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. Chapter 13 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. 226 Chapter 13 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

Load more