds also occur, to a certain degree, when using atomic spectrometric methods such as AAS [146,147], ICP- OES [148,149] and ICP-MS [150,151]. As noted above, nitric acid is the most frequently utilized sample dissolution medium. Unfortunately, the carbon contained in organic materials is only partly converted to CO2 by HNO3 at temperatures of up to 200 oC (maximum operating temperature of PTFE vessels) [26]. In these cases, extending the digestion time and increasing the quantity of nitric acid does not improve the extent of decomposition. In principle, temperature and digestion time ultimately determine the effectiveness of a digestion, with residual carbon content serving as a useful measure of quantitative assessment [152-155]; in other words, the highest temperatures are required to achieve a decomposition as complete as possible [156,157]. It should be noted here that the usefulness of the decomposition technique should be judged not from a visual point of view, because it often happens that a clear, colorless solution, indistinguishable from water, still contains significant amounts of carbon. In closed systems the pressure depends not only on the temperature but also on the type and quantity of the sample, the size of the vessel, and the nature and quantity of the decomposition reagent. This pressure is not responsible for the decomposition quality, but nevertheless it should be controlled automatically. Würfels et al. [157-159] described the extremely strong impact from residual organic compounds on elemental determinations by means of inverse voltammetry and demonstrated that a temperature of 300 - 320 oC is necessary for pressurized sample digestion with pure nitric acid to obtain a solution containing less than 0.1% carbon. Otherwise, trace elements cannot be determined with inverse voltammetry. This was confirmed by Wasilewska et al. [160], who showed that for complete oxidation of organic compounds with nitric acid the decomposition temperature should be raised to 300 oC. The influence of the digestion equipment (either thermal or microwave) is negligible if the digestion time employed is long enough to reach the steady state temperature. Sample digestion with nitric acid between 220 and 250 oC (most commercial equipment is able to fulfil this prerequisite) leads to residual carbon contents in the low percentage range. The mode of heating of the digestion vessels is more and more supplanted by microwave technology, therefore microwave-assisted wet digestion is a frequently used sample preparation technique for trace element determinations in organic materials. Studies of the residual carbon content (RCC) as a measure of decomposition efficiency have been undertaken [134,161-165]. Using gas chromatography, Stoeppler et al. [152] quantified the ashing ability of conventional pressurized decomposition. Differences between the carbon content in the original sample and the carbon converted to CO2 showed that the investigated biological and environmental samples were not completely ashed with nitric acid. Würfels and Jackwerth [166] determined the residual carbon in samples digested under pressure or evaporated with HNO3. In most cases, microwave digestion of biological material was incomplete. Subsequently, the undigested

245 Chapter 13 compounds were identified [156]. Parallel to Würfels and Jackwerth’s studies [166], the residual organic species in nitric acid digests of bovine liver were identified by Pratt et al. [167]. Kingston and Jassie [168] evaluated the dissolution of several biological and botanical samples wet digested with HNO3. Free amino acid concentrations of human urine samples were typically reduced by a factor of 105. This reflects the comparative efficiency of protein hydrolysis, and is not necessarily equivalent to the total carbon oxidation efficiency. Nakashima et al. [163] investigated the digestion efficiency of HNO3-HClO4 mixtures. The total RCC in a number of digested marine biological reference material (NRCC TORT-1) solutions was determined and used as a relative measure of the efficiency of various digestion schemes. Two-stage microwave digestions (i.e., conventional digestion followed by cooling, venting of excess of gas from the bomb, re-capping and re-heating) were superior to single-stage digestions. However, even the two-stage procedures were not complete and 24% carbon remained. The determination of residual carbon in digests of biological material with simultaneous ICP-OES analysis was described by Que Hee and Boyle [164] and Krushevska et al. [165]. The oxidation efficiencies of different dry and wet ashing procedures for milk samples were compared by Krushevska et al. [169], who noted that the residual carbon concentrations obtained with medium-pressure microwave digestions varied between 5 and 15 %. Oxidizing mixtures of H2O2 or H2SO4 with HNO3 applied in a medium pressure (11 bar) microwave system did not yield a digestion efficiency higher than that for pure nitric acid (total acid volume was kept constant). Thus, until now, no suitable closed low- or medium-pressure microwave heated oxidation system has been available to completely decompose biological samples leaving no carbon residue. In spite of that, the safest way to obtain total mineralization is to complete these decomposition techniques with the addition of perchloric acid followed by heating until white perchloric fumes appear. However, with the high pressure/temperature focused microwave heated TFM-Teflon bomb device, organic material is totally oxidized with nitric acid in a single-step procedure [101,103] (the closed TFM-Teflon focused microwave heated bomb enables very high pressure and temperature to be reached). Continuous or stopped flow on-line microwave heated digestion appears very attractive since it can increase the sample throughput and extend the automation of sample handling. However, on-line microwave heated digestion was a priori expected to be associated with the problem of incomplete digestion of organic matter, since complete decomposition can only be achieved under vigorous conditions requiring high temperature and pressure. Matusiewicz and Sturgeon [170] critically evaluated on-line and high-pressure/temperature closed vessel techniques with regard to efficiency of digestion. The completeness of destruction of biological materials (Standard and Certified Reference Materials) was characterized in terms of their RCC in the solution following digestion. Pressurized decomposition in a TFM-Teflon vessel was the most effective procedure (organic material was totally oxidized with nitric acid in a single- step procedure), whereas urine and sewage plant effluent were incompletely decomposed (between 56 to 72%) with on-line microwave heated digestion using nitric acid, nitric acid and hydrogen peroxide, and peroxydisulphate oxidation. Very recently, the residual weight of a bottom anti-reflective coating (BARC) sample was successfully used as an indicator to evaluate the digestion kinetics [171]. The weight degradation rate was independent of the sample weight under various temperatures, but was strongly dependent on the digestion acid volume and the digestion temperature. Mathematical modeling for prediction of digestion efficiency for the BARC sample was achieved by employing digestion kinetics as the backbone. With empirical fitting of a pre- exponential factor, a novel equation incorporating the temperature, acid digestion

246 Chapter 13 volume, digestion time and sample weight was developed. As a result, appropriate digestion parameters could be logically evaluated using the resultant model to achieve the desired digestion efficiency. Hydrogen peroxide is a very popular oxidizing reagent as it is converted to water and oxygen during the oxidation of biological material [134,172-174]. No acid corrosion of the digestion vessel PTFE walls, no formation of insoluble salts with an acid anion, and no change of the sample matrix by an acid are additional advantages. Because of its strong oxidation power, only small amounts of H2O2 need be used, so that concentrated sample solutions can be obtained. Furthermore, high purity H2O2 is available and low blank concentration values (and thus low detection limits) can be achieved. However, explosion can occur if too much H2O2 is present. In addition, experiments with HNO3-H2O2 mixtures conducted by Matusiewicz et al. [134] showed that all versions of pressurized microwave digestion with HNO3 and H2O2 gave incomplete digestion. No significant improvement in the efficiency was achieved with 50% H2O2. The extension of this observation to medium-pressure and high temperature microwave heating provided verification of this observation [175]. Nitric acid digestion with the addition of H2O2 did not enhance digestion efficiency in this study compared to use of only HNO3. Wet acid digestion of aquatic samples is one of the most efficient processes; however, it is time consuming and there is risk of contamination by the reagents used. Thus, an alternative oxidizing reagent is desirable to completely and safely decompose organic carbon residues. The application of ozone seems to have advantages in this respect. It was found that ozone is very effective in destroying natural organic compounds [176-178], and has the potential to be used as an additional decomposition and/or finishing reagent [179]. Improvement in the pressurized microwave-assisted digestion procedure was achieved by adding optimum concentrations of strong oxidizing agents such as ozone [175]. The digestion efficiency of an optimal nitric acid system was improved by 13% by addition of ozone, with the further advantage that the agent does not contribute to the blank. A single digestion procedure is often insufficient for the complete decomposition of a complex matrix, leading some authors to recommend a combination of two or more techniques. Two examples will suffice to illustrate the principle [143,104]. First, pressure digestion followed by UV photolysis. Thus, it has been shown that analysis of olive leaves for heavy metals by voltammetric methods leads to distorted results after “pressure digestion” alone. Reliable data can be obtained only by supplementing the digestion with UV irradiation to ensure adequate decomposition of the matrix [143]. Secondly, a novel microwave-assisted high-temperature UV digestion procedure was developed for the accelerated decomposition of interfering dissolved organic carbon prior to trace element determination in liquid samples. This new technique significantly improved the performance of the process of UV digestion (oxidation) and is especially useful for ultratrace analysis due to its extremely low risk of contamination [104]. In order to investigate the completeness of dissolution of inorganic materials, the recovery (or incomplete recovery), and accuracy of major, minor and trace element determinations is usually applied. If silicates are present, which is usually the major inorganic component of many matrices (i.e., soils, sediments, sludges, ceramics and other similar samples) the use of HF to achieve complete dissolution is mandatory [180,181].

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5.6 Comparison of wet digestion techniques A careful comparison of several digestion techniques is the only way of assuring accurate results, particularly when little experience is available with respect to the digestion of a specific matrix, or existing reports are contradictory. The analyst must choose the sample preparation technique carefully to ensure that the system is optimal for the analyses at hand. However, there is still no universal sample preparation system. With respect to requirements specific to contamination or losses through volatilization or retention, convection heated or microwave-assisted wet digestion, quartz-lined high- pressure wet digestion, UV digestion and vapor-phase acid digestion seem to be the best choice. However, all of these techniques require considerable investment in apparatus. Digestion of samples in an open vessel presents a serious risk of significant analyte loss, despite the use of a reflux condenser. As far as economic aspects are concerned (low procurement, short digestion time, high sample throughput), microwave-assisted wet digestion, and especially microwave-assisted pressurized on-line digestion, appear to rank high. According to completion of the digestion, complete degradation of many samples is achieved only through high-pressure, high-temperature Teflon- or quartz- lined pressure vessel digestion, or by combination of a closed wet digestion system with UV irradiation. Table 4 summarizes the advantages and disadvantages of the wet digestion techniques discussed in section WET ACID DIGESTION PROCEDURES with respect to losses of analytes, blank levels, contamination problems, sample size, digestion time, degree of digestion, and economic aspects.

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TABLE 4 Advantages and disadvantages of wet digestion methods

Digestion technique Possible way of Source Sample size (g) Maximum Digestion Degree of Economical losses of blank organic inorganic temp pressure time digestion aspects (oC) (bar) Open systems Conventional heating Volatilization Acids, <5 <10 <400 Several Incomplete Inexpensive, needs Vessels, hours supervision Air Microwave heating Volatilization Acids, <5 <10 <400 <1h Incomplete Inexpensive, needs Vessels, Air supervision UV digestion None liquid <90 Several High Inexpensive, needs hours supervision 249 Closed systems Conventional heating Retention Acids (low) <0.5 <3 <320 <150 Several High Needs no hours supervision Microwave heating Retention Acids (low) <0.5 <3 <300 <200 <1h High Expensive, needs no supervision Flow systems Conventional heating Incomplete Acids (low) <0.1 <0.1 <320 >300 Several High Expensive, needs digestion (slurry) (slurry) minutes no supervision UV on-line digestion Incomplete None liquid <90 Several High Inexpensive, needs digestion minutes no supervision Microwave heating Incomplete Acids (low) <0.1 <0.3 <250 <40 Several High Expensive, needs digestion (slurry) (slurry) minutes no supervision Vapor-phase acid digestion None None <0.1 <0.1 <200 <20 <1h High Needs no supervision

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5.7 Digestion systems (instrumentation, equipment, automation) Presently, the instrumentation market offers many devices to make wet digestion more efficient and easier to manage by means of possible automation, but this is achieved principally with microwave energy. Wet digestions in open vessels are undertaken with or without refluxing. Since it is very critical to adhere very closely to the optimized time and temperature digestion parameters, mechanization of the digestion not only leads to higher sample throughput with less human intervention but also to the avoidance of errors. The simplest form of mechanization can be implemented through a time (programmable timer) and temperature (via an autotransformer) controlled heating block. There are many models of heating blocks on the market. A greater degree of mechanization would also incorporate control of reagent reflux during digestion. These procedures operate batch-wise. Continuous sample handling has some advantages over discontinuous handling; the former generally better match analytical needs. The automated wet digestion device (VAO, Anton Paar, Austria) is such a continuously operating digestion system, an ideal instrument for laboratories requiring high throughput of similar samples with which all methods of wet chemical digestions can be performed [182]. With the help of a microprocessor, all important digestion parameters are controlled. Automation controls the time-temperature/pressure program for sample digestion, so that different sample materials can be processed under optimum conditions. The loading or charging of the high pressure asher with sample material is achieved manually. A fully automated version of this high pressure asher is not available. Berghof pressure digestion systems [183] serve for sample preparation of inorganic and organic matrices at high temperature (max. 200 – 250 oC) and high pressure (max. 100 and 200 bar) in pure, isostatically pressed PTFE or quartz vessels. As noted already, three basic types of microwave-assisted digestion systems have evolved: atmospheric pressure, elevated pressure (closed vessel), and flow- through, working in the two most common modes: multimode cavity and focused-type (waveguide). Reviews of commercially available microwave-assisted digestion systems and vessels (summary of the vessels, ovens and oven systems) are given in [29- 32,35,37,41,184-186] together with specifications and features for elevated-pressure, atmospheric pressure and flow-through units. The reader should also consult Chapter 8 of this volume. The simplicity and efficacy of microwave digestion easily lends itself to automation and robotics. Systems have been developed which are capable of weighing samples, adding acids, capping and uncapping vessels, accomplishing microwave- assisted digestion, diluting digestates, transferring vessels, and even cleaning and reusing the vessels. Once such a system is operational, the only things the analyst has to do is supply and place the representative sample(s) in locations recognized by the system and then initiate the controlling program (Table 18 in Ref. [35] primarily summarizes the application and functioning of these systems).

5.8 Safety of acid digestions The reagents, instruments, and operations employed in the digestion of materials are potentially hazardous, even when used as directed. The operator must always be properly protected with a laboratory coat, gloves, and safety glasses or, better still, face protection. Some concentrated fuming acids (HF, HNO3, HCl), are to be handled only in a well-ventilated hood. Oxidizing acids (HNO3, HClO4) are more hazardous than nonoxidizing acids (HCl, H3PO4, HF), being more prone to explosion, especially in the presence of reducing agents, such as organic matter. Perchloric acid is oxidizes only

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when it is concentrated and hot; it must never be brought into contact with organic matter unless diluted with nitric acid. Acid digestion must be conducted in a fume cupboard with efficient scrubbers installed. The evaporation of perchloric acid is to be performed only in an appropriate stainless-steel, stoneware, or polypropylene hood, with washing facilities to eliminate any perchlorate deposit. Great care should be taken when using “pressure digestion” methods. Pressure digestion vessels (bombs) contain the acid fumes and are useful for rapid, one-step digestions without losses. But, again, there are restrictions; in some reactions (especially spontaneous) potentially explosive gases are produced that exceed the safety limits of the vessels. For instance, nitric acid and especially the spontaneous HNO3 and H2O2 digestion of organic matter in a closed vessel, may result in explosion due to unintended pressure build-up within the vessel. These systems produce high-pressure spikes, which can be avoided by decreasing the sample weight or by applying a gradual temperature increase. Microwave-assisted sample digestion has its own safety requirements. As a result of the direct energy absorption and rapid heating, microwave techniques introduce unique safety considerations that are not encountered in other methods. Differences in conditions between traditional laboratory practices and microwave-implemented methods should be examined before microwave energy is applied to heat reagents or samples. An excellent summary of this aspect is extensively reviewed in [30, 31].

6 CONCLUSIONS AND FUTURE TRENDS The chief methods used for the digestion of organic and inorganic samples have been evaluated. A brief summary of applications of these techniques to various sample matrices is presented in Table 5. The variety of approaches currently available for the digestion of solid and liquid samples allows the most suitable method to be selected for each application, depending on both the matrix and type of analyte, and the subsequent steps to be developed in order to complete the analytical process. In spite of that, it is fair to point out that sample digestion must not be looked at as an isolated step, but one that needs to be integrated into the entire analytical process.

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TABLE 5 Summary of applications of total wet digestion procedures to the analysis of materials (determination of elements)

Material/Matrix/Sample Required acid(s)* Digestion technique (mode=) Reference Water(s) H2O2, HNO3 UV radiation [49] Environmental samples coal HNO3, HCl, HF open or closed system [20,31,35,38] coal fly ash aqua regia& + HF# open or closed system [20,31,35,38] dust aqua regia + HF open or closed system [20,31,35,38] catalysts aqua regia open systems [20,21] Waste materials sewage sludge HNO3, HCl open or closed or flow systems [18,31] waste water HNO3 flow systems [31,39] Biological samples botanicals HNO3 + H2O2 + HF open or closed system [15,16,18,31] plants HNO3 + H2O2 + HF open or closed system [15,16,18,31] clinical HNO3 open or closed system [15,16,18,31] marine HNO3 open or closed system [15,16,18,31] Forensic HNO3 open or closed system [41] Food(s) HNO3 open or closed system [31,41] Beverages HNO3, H2O2 open or closed or flow systems [41] Silicates soils aqua regia + HF open and/or closed systems [19-22,35] sediments aqua regia + HF open and/or closed systems [19-22,35] glasses HF open systems [19-22] Geological samples rocks aqua regia + HF@ open or closed systems [20-22,31,33] ores aqua regia + HF open or closed systems [20-22,31,33] minerals HF + H2SO4, HCl open systems [20-22,31,33] Petroleum products fuels HNO3 + HCl open or closed system [23,31] oils HNO3 + HCl open or closed system [23,31] Drugs and pharmaceuticals HCl, HNO3 open systems [41] Metals ferrous HNO3 + (HF or HNO3 or H2SO4) open systems [41] non-ferrous HCl or HNO3 or HF open systems [41] alloys aqua regia + HF open systems [41] ^ steels HCl + HNO3, HClO4 open systems [41] Chemicals HCl, HNO3, HF, H2SO4 open or closed systems [20,23] Polymers HCl, HNO3, HF, H2SO4 open or closed systems 23,41] Refractory compounds% ceramics HNO3, HCl, HF, H2SO4, H2O2 open or closed systems [20] composites HNO3, HCl, HF, H2SO4, H2O2 open or closed systems [20] Nuclear materials HNO3 or HCl, H3PO4, HClO4 open or closed systems [21] * Concentrated acids are usually employed; H2O2 is 30 %; in most cases alternative digestions are possible depending on requirements of analyst =Conventional or microwave &Unstable #Use only Teflon vessels, the addition of HF is required to obtain quantitative recoveries for Cr @ Addition of H3BO3 to neutralize the HF by forming tetrafluoro-boric acid ^Danger of explosion %Certain refractory materials are not decomposed; these must be solubilized by fusion

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Attention has been focused on digestion at elevated temperature and pressure. High-pressure digestion with its large digestion temperature range is the most universal digestion system at present, and is the technique of choice for the vast majority of both inorganic and organic materials. New ways to further increase the efficiency of sample preparation should continue with development of hyphenated digestion techniques. A novel, microwave-assisted, high-temperature UV-digestion for accelerated decomposition of dissolved organic compounds or slurries was developed [104]. This new technique is ideal for extreme trace analysis due to the low blank values and low acid concentration. In addition, this digestion method can be used for the determination of non-metals by ion chromatography. Alternatively, within the limits of the Teflon- lined digestion vessels, improvement in the digestion efficiency can be achieved by adding optimum concentrations of strong oxidizing agents, such as ozone or oxygen, which appear to be efficient digestion agents for the treatment of biological material. Again, this has the advantage that the agent does not contribute to the analysis blank. It should be mentioned that vapor-phase acid digestion offers an alternative solution to these problems: reduced concentration of acid in the digestate and the possibility of using a technical grade acid without any deterioration of the analytical blank. Another example where significant improvement in digestion and dissolution was obtained is the use of a reactor that combines microwave and ultrasound energy [48]. It is expected that these two methods could open a new research field “combined digestion techniques”. It can be said with certainty that the majority of all digestions will be performed in the future by means of microwave-assistance. Progress has been made over the past several years in reducing systematic errors and improving detection limits with microwave digestion, as well as its automation. A noticeable trend toward pressurized closed vessel systems permitting high temperature decomposition compatible with trace analysis has occurred. While some researchers advocate high-pressure (>100 bar) digestion at 250 – 300 oC to destroy interferences in refractory compounds, manufacturers are working to devise sample vessels that can withstand these conditions. There has been a growing trend in recent years towards development of fully automated on-line analysis techniques. Microwave-assisted high pressure flow digestion system with PTFE or PFA tubes for digestion temperatures up to 250 oC open up new possibilities for fully automated sample preparation [119]. On the other hand, the development of new high-temperature/high-pressure flow digestion systems that incorporate resistively heated capillaries for the continuous digestion of various samples coupled with atomic spectrometric instruments has arisen [107-109]. It is predicted that flow systems will become dominant for liquid samples and slurries and extend the analytical capabilities of instrumental methods by combining sample preparation with simultaneous analysis using only micrograms of sample and microliters of reagents. The final goal of these studies should be the adaptation of standard batch digestion methods to on-line systems combining flow-through digestion directly to analyzers. It is evident that wet digestion methods will remain a fertile area for development. New digestion techniques need to be designed which address the limitations of the instrumentation and maximize its potential. Development trends for conventional and microwave instruments will focus on sample throughput, enhanced vessel performance specifications, the use of new materials, further refinement of in situ vessel control (direct temperature and pressure, incident and reflected microwave power), and computer controlled sample digesters with automated capability.

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Finally, the development of automated methods for wet digestion of solid samples without human participation can only be achieved with the use of a robotic station. Nevertheless, a number of auxiliary energies and commercially available modules can facilitate and/or accelerate one of the most time consuming steps of the analytical process, i.e., to obtain the analyte(s) from a solid sample in the form of a solution.

REFERENCES [1] Duflos A., Hand. d. pharm. chem. Praxis, 2. Aufl., I. Max u. Comp., Breslau, 534 (1938) [2] Fresenius R., and v. Babo L., Justus Liebigs Ann. Chem., 49, 287 (1844) [3] Danger F.P., and Flandin C., C.R. Acad. Sci., 12, 1089 (1841) [4] Carius L., Justus Liebigs Ann. Chem., 116, 1 (1860); 136, 129 (1865) [5] Kjeldahl J., Z. Anal. Chem., 22, 366 (1883) [6] Classen A., and Bauer O., Ber. Dtsch. Chem. Ges., 16, 1061 (1884) [7] Stcherbak A., Arch. Med. Exp., 5, 309 (1893) [8] Van Slyke D.D., Anal. Chem., 26, 1706 (1954) [9] Polley D., and Miller V.L., Anal. Chem., 27, 1162 (1955) [10] Fenton H.J.H., J. Chem. Soc. (London), 65, 899 (1894) [11] Sansoni B., Sigmund O., Bauer-Schreiber E., Wiegand W., and Perrera L., Angew. Chem., 73, 763 (1961) [12] Sansoni B., and Kracke W., Z. Anal. Chem., 243, 209 (1968) [13] Pure Appl. Chem., 60, 1461 (1988) [14] Pure Appl. Chem., 61, 1139(1989) [15] Gorsuch T.T., The Destruction of Organic Matter, Pergamon Press, Oxford, 1970 [16] Sansoni B., and Panday V.K., Ashing in Trace Element Analysis of Biological Material in: Analytical Techniques for Heavy Metals in Biological Fluids, S. Fachetti(ed), Elsevier, Amsterdam 1983, 91 [17] Subramanian K.S., Spectrochim. Acta, B51, 291 (1996) [18] Iyengar G.V., Subramanian K.S, and Woittiez J.R.W., Sample Decomposition in: Element Analysis of Biological Samples. Principles and Practice, CRC Press, Boca Raton, 1998 103 [19] Šulcek Z., Povondra P., and Doležal J., CRC Crit. Rev. Anal. Chem., 6, 255 (1977) [20] Šulcek Z., and Povondra P., Methods of Decomposition in Inorganic Analysis, CRC Press, Inc., Boca Raton 1989 [21] Povondra P., and Šulcek Z., Modern Methods of Decomposition of Inorganic Substances in: Instrumentation in Analytical Chemistry, J. Zýka(ed), Ellis Horwood, New York 1991, 188 [22] Chao T.T., and Sanzolone R.F., J. Geochem. Explor., 44, 65 (1992) [23] Bock R., A Handbook of Decomposition Methods in Analytical Chemistry, translated and revised by I.L. Marr, International Textbook Company, Glasgow 1979 [24] Bajo S., Dissolution of Matrices in: Preconcentration Techniques for Trace Elements, Z.B. Alfassi and C.M. Wai(eds), CRC Press, Boca Raton 1992, 3 [25] Vandecasteele C., and Block C.B., Sample Preparation in: Modern Methods for Trace Element Determination, John Wiley & Sons, Chichester 1993, 9 [26] Sampling and Sample Preparation, M. Stoeppler(ed), Springer, Berlin, 1997 [27] Hoenig M., and de Kersabiec A-M., Spectrochim. Acta, B51, 1297 (1996) [28] Hoenig M., Talanta, 54, 1021 (2001)

254 Chapter 13

[29] Krakovská E., and Kuss H-M., Rozklady v Analitickej Chémii (in Slovak), VIENALA, Košice, Slovakia, 2001 [30] Introduction to Microwave Sample Preparation. Theory and Practice, H.M. Kingston and L.B. Jassie(eds), ACS, Washington, DC, 1988 [31] Microwave-Enhanced Chemistry. Fundamentals, Sample Preparation, and Applications, H.M.”Skip” Kingston and S.J. Haswell(eds), ACS, Washington, DC, 1997 [32] Matusiewicz H., and Sturgeon R.E., Prog. Analyt. Spectrosc., 12, 21 (1989) [33] Kuss H-M., Fresenius J. Anal. Chem., 343, 788 (1992) [34] Zlotorzynski A., Crit. Rev. Anal. Chem., 25, 43 (1995) [35] Smith F.E., and Arsenault E.A., Talanta, 43, 1207 (1996) [36] Burguera M., and Burguera J.L., Quim. Anal., 15, 112 (1996) [37] Kingston H.M.”Skip”., and Walter P.J., The Art and Science of Microwave Sample Preparations for Trace and Ultratrace Elemental Analysis in: Inductively Coupled Plasma Mass Spectrometry A. Montaser(ed), Wiley-VCH, New York 1998, 33. [38] Lamble K.J., and Hill S.J., Analyst, 123, 103R (1998) [39] Burguera M., and Burguera J.L., Anal. Chim. Acta, 366, 63 (1998) [40] Chakraborty R., Das A.K., Cervera M.L., and de la Guardia M., Fresenius J. Anal. Chem., 355, 99 (1996) [41] Encyclopedia of Analytical Chemistry, R.A. Meyers(ed), John Wiley&Sons Ltd., Chichester 2000 [42] Kuehner E.C., Alvarez R., Paulsen P.J., and Murphy T.J., Anal. Chem., 44, 2051 (1972) [43] Barnes R.M., Quináia S.P., Nóbrega J.A., and Blanco T., Spectrochim. Acta, B53, 769 (1998) [44] Bethge P.O., Anal. Chim. Acta, 10, 317 (1954) [45] Nadkarni R.A., Anal. Chem., 56, 2233 (1984) [46] Abu-Samra A., Morris J.S., and Koirtyohann S.R., Anal. Chem., 47, 1475 (1975) [47] Nóbrega J.A., Trevizan L.C., Araújo G.C.L., and Nogueira A.R.A., Spectrochim. Acta, B57, 1855 (2002) [48] Lagha A., Chemat S., Bartels P.V., and Chemat F., Analusis, 27, 452 (1999) [49] Golimowski J., and Golimowska K., Anal. Chim. Acta, 325, 111 (1996) [50] Trapido M., Hirvonen A., Veressinina Y., Hentunen J., and Munter R., Ozone Sci. Eng., 19, 75 (1997) [51] Mitscherlich A., J. Prakt. Chem., 81, 108 (1860) [52] Carius G.L., Ber. Dtsch. Chem. Ges., 3, 697 (1870) [53] Gordon C.L., J. Res. Natl. Bur. Stand., 30, 107 (1943) [54] Wichers E., Schlecht W.G., and Gordon C.L., J. Res. Natl. Bur. Stand., 33, 363 (1944) [55] Wichers E., Schlecht W.G., and Gordon C.L., J. Res. Natl. Bur. Stand., 33, 451 (1944) [56] Long S.E., and Kelly W.R., Anal. Chem., 74, 1477(2002) [57] Jannasch P., Z. Anorg. Allgem. Chem., 6, 72 (1894) [58] May E., and Rowe J.J., Anal. Chim. Acta, 33, 648 (1965) [59] Jackwerth E., and Gomišček S., Pure Appl. Chem., 56, 479 (1984) [60] Ito J., Bull. Chem. Soc. Japan, 35, 225 (1962) [61] Langmyhr F.J., and Sveen S., Anal. Chim. Acta, 32, 1 (1965) [62] Bernas B., Anal. Chem., 40, 1682 (1968) [63] Okamoto K., and Fuwa K., Anal. Chem., 56, 1758 (1984)

255 Chapter 13

[64] Ravey M., Farberman B., Hendel I., Epstein S., and Shemer R., Anal. Chem., 67, 2296 (1995) [65] Uhrberg R., Anal. Chem., 54, 1906 (1982) [66] Kotz L., Henze G., Kaiser G., Pahlke S., Veber M., and Tölg G., Talanta, 26, 681 (1979) [67] Stoeppler M., and Backhaus F., Fresenius Z. Anal. Chem., 291, 116 (1978) [68] Kotz L., Kaiser G., Tschöpel P., and Tölg G., Z. Anal. Chem., 260, 207 (1972) [69] Tomljanovic M., and Grobenski Z., At. Absorption Newslett., 14, 52 (1975) [70] Uchida T., Kojima I., and Iida C., Anal. Chim. Acta, 116, 205 (1980) [71] Stoeppler M., Müller K.P., and Backhaus F., Fresenius Z. Anal. Chem., 297, 107 (1979) [72] Takenaka M., Kozuka S., Hayashi M., and Endo H., Analyst, 122, 129 (1997) [73] Lechler P.J., Desilets M.O., and Cherne F.J., Analyst, 113, 201 (1988) [74] Knapp G., ICP Information Newslett., 10, 91 (1984) [75] Knapp G., Fresenius Z. Anal. Chem., 317, 213 (1984) [76] Knapp G., Intern. J. Environ. Anal. Chem., 22, 71 (1985) [77] Strenger S., and Hirner A.V., Fresenius J. Anal. Chem., 371, 831 (2001) [78] Kettisch P., Maichin B., Zischka M., and Knapp G., Trends in Sample Preparation 2002. Development and Application (Conference, Abstract T-11), Seggau, Austria, 2002 [79] Hale M., Thompson M., and Lovell J., Analyst, 110, 225 (1985) [80] Eggimann D.W., and Betzer P.R., Anal. Chem., 48, 886 (1976) [81] Adrian W.J., At. Absorption Newslett., 10, 96 (1971) [82] Kuennen R.W., Wolnik K.A., Fricke F.L., and Caruso J.A., Anal. Chem., 54, 2146 (1982) [83] May K., and Stoeppler M., Fresenius Z. Anal. Chem., 317, 248 (1984) [84] Matusiewicz H., Chem. Anal. (Warsaw), 28, 439 (1983) [85] Matthes S.A., Farrell R.F., and Mackie A.J., Tech. Prog. Rep – US Bur. Mines, 120, 9 (1983) [86] Smith F., Cousins B., Bozic J., and Flora W., Anal. Chim. Acta, 177, 243 (1985) [87] Buresch O., Hönle W., Haid U., and v. Schnering H.G., Fresenius Z. Anal. Chem., 328, 82 (1987) [88] Alvarado J., León L.E., López F., and Lima C., J. Anal. At. Spectrom., 3, 135 (1988) [89] Kojima I., Uchida T., and Iida C., Anal. Sci., 4, 211 (1988) [90] Kingston H.M., and Jassie L.B., Anal. Chem., 58, 2534 (1986) [91] Xu L.Q., and Shen W.X., Fresenius Z. Anal. Chem., 332, 45 (1988) [92] Nicholson J.R.P., Savory M.G., Savory J., and Wills M.R., Clin. Chem., 35, 488 (1989) [93] Baldwin S., Deaker M., and Maher W., Analyst, 119, 1701 (1994) [94] Reid H.J., Greenfield S., Edmonds T.E., and Kapdi R.M., Analyst, 118, 1299 (1993) [95] Reid H.J., Greenfield S., and Edmonds T.E., Analyst, 118, 443 (1993) [96] Heltai G., and Percsich K., Talanta, 41, 1067 (1994) [97] Pougnet M., Michelson S., and Downing B., J. Microwave Power Electromagnetic Energy, 26, 140 (1991) [98] Pougnet M., Downing B., and Michelson S., J. Microwave Power Electromagnetic Energy, 28, 18 (1993) [99] Légère G., and Salin E.D., Appl. Spectrosc., 49, 14A (1995) [100] Légère G., and Salin E.D., Anal. Chem., 70, 5029 (1998)

256 Chapter 13

[101] Matusiewicz H., Anal. Chem., 66, 751 (1994) [102] Lautenschläger W., Apparatus for Performing Chemical and Physical Reactions, U.S. Patent 5,382,414, Jan. 17, 1995 [103] Matusiewicz H., Anal. Chem., 71, 3145 (1999) [104] Florian D., and Knapp G., Anal. Chem., 73, 1515 (2001) [105] Allen E., Automation in Analytical Chemistry, Technicon Symposia p.247, 1966 [106] Gluodenis T.J., and Tyson J.F., J. Anal. At. Spectrom., 7, 301 (1992) [107] Gräber C., and Berndt H., J. Anal. At. Spectrom., 14, 683 (1999) [108] Haiber S., and Berndt H., Fresenius J. Anal. Chem., 368, 52 (2000) [109] Jacob P., and Berndt H., J. Anal. At. Spectrom., 17, 1615 (2002) [110] Fernandes S.M.V., Lima J.L.F.C., and Rangel A.O.S.S., Fresenius J. Anal. Chem., 366, 112 (2000) [111] Burguera J.L., and Burguera M., Flow injection systems for on-line sample dissolution/decomposition in: Flow Analysis with Atomic Spectrometric Detectors, A. Sanz-Medel(ed), chapter 5, Elsevier, Amsterdam, 1999, 135 [112] Burguera M., Burguera J.L., and Alarcón O.M., Anal. Chim. Acta, 179, 351 (1986) [113] Carbonell V., de la Guardia M., Salvador A., Burguera J.L., and Burguera M., Anal. Chim. Acta, 238, 417 (1990) [114] Karanassios V., Li F.H., Liu B., and Salin E.D., J. Anal. At. Spectrom., 6, 457 (1991) [115] Haswell S.J., and Barclay D., Analyst, 117, 117 (1992) [116] Burguera M., Burguera J.L., and Alarcón O.M., Anal. Chim. Acta, 214, 421 (1988) [117] Martines Stewart L.J., and Barnes R.M., Analyst, 119, 1003 (1994) [118] Sturgeon R.E., Willie S.N., Methven B.A., Lam J.W., and Matusiewicz H., J. Anal. At. Spectrom., 10, 981 (1995) [119] Pichler U., Haase A., Knapp G., and Michaelis M., Anal. Chem., 71, 4050 (1999) [120] Mason C.J., Coe G., Edwards M., and Riby P., Analyst, 125, 1875 (2000) [121] Matusiewicz H., Spectroscopy, 6, 38 (1991) [122] Zilbershtein Kh.I., Piriutko M.M., Jevtushenko T.P., Sacharnova I.L., and Nikitina O.N., Zavod. Lab., 25, 1474 (1959) [123] Mitchell J.W., and Nash D.L., Anal. Chem., 46, 326 (1974) [124] Thomas A.D., and Smythe L.E., Talanta, 20, 469 (1973) [125] Klitenick M.A., Frederickson C.J., and Manton W.I., Anal. Chem., 55, 921 (1983) [126] Woolley J.F., Analyst, 100, 896 (1975) [127] Karpov Yu.A., and Orlova V.A., Vysokochist. Veshchestva, 2, 40 (1990) [128] Marinescu D.M., Analusis, 13, 469 (1985) [129] Kojima I., Jinno F., Noda Y., and Iida C., Anal. Chim. Acta, 245, 35 (1991) [130] Matusiewicz H., Chem. Anal. (Warsaw), 33, 173 (1988) [131] Matusiewicz H., Chem. Anal. (Warsaw), 28, 439 (1983) [132] Pimenov V.G., Pronchatov A.N., Maksimov G.A., Shishov V.N., Shcheplyagin E.M., and Krasnova S.G., Zh. Anal. Khim., 39, 1636 (1984) [133] Amaarsiriwardena D., Krushevska A., Argentine M., and Barnes R.M., Analyst, 119, 1017 (1994) [134] Matusiewicz H., Sturgeon R.E., and Berman S.S., J. Anal. At. Spectrom., 4, 323 (1989) [135] Matusiewicz H., Sturgeon R.E., and Berman S.S., J. Anal. At. Spectrom., 6, 283 (1991) [136] Matusiewicz H., J. Anal. At. Spectrom., 4, 265 (1989)

257 Chapter 13

[137] Amarasiriwardena D., Krushevska A., and Barnes R.M., Appl. Spectrosc., 52, 900 (1998) [138] Czégény Zs., Berente B., Óvári M., Garcia Tapia M, and Záray Gy., Microchem. J., 59, 100 (1998) [139] Eilola K., and Perämäki P., Fresenius J. Anal. Chem., 369, 107 (2001) [140] Han Y., Kingston H.M., Richter R.C., and Pirola C., Anal. Chem., 73, 1106 (2001) [141] Araújo G.C.L., Nogueira A.R.A., and Nóbrega J.A., Analyst, 125, 1861 (2000) [142] Würfels M., Jackwerth E., and Stoeppler M., Fresenius Z. Anal. Chem., 329, 459 (1987) [143] Hertz J., and Pani R., Fresenius Z. Anal. Chem., 328, 487 (1987) [144] Schramel P., and Hasse S., Fresenius J. Anal. Chem., 346, 794 (1993) [145] Reid H.J., Greenfield S., and Edmonds T.E., Analyst, 120, 1543 (1995) [146] Nève J., Hanocq M., Molle L., and Lefebvre G., Analyst, 107, 934 (1982) [147] Welz B., Melcher M., and Nève J., Anal. Chim. Acta, 165, 131 (1984) [148] Knapp G., Maichin B., and Baumgartner U., At. Spectrosc., 19, 220 (1998) [149] Machat J., Otruba V., and Kanicky V., J. Anal. At. Spectrom., 17, 1096 (2002) [150] Ashley D., At. Spectrosc., 13, 169 (1992) [151] Begerov J., Turfeld M., and Duneman L., J. Anal. At. Spectrom., 12, 1095 (1997) [152] Stoeppler M., Müller K.P., and Backhaus F., Fresenius Z. Anal. Chem., 297, 107 (1979) [153] Matusiewicz H., Golik B., and Suszka A., Chem. Anal. (Warsaw), 44, 559 (1999) [154] Carrilho E.N.V.M., Nogueira A.R.A., Nóbrega J.A., de Souza G.B., and Cruz G.M., Fresenius J. Anal. Chem., 371, 536 (2001) [155] Costa L.M., Silva F.V., Gouveia S.T., Nogueira A.R.A., and Nóbrega J.A., Spectrochim. Acta, B56, 1981 (2001) [156] Würfels M., Jackwerth E., and Stoeppler M., Anal. Chim. Acta, 226, 1 (1989) [157] Würfels M., Jackwerth E., and Stoeppler M., Anal. Chim. Acta, 226, 17 (1989) [158] Würfels M., Jackwerth E., and Stoeppler M., Fresenius J. Anal. Chem., 329, 459 (1987) [159] Würfels M., Marine Chem., 28, 259 (1989) [160] Wasilewska M., Goessler W., Zischka M., Maichin B., and Knapp G., J. Anal. At. Spectrom., 17, 1121 (2002) [161] Matusiewicz H., Suszka A., and Ciszewski A., Acta Chim. Hung., 128, 849 (1991) [162] Krushevska A., Barnes R.M., Amarasiriwaradena C.J., Foner H., and Martines L., J. Anal. At. Spectrom., 7, 851 (1992) [163] Nakashima S., Sturgeon R.E., Willie S.N., and Berman S.S., Analyst, 113, 159 (1988) [164] Hee S.S.Q., and Boyle J.R., Anal. Chem., 60, 1033 (1988) [165] Krushevska A., Barnes R.M., and Amarasiriwaradena C.J., Analyst, 118, 1175 (1993) [166] Würfels M., and Jackwerth E., Fresenius Z. Anal. Chem., 322, 354 (1985) [167] Pratt K.W., Kingston H.M., MacCrehan W.A., and Koch W.F., Anal. Chem., 60, 2024 (1988) [168] Kingston H.M., and Jassie L.B., Anal. Chem., 58, 2534 (1986) [169] Krushevska A., Barnes R.M., Amarasiriwaradena C.J., Foner H., and Martines L., J. Anal. At. Spectrom., 7, 845 (1992) [170] Matusiewicz H., and Sturgeon R.E., Fresenius J. Anal. Chem., 349, 428 (1994) [171] Ko F-H., and Chen H-L., J. Anal. At. Spectrom., 16, 1337 (2001)

258 Chapter 13

[172] Matusiewicz H., and Barnes R.M., Anal. Chem., 57, 406 (1985) [173] Sah R.S., and Miller R.O., Anal. Chem., 64, 230 (1992) [174] Veschetti E., Maresca D., Cutilli D., Santarsiero A., and Ottaviani M., Microchem. J., 67, 171 (2000) [175] Matusiewicz H., Chem. Anal. (Warsaw), 46, 897 (2001) [176] Clem R.G., and Hodgson A.T., Anal. Chem., 50, 102 (1978) [177] Filipović-Kovačević Ž., and Sipos L., Talanta, 45, 843 (1998) [178] Sasaki K., and Pacey G.E., Talanta, 50, 175 (1999) [179] Jiang W., Chalk S.J., and Kingston H.M.”Skip”., Analyst, 122, 211 (1997) [180] Schramel P., Lill G., and Seif R., Fresenius Z. Anal. Chem., 326, 135 (1987) [181] Matusiewicz H., Mikrochim. Acta, 111, 71 (1993) [182] Budna K.W., and Knapp G., Fresenius Z. Anal. Chem., 294, 122 (1979) [183] Stainless steel pressure vessels with Teflon inserts, Berghof Laborprodukte GmbH, Eningen, Germany [184] Matusiewicz H., Development of High-Pressure Closed-Vessel Systems for Microwave-Assisted Sample Digestion in: Microwave-Enhanced Chemistry. Fundamentals, Sample Preparation, and Applications, H.M. (Skip) Kingston and S.J. Haswell (eds.), Chapter 4, Am. Chem. Soc., Washington, 1997, 353 [185] Erickson B., Anal. Chem., 70, 467A (1998) [186] Richter R.C., Link D., and Kingston H.M.”Skip”., Anal. Chem., 73, 31A (2001)

259