ANALYTICAL SCIENCES APRIL 2006, VOL. 22 503 2006 © The Japan Society for Analytical Chemistry

Reviews Solid-Phase Extraction on Alkyl-bonded Silica Gels in Inorganic Analysis

Boris Ya. SPIVAKOV,† Galina I. MALOFEEVA, and Oleg M. PETRUKHIN

Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow 119991, Russia

Solid-phase extraction (SPE) is an effective tool for the preconcentration of trace elements and their separation from various sample constituents. Octadecyl and other alkyl-bonded silica gels are most widely used for these purposes. The fundamentals of the SPE of inorganic ions are reviewed and compared with those of related techniques (liquid–liquid extraction and reversed-phase liquid chromatography). The extraction of ions in the form of chelate compounds, inorganic salts solvated by neutral reagents, and ion-pair compounds is considered. Numerous applications of SPE to the separation and preconcentration of different elements and their species, including on-line combinations with instrumental determination techniques, are described and tabulated.

(Received October 20, 2004; Accepted April 25, 2005)

1 Introduction 503 6 Application of SPE to Metal Preconcentration 510 2 Theory of Solid-Phase Extraction and Related 7 Solid-Phase Extraction in Element Speciation Techniques 504 Analysis 511 3 Chelate Compounds 505 8 Conclusions 515 4 Coordinatively Solvated Compounds 509 9 References 515 5 Ion-Pair Extraction 509

all, these are techniques based on the use of chemically bonded 1 Introduction comlexing groupings.10 Commercially available Kelex-100, containing iminodiacetate functions is one of best known Preconcentration and separation processes are often needed for adsorbents of this kind.7,8 A number of adsorption materials the determination of trace elements in environmental samples, with various classical chelating groups as well as with high-purity materials and other matrices.1–4 Solid-phase macrocycles attached to silica gel by a chemical bond are extraction (SPE) with the use of alkyl-bonded and other surface- applied.9,10 There are some new technologies that have modified silica gels has been used in the last 15 years as an appeared in SPE and SPE-related products. Solid-phase alternative to liquid–liquid extraction in the analysis of various microextraction (SPME) involves the sorption of analytes onto a samples. The method is simple; it enables the rapid and microfiber, which is made of a fused-silica fiber coated with a complete isolation of the analytes of interest from a complex hydrophobic polymer. SPME is usually followed by direct matrix with a preconcentration factor of several orders of desorption in the inlet of a gas chromatograph.11,12 This method magnitude to be achieved. A surface-modified adsorbent is a can be considered as a microvariant of extraction support bearing at the surface of a chemically bonded chromatography.13 There are some new phases for SPE “monomolecular” layer of adsorbing groups, which provides including highly cross-linked copolymers, graphitized carbon fast kinetics of the recovery and elution of the molecules and for adsorbing polar organic compounds, internal reversed-phase ions under study.5–9 SPE by use of alkyl-bonded silica gels has sorbents for clean-up of complex biological materials, etc.6 been widely applied in the environmental analysis of Alkyl-bonded sorbents have been mainly studied for SPE in polyaromatic hydrocarbons, pesticides, substances of inorganic analysis; our attention in this paper will be focused on biochemical origin and other organic compounds. However, its such materials. The most simple mode of metal preconcentration application in metal extraction and preconcentration has been by SPE is based on the addition of a soluble complexing reagent much less extensive, although the capabilities of the technique to the test solution, followed by the adsorption of metal in inorganic analysis also appear to be very promising. complexes formed on an alkyl-bonded material. Another mode To avoid confusion, it should be noted that different in SPE involves a preliminary dynamic modification of a adsorption methods are named solid-phase extraction.6 First of similar adsorbent, e.g. by a solution of a chelating agent, which is retained at the material surface during the following † To whom correspondence should be addressed. separation process due to non-covalent interactions. Thus- E-mail: [email protected] prepared sorbents possess the properties of materials bearing 504 ANALYTICAL SCIENCES APRIL 2006, VOL. 22 covalently-bonded chelating functions. Both modes of SPE are a physicochemical process can be expressed as additive partial examined in this paper. energies related to separate fragments of the molecules or process stages.16,17 The correlation analysis is widely applied in chromatography as well as in liquid–liquid extraction, and up to 2 Theory of Solid-Phase Extraction and Related now a great number of data are available that make it possible to Techniques discuss and moreover to predict the chromatographic behavior of compounds, depending on their properties and on the Like most of separation methods, SPE is based on the chromatographic system used.18 Reversed-phase high- distribution of a solute between two phases. Both batch and performance liquid chromatography (RP HPLC) has been column variants of SPE can be used for preconcentration mainly used for organic separations, and the great majority of purposes. The column mode can be considered as a version of data has been obtained for organic substances. We will mention reversed phase liquid–solid chromatography (RP LSC).13 In here only some results, which may have interest concerning this fact, alkyl-bonded stationary phases with the alkyl length nc = 8, work. 16, or 18 and different grain sizes are most often used. The retention (k) first increases exponentially with the Chromatographic retention is conveniently described by the hydrocarbon chain length; that is, log k enhances with capacity factor, k, which is defined as ki = qi,s/qi,m, where qi,s and increasing nc. However, if the chain lengthens further, the qi,m denote the total quantity of a solute (i) to be present in the retention rise becomes less pronounced and, to a first stationary phase (s) and the mobile phase (m), respectively.14,15 approximation, one may suppose that the capacity factor tends

The higher is the k value, the better is the solute retained, and to be independent of the chain length at nc between 14 and 22. the later is eluted from the column. Correspondingly, the A bend in the log k – nc dependence or the boundary value of nc separation selectivity can be defined as the relative retention of is dependent on the test sample. It has been shown two solutes, βi,j, which is sometimes called the separation factor, experimentally that this value increases with the molecular mass expressed in terms of the capacity factors, βi,j = kj/ki. If the of sample molecules. Such a dependence violates simple preconcentration of a group of dissolved ions or molecules is considerations of the interaction between the sample molecules required, the k values for all of the solutes should be as close as and an alkyl-bonded surface. One could expect a linear increase possible. of k with nc if such a surface would behave like a liquid. Also Two approaches can be applied to the treatment of SPE on the contrary, the k value should not depend on nc in case of a experimental data. The first one may be based on the Snyder solid adsorption surface.18 “competition” model, which describes the distribution of a The partition of substances between two liquid phases is solute between liquid and solid phases.14,15 In this model it is important for various fields of science and technology. For assumed that the solid surface is covered with mobile-phase example, a priori estimation of the possibility of substance molecules, and that solute molecules have to compete with the permeation into cells is of importance for biological studies. solvent molecules in this adsorption layer to occupy an The distribution between water and n-octanol describes quite adsorption site. It is the difference between the affinity of the adequately the membrane permeability. A database including mobile phase and that of solute for the stationary phase that several thousand partition constants is now available for this determines the retention in LSC and, therefore, in SPE standard two-phase system.19–21 A statistical analysis of the according to the competition model. Snyder14 formulated the experimental data has made it possible to estimate the following equation that interrelates the distribution coefficient hydrophobicity (or hydrophilicity) factors for discrete fragments

KD = ci,s/ci,m (c denotes concentration in one phase) with the of organic molecules. The correlations between capacity factors adsorption area of the solute molecule Ai and the adsorption and the hydrophobic/hydrophilic balance for organic substances 0 energy of the solute on a standard adsorbent Si : verify an assumption that the capacity factor is, to a first approximation, proportional to the molecular volume of the α 0 0 log KD = log Va + (Si – AiE ), (1) compound. A more detailed examination shows that the capacity factor is a function of the hydrophilic/lipophilic where Va is the volume of the adsorbed solvent per gram of the balance for the compound. A number of equations are known stationary phase and α is the adsorbent activity. that enable the capacity factors as functions of various In the frames of the second model, which is more practical for parameters characterizing different compound properties to be our further speculations, the distribution of a solute in SPE can evaluated.17,18 be considered as a partition between two liquid phases. By The solvent effect in Snyder’s equation is described by the definition, the capacity factor is a dimensionless quantity, which adsorption energy of the solvent per unit area, usually referred is in this case described by to as the solvent strength.13,14,18 Obviously, the adsorption energy depends not only on the solvent, but also on the

k = KDVs/Vm, (2) adsorbent. For a hydrophobic adsorbent, e.g. carbon, alkanes have the highest and methanol has the lowest values of the where Vs and Vm are the volumes of the stationary and liquid solvent strength. This is why high distribution coefficients in phases, respectively. It is assumed in this model that the SPE are achieved in the use of alkyl-bonded adsorbents as a analyte-containing phase is a homogeneous solution. Because stationary phase and an aqueous solution as a mobile phase that relative values are used in the calculations, in both cases the makes it possible to apply short columns, and even filters for experimental values of the capacity factors allow us to discuss preconcentration of organic compounds. It is also worth noting the dependence of the chromatographic efficiency on both the that such materials possess rather low volume capacities with properties of the stationary phase and the solutes to be separated respect to solutes in RP HPLC, where a solid or pseudosolid and the experimental conditions. surface works as the stationary phase. Ion-pair RP HPLC is Methods of correlation analysis are used for this purpose, used for the separation of substances in the form of ion pairs. In which may be used if the principle of the linearity of free this case, the hydrophobic/hydrophilic balance is still of energies (PLFE) is valid, i.e., if the free energy of a chemical or importance, but the dependence on the solvent and experimental ANALYTICAL SCIENCES APRIL 2006, VOL. 22 505 conditions becomes more complex.15,18 The behavior of metal complexes as well as organic substances in RP HPLC depends on the molecular volume and the hydrophilic/lipophilic balance for the compound as a whole. This has been shown experimentally in chromatographic studies of metal alkyldithiocarbamates, alkyldithiophosphates21–23 and other chelate compounds.24,25 Because during the chromatographic separation of metal chelates, not only the distribution between the mobile and stationary phases, but also complex formation and dissociation occur; for normal and reversed-phase HPLC it is also important that the behavior of neutral chelates and ion pairs involving charged chelates is dependent on the coordination saturation, stability and kinetic inertness of the compounds.25 Fig. 1 Dependence of the solid-phase extraction percentage of Cu2+ The alkylated phases belong to so-called boundary phases. at pH 5.0 on the number of carbon atoms in alkyl groups bonded to × –2 44 Their properties are determined by the matrix (silica gel in our the surface of silica gel in the presence of 1 10 M acetylacetone. case), alkylation density, length of “legs” and nature of the functional groups.10 Aqueous silica gel bears OH– functional groups (hydroxylated silica gel). The properties of extracting ligand) complexes, which do not contain water hydroxylated silica gel have been thoroughly studied. This molecules in the first metal coordination sphere, and adsorbent is widely used, e.g., for the separation of metal coordinatively unsaturated, and therefore hydrated compounds. chelates by normal-phase chromatography.26–28 It is worth The behaviors of the complexes of these two types in LLE are noting that silica gel is a hard reagent in terms of the principle in principle different.31–33,35 of hard and soft acids and bases (HSAB).29,30 Thus, alkyl- The extraction constants for any metal chelates and, therefore, bonded silica gels possess the properties of both a hydrophobic the metal recovery and extraction selectivity are functions of adsorbent and a hydroxylated silica gel. stability and partition constants for the chelate compounds.33–35,37 With increasing the alkyl length, the hydroxyl groups become Because similar metal chelates are used in LLE and SPE, it less available sterically. Nowadays, only commercial phases seemed to be obvious that the behavior of metal complexes, with a hydrocarbon chain length of C16 – C18 are practically depending on their thermodynamic stability, should be common used. The reactivity of residual hydroxyl groups is reduced to for both techniques, and that the HSAB principle could be an acceptable level in adsorbents with such alkyl groups. It applied to the prediction of metal behavior in SPE.35 The should be taken into account that complete alkylation is partition constants for coordinatively saturated hydrophobic impossible, and the conventional adsorbents have 50% “free”, chelates are mainly dependent on the energy of cavity formation although difficultly availably hydroxyl groups.10 and on the formation of hydrogen bonds between Liquid–liquid (solvent) extraction (LLE) is very widely electronegative atoms of chelate molecule and water molecules applied to metal separation and enrichment. The technique is at the cavity surface,36,37 which is on the hydration energy, based on the formation of metal complexes and their which is the same in the LLE and SPE systems. An alkyl- distribution between an aqueous phase and an organic solvent. bonded surface is hydrophobic, and one can also suppose that LLE depends on the stability and partition constants of metal the solubility of coordinatively saturated metal chelates in complexes. One of the essential differences of the LLE hydrocarbons and the energy of adsorption on alkyl-bonded methods from chromatographic ones, particularly from those silica gel are interrelated. The study of the distribution of based on the use of surface-modified adsorbents, consists in the copper acetylacetonate between an aqueous solution and possibility of the extraction of large metal amounts. This is why alkylated silica gel has shown that both the hydrophobicity of LLE is widely used in various technologies, but not only on a silica gel phases and the extraction percentage for Cu2+ increase laboratory scale, and a huge amount of information on the with the alkyl chain length (Fig. 1). chemistry of liquid–liquid extraction of various compounds has In fact, the electron paramagnetic resonance spectra of copper been accumulated and generalized in a number of reviews, diethyldithiocarbamate were shown to be identical for this monographs and reference books (e.g., see Refs. 31 – 34). chelate compound adsorbed at the surface of hexadecyl silica Noticeable similaries of LLE and SPE allowed us to hope that a gel and dissolved in an organic solvent.38 This means that non- variety of different metal compounds used in LLE could be covalent interactions are responsible for the total interactions employed in SPE. The authors of this paper pay attention not between the chelate molecules and the organic solvent only to the SPE of metal chelates, which have been mainly molecules or the surface hydrocarbon groups of modified silica used, but also to the extraction of metal compounds of other gel. The investigation of copper(II) dialkyldithiophosphates 2 types investigated and applied in conventional LLE. adsorption at C16-bonded silica has shown that the area (cm ) occupied by a complex molecule increases with the molecular mass of the reagent alkyl group, and is equal to 9.8 × 10–14 –13 –13 39 3 Chelate Compounds (alkyl = C2H5), 1.2 × 10 (C4H9), and 6.7 × 10 (C6H13). The distribution coefficients of the complexes in LLE and SPE Compounds of different types used in LLE are usually divided systems follow the same order. into two big groups: neutral compounds and ion associates or A comparison of some properties of organic solvents (e.g. pairs.32,33 Against neutral compounds, complexes formed by characterized by solubility parameters) with those of alkyl-silicas bidentate monobasic chelating reagents, which belong to cation- and the experimental results mentioned above suggest that the exchange extractants, and by neutral extractants, should be first variety of data on liquid–liquid extraction of metal chelates may of all mentioned. Neutral metal chelates may also be divided be used to select the reagents and conditions for the SPE into two types: coordinatively saturated (with respect to preconcentration of metal ions. This can be illustrated by taking 506 ANALYTICAL SCIENCES APRIL 2006, VOL. 22

Fig. 2 Dependences of the distribution coefficients of Hg2+ (1), 2+ 2+ 2+ Cu (2), Cd (3) and Zn (4) extracted on a 1-ml Diapak C16 cartridge from 10 ml of aqueous solution containing 1 × 10–3 M dibutyldithiophosphate on the solution acidity. Flow rate, 1 ml/min.

as examples the behavior of metal dialkyldithiphosphates,40–43 acetylacetonates,44–47 and tenoyltrifluoroacetonates48 in LLE and SPE. In a discussion of the experimental data, one can also assume that the models applicable to LLE can be used for describing the formation and distribution of metal chelates in Fig. 3 Dependences of the distribution coefficients of Zn2+ in batch SPE. extraction with 100 mg of Diapak C16 from 10 ml of an aqueous The solvent extraction of neutral coordinatively saturated solution containing potassium dialkyldithiophosphate with alkyl chelate compounds is described by the equation31,33,35 chains (R) of different length.

n+ —– —–– + M + nHA = MAn + nH (3) with the extraction constant, Kex: transferred to the non-aqueous phase, as in the case of LLE. The data on SPE of zinc dialkyldithiophosphates using C16- —–– + n [MAn][H ] bonded silica gel and aqueous solutions of potassium salts of Kex = ——————– . (4) [Mn+][HA]n dialkyldithiophosphoric acids with alkyl groups from C2H5 to 42 C10H21 have made it possible to compare the dependences of The extraction constant can be written in the form metal extraction on the molecular mass of the reagent and the complexes. Upon increasing the molecular mass of the β –1 log Kex = log( MAnPMAn) – nlog(PHAKHA ). (5) reagents, the extraction curves are shifted to the acidic region and the maximum values of distribution coefficients increase For a system containing dialkyldithiophosphate, M is the n- (Fig. 3). This effect is most drastic in passing from zinc charged metal cation, A is the single-charged anion of diethyldithiophosphate to zinc diisopropyldithiophosphate. β dialkyldithiophosphoric acid under study, MAn is the stability With increasing the reagent concentration, the distribution constant of metal dialkyldithiophosphate in water, PMAn is the coefficients of zinc increase for all of the dialkyldithiophosphates partition constant of the compound, KHA is the dissociation considered (except for diethyldithiophosphate). The constant of dialkyldithiophosphoric acid, and PHA is the partition equilibrium values of the distribution coefficients, which can be constant of non-dissociated acid (the line at the top indicates assumed to be equal to the partition constants of the neutral that the species occurs in the non-aqueous phase, which is chelate (PMAn), enhance on going from the reagent with C3H7 to assumed to be a homogeneous solution). the reagent with C5H11 groups and then remain constant: Studies on solid-phase extraction of dialkyldithiophosphates 2+ 2+ 2+ 2+ 42 formed by Hg , Cd , Cu , Zn , which are soft Lewis acids R: C3H7 C4H9 C5H11 C6H13 C8H17 readily extracted into an organic solvent, and by Mn2+, Ni2+, log P (±0.5)a: 2.94 3.05 3.80 3.80 3.80 Co2+ (borderline acids),40–43 have shown that the pH- a. Mean of 5 measurements. dependences of extraction with C16 or C18-bonded silica gels are in accordance with the complex stabilities expected from the Investigations of SPE of Ni2+, Co2+, and Mn2+ using 0.5% HSAB principle and with the sequence of the distribution aqueous solutions of ammonium dialkyldithiophosphates with 49–53 coefficients in LLE systems. For example, the complete alkyls from C2H5 to C6H13 and C18-bonded silica gel have also recoveries of mercury and copper in the studied range of the shown that the extractability increases with the chain length of solution acidity (from pH 6 to 1 M HCl) are obtained at the the substituent groups in the reagents and is quantitative for concentration of sodium dibutyldithiophosphate 1 × 10–3 M. alkyl groups larger than butyl at pH 3. The branching of the For cadmium and zinc, quantitative recoveries are achieved at alkyl chains affects the recoveries as well.40,41,43 As in the pH > 3 (Fig. 2). For zinc dibutyl- (Fig. 3c, curve 3), dipentyl- systems for zinc dialkyldithiophosphates, described above (Fig. 3), (Fig. 3d, curve 4), and dihexyldithiophosphates (Fig. 3e, curve the recoveries of Ni2+, Co2+, and Mn2+ dipentyldithiophosphates 3), the slope of the pH-dependence of log D is equal to 2. It can are better at higher reagent concentrations. In contrast, the 2+ 2+ 2+ 3+ be assumed that the complex of the composition ZnA2 is extractability for Cu , Pb , Cd , and Fe decreases with ANALYTICAL SCIENCES APRIL 2006, VOL. 22 507

Fig. 5 pH-Dependences of the distribution coefficients of metal β Fig. 4 Correlation between the distribution coefficients of - ions extracted with C18-bonded silica gel from an aqueous solution diketones (listed in Ref. 47) and the number of carbon atoms in containing 5 × 10–3 M acetylacetone. Fe3+ (1), Cu2+ (2), Be2+ (3), Al3+ methylene groups in their molecules.47 Solid-phase extraction (1) (4), Pb2+ (5), Zn2+ (6). with C18-bonded silica gel and liquid–liquid extraction (2) with benzene from aqueous solution of 1 × 10–4 M β-diketone (see Ref. 5 in the Ref. 47).

results show that the metal ions can be adsorbed on C18-bonded silica gel as charged complexes (contrary to solvent extraction increasing the reagent concentrations. The authors40 did not in which they are extracted as non-charged metal compounds), explain that difference, but proposed to use it for separating the which is explained by interactions with residual adsorbent elements into two groups. silanols.45 The dissociation of metal β-diketonates at a silica gel Interesting data on the distribution of some oxygen-bearing surface has been studied in detail. The degree of chelate reagents and their metal complexes between aqueous solution dissociation with the subsequent formation of surface 44–47 and C18-bonded silica gel have been reported. The effect of complexes depends on the electronegativities of the central a hydrophobic substituent to β-diketones on the adsorption by metal atom in the complex and the substituent to the β-diketone 25,26 C18 silica was investigated, and the adsorption behavior was molecule. compared with the data on solvent extraction.47 The partition Again, one could expect that the complex stability orders for constants for twelve β-diketones, from acetylacetone to 4,6- the same metal ions should be comparable in the SPE and LLE dodecanedione, were measured. The correlation between log systems. In fact, the sequence of pH-dependences of metal

PHA and the number of carbon atoms of the β-diketones are adsorption presented in Fig. 5 and that of the metal extraction shown in Fig. 4 along with the correlation obtained for the into benzene are very similar.46 benzene–water extraction equilibria. The slope of 0.38 (log As in the case of the above-mentioned dialkyl-

PHA/number of carbon atoms) for adsorption on C18 silica is not dithiophosphates, the extractability of metal diethyldithio- much different from the value for the benzene–water extraction carbamates in both the LLE and SPE systems54 follows the of β-diketones, and which obeys the PLFE.19,20,47 Bearing in order of stability constants: Cu2+ > Ni2+ > Pb2+ > Cd2+. Metal mind that the hydrophobicity characteristics are also dependent dithiocarbamate complexes are coordinatively saturated, readily on the solvent nature, one may be surprised by close values of soluble in organic solvents of different nature, and the optimal the methylene group parameters for the compared SPE and LLE conditions for their SPE and elution with an organic solvent systems. This fact says once more that the partition constants of (Table 1) can be predicted on the basis of LLE data.55 However, compounds distributing between an aqueous solution and an this may not be the case for coordinatively unsaturated organic solvent or an alkyl-bonded phase are strongly dependent compounds if their nature is not taken into consideration. on the hydration energies.47 Unsaturated metal chelates are well extracted only by Unlike metal complexes formed with sulfur-containing coordinatively active solvents, for example, alcohols and chelating agents, the metal chelates with oxygen-bearing ligands ketones. The solvent extraction of such compounds is possible can be retained not only by hydrophobic alkyl chains bonded on but in the presence of specially added electron-donor bases the silica gel surface, but due to interactions with residual (synergistic reagents). In this case, hydrophobic mixed-ligand silanol groups on the surface as well.45 The adsorption complexes are formed, the solubility and extraction behavior of behaviors of Al3+, Be2+, Cd2+, Co2+, Cr3+, Cu2+, Fe3+, Mn2+, Ni2+, which are similar to those of coordinatively saturated chelates. Pb2+ acetylacetonates and of different copper β-diketonates on In systems with coordinatively inactive solvents (e.g., carbon 44–47 C18-bonded silica gel were studied. To interpret the obtained tetrachloride, hexane, chloroform) unsaturated chelates being data, the authors46 also supposed that the adsorption equilibria poorly soluble in both organic and inorganic phases often float could be considered as the distribution equilibria of metal at the interface and are thus lost. Solvent extraction is based on chelates between two liquid phases. Plots of the linear the formation of compounds soluble in organic solvents and on relationships between log D and pH for the metal their distribution between the aqueous and organic phases, acetylacetonates (Fig. 5)46 were found to have lower slopes than whereas in SPE not only a distribution but a filtration those expected from the stoichiometric ratios for the neutral mechanism of extraction is realized as well, i.e., both types of chelate compounds. For copper chelates, the slope increases complexes, hydrophobic and hydrated, can be retained by the with the β-diketone molecular mass, changing from one to two non-aqueous phase. The difference in their behavior can occur upon going from acetylacetone to 4,6-decanedione. These during elution. 508 ANALYTICAL SCIENCES APRIL 2006, VOL. 22

Table 1 Extraction percentage of some metal ions as Table 2 Extraction percentage of some metal ions as diethyldithiocarbamates on a C16-bonded silica gel column from cupferronates and 1-phenyl-3-methyl-4-benzoyl-5-pyrazolonates a aqueous solution and their elution percentage by chloroform on a C16-bonded silica gel column from aqueous solution and their elution percentage with an organic solventa Metal ion Extraction Elution Metal ion Extraction Elution Metal ion Extraction Eluent Elution Bi3+ >99 96 Cu2+ >99 92 Zn2+ >99 95 Co2+ 98 98 Cupferronates Mn2+ 94 98 Au3+ 94 — Zn2+ > 99 Acetone >99 > 99 CCl4 <1 × a. Conditions: column size, 3.5 0.2 cm; metal solution volume, 25 Eu3+ > 99 Acetone >99 ml; NaDEDTC concentration, 0.025 M; pH, 6.2 – 6.5; eluent volume, > 99 CCl4 <1 1 ml; flow rate, 0.4 ml/min. Fe3+ 94 Cloroform >99 96 Acetone <1 PMBP chelates Zn2+ 95 Acetone 92 Table 2 presents the results of a comparative study of solid- 96 CCl4 <1 phase extraction and subsequent elution of some metals, which Mn2+ 97 Acetone >99 form chelate compounds with cupferron and 1-phenyl-3-methyl- 91 CCl4 <1 2+ 4-benzoylpyrazolone-5 (PMBP). Among the complexes Co 98 Acetone >99 studied, only iron cupferronate is a coordinatively saturated 98 CCl4 <1 Eu3+ 95 Acetone 90 compound which, like metal dithiocarbamates (Table 1), is 97 CCl4 <1 completely eluted which both chloroform and acetone. The coordinatively unsaturated Zn2+ and Eu3+ cupferronates and the a. Conditions: column size, 3.5 × 0.2 cm; metal solution volume, 10 chelates of Zn2+, Mn2+, Co2+, and Eu3+ with PMBP are ml; reagent concentrations, 10–2 M (cupferron) and 10–4 M (PMBP); quantitatively extracted but the elution is achieved only with pH, 6.2 – 6.5; eluent volume, 1 ml; flow rate, 0.4 ml/min. acetone (Table 2).55 The coordination saturation and corresponding electron- acceptor capacity (with respect to bases including water) of the time in SPE. This is an important advantage of the SPE modes metal chelates is determined by the nature of the central atom of based on the addition of an extracting reagent to a sample the complex and the donor atoms of the chelate-forming reagent solution prior to the extraction stage. Higher flow rates may be as well as the structure of the complex as a whole. The acceptor employed in the use of non-modified reversed phases when the capacity of the chelates increases in the following order of the complexation processes involved are not fast enough. Besides, donor atoms of the chelate-forming monobasic bidentate there is no need to use an additional procedure of preliminary reagents: S,S < N,S < O,S < N,N < N,O < O,O.34 Therefore, the reagent modification. same metals can form complexes that are coordinatively However, such an extraction mode has a limitation connected saturated with respect to water (for example, with anions of with poor reagent solubility in aqueous solutions. For diethyldithiocarbamic acid) or coordinatively unsaturated examples, this is the case in the use of 8-hydroxyquinoline complexes (e.g., with cupferron). This may explain such a (oxine) and 5,7-dibromooxine for SPE of zinc and manganese different behavior of the Mn2+, Zn2+, and Co2+ ions.55 If the water solubility of the reagent selected for SPE is diethyldithiocarbamates on one hand and that of Zn2+ insufficient to provide complete recovery of the metal ions of cupferronate and the Mn2+, Zn2+, and Co2+ chelates with PMBP interest, it is recommended to use an alkyl-bonded phase on the other hand. premodified with a reagent. Not only aqueous reagent As in LLE,33 systems with mixed complexes formed by a solutions, but also solutions of a reagent in an organic solvent or chelating reagent and a neutral electron-donor agent can be in a water–organic solvent mixture, can be used for the successfully used in SPE.55 Thus, the application of a mixture modification. Another advantage of a modified sorbent is no of PMBP and trioctylphosphine oxide (TOPO) or necessity of excess reagent in the flow passing through a tetraphenylmethylene diphosphine (TPMDP) instead of cartridge or a column that may be of importance for blank corresponding individual reagents results in a non-additive experiment. A number of different organic reagents applied to increase in the distribution coefficients for Eu3+ and Am3+ the preconcentration of metal ions in the form of neutral chelate adsorbed on C16-bonded silica. Such mixtures enable the compounds have been modified on reversed phases and used for complete recovery of both elements from acidic solutions in the analytical purposes.57–83 A comparison of the retention of presence of high concentrations of alkali and alkaline-earth various reagents on C1-, C2-, C8-, C16-, phenyl-, and nitrile- elements to be achieved.56 bonded silica gels has shown that the most hydrophobic phenyl- 58 Some interesting results were obtained in studies of the and particularly C16-bearing phase are the best supports. In behavior of platinum-group metals adsorbed from weakly acidic most cases, the reagent immobilization does not change the chloride solutions containing potassium dialkyldithiophosphates.57 complex-forming properties of the reagent. The pH- Conditions were found for the solid-phase extraction (in a dependences for SPE are generally consistent with the literature dynamic mode using a cartridge with C16-bonded silica gel) of data on LLE with the same reagents, but show a small shift to not only Pd2+ and Pt4+ but Ir4+ as well. The complexes of the higher pH values. This shift is probably due to the micro- latter metal ion occupy the extreme position in the lability series environment on the solid-phase material, as compared with the of platinum-group metal compounds, including kinetically very bulk liquid–liquid environment,63 where the pH can easily be inert chloride complexes. Iridium and platinum can be recovered maintained. from aqueous chloride solutions by SPE as dialkyldithiophosphates Two interesting approaches to changing the selectivity of under milder conditions than in conventional LLE (at room solid-phase extraction of metal ions have been proposed. The temperature and without addition of a labilizing agent), because first approach is based on using mixed sorbents, which may be the complexation and adsorption processes can be separated in obtained, e.g., by the simultaneous immobilization of two ANALYTICAL SCIENCES APRIL 2006, VOL. 22 509

60 88,89 complexing reagents on C16- or phenyl-bonded silica gels. A LLE. The data on SPE of europium with reagents of the mixture of two reagents, L1 and L2, is chosen, where L1 is a second group are presented in Fig. 6b. Quantitative extraction “soft” ligand, e.g., containing donor sulfur atoms, and L2 is a at the reagent concentrations studied is achieved only with “hard” ligand containing donor oxygen atoms. Depending on Ph2Et2 and Ph2Bu2. The morpholino derivative is the weakest the selection of the pair of reagents and their concentrations, the extractant, which is also in accordance with the LLE data. A mixed sorbent can provide different retention orders for the low recovery with the iso-amyl derivative is due to its low same group of metal ions.60 Another approach to improving the concentration. In general, the order of the extraction power for selectivity is based on the use of exchange reactions between all of the reagents studied are similar in the LLE and SPE 90 the metal M1 ion in solution and the metal M2 complex systems. 59 premodified on alkyl-bonded silica gel. Selective SPE-AAS It follows from the results obtained that An2Et2, Tol2Et2, and procedures were developed based on the exchange reactions Ph2Et2 exhibit sufficient water solubility and the best extraction 3+ 2+ 2+ between Fe and Zn(D2EHP)2, Cu and Co(PMBP)2, Cu and efficiency with respect to europium. Under the chosen 59 Pb(DEDTC)2 using C16-bonded silica gel as adsorbent. conditions, these reagents are suitable for the recovery of Premodified sorbents can also be used in different test americium as well (Fig. 6a). They can be used for the methods.84–86 preconcentration of transplutonium elements, e.g., in the analysis of technological and environmental samples. The recovery of Eu3+ and Am3+ by SPE in the form of complexes 4 Coordinatively Solvated Compounds with tetraphenylmethylenediphosphine dioxide (TPMDPD) was studied. TPMDPD is suitable for concentrating these elements

Neutral mixed-ligand complexes MXnLm (coordinatively from nitric acid solutions in the acidity range from pH 5 to – – – – – 91 solvated compounds), where X = NO3 , Cl , Br , I , etc., and L HNO3 concentration of 6 M. is a neutral extracting reagent, are widely used in metal extraction methods using traditional LLE systems.32,33 Oxygen- bearing reagents containing one or two phosphoryl groups are 5 Ion-Pair Extraction applied to extract hard metal ions, such as actinides and rare earth elements for solving various radiochemical problems.87 Ion-pair compounds can also be used for metal extraction in The possibility of utilizing such reagents in SPE was shown.88 LLE and SPE systems. Various compounds of this type may be 3+ 3+ The solid-phase extraction of Eu and Am on a C16-bonded applied because metal ions can form different cationic and silica gel cartridge from aqueous solution containing a diaryl- anionic complexes of organic and inorganic ligands, which can (dialkylcarbamoylmethyl)phosphine oxide (CMPO) was be extracted into a non-aqueous phase in the presence of ion- studied using two groups of CMPO with the general formula pairing reagents. We have shown the possibility to use cationic

R′R″P(O)–CH2–C(O)NR′′′2. The first group included reagents metal complexes in the form of ion pairs, taking the systems with the same substituent to the nitrogen atom R′′′ = ethyl (Et) strontium–dicyclohexano-18-crown-6(DCH18C6)–ion-pairing and various substituents to the phosphorus atoms: R′ = R″ = reagent as an example.92 anisyl (An), tolyl (Tol), phenyl (Ph), tert-butyl (t-Bu), butyl The extraction of strontium is based on its complexation with

(Bu), and butoxy (BuO), and also R′ = Ph and R″ = PhEtEt2 DCH18C6 (L), which occurs in the aqueous phase, (Fig. 6a). The second group of reagents included those with the —– same substituent to the phosphorus atom and different M + HL+ = ML + H+. (9) substituents to the nitrogen atom: R′ = R″ = Ph, R″ = Bu, Et, iso-amyl, or octyl (Oct), NR′′′2 = piperidino (Pip), morpholino Taking into account that at pH < 1 DCH18C6 can be (M) (Fig. 6b). protonated,93 it can be shown that the distribution coefficient of

The extraction of coordinatively solvated compounds of metal, KD, in such systems is given by trivalent metals from aqueous nitric acid solution into a –1 –n n n n hydrophobic phase (an organic solvent or a sorbent in SPE), KD = βMLβMLAKb,LKa,HAPMLAPHA[HA] [H] , (10) taking into account the protonation of the reagent molecules L, z+ can be described by the equation where βML is the formation constant of complexes ML , βMLA is z+ – the association constant of the ion-pair complexes ML A , Kb,L, 3+ – + ————— + M + 3NO3 + nHL = M(NO3)3Ln + nH . (6) is the protonation constant of DCH18C6 (L), Ka,HA, is the dissociation constant of acid HA, PMLA and PHA are the The extraction constant is expressed as distribution constants of ion-pair complexes and acid HA, respectively. ————— + [M(NO3)Ln][H ] Equation (10) demonstrates an intricate dependence of the Kex = . (7) ————————3+ – + n [M ][NO3 ][HL ] distribution of a recovered metal on a large number of factors, some of which are interrelated. Thus, the MLAn recovery is a The distribution coefficient in such systems is given by function of its partition and dissociation constants. The influence of other factors depends on the recovery conditions. 3 + n + –n KD = βPKa[NO3] [LH ] [H ] , (8) The dependence derived holds for the linear portion of the isotherm of MLAn distribution in SPE. where β and P are the stability and partition constants of the Studies on ion-pair reversed-phase chromatography have solvated compound and Ka is the protonation constant of the shown that the chromatographic behavior of such compounds reagent. depends on both the chelate stability and on a number of other Figure 6a shows the dependence of solid-phase extraction of characteristics relevant to the chelate, ion-pairing reagent, and Eu3+ with reagents of the first group on the nitric acid mobile phase.15,25 The data on the solid-phase extraction of the concentration in the initial aqueous solution. The ranges of the Sr-DCH18C6 complex also show that the model correctly maximum recovery of europium by SPE coincide with those in describes the process under consideration. As mentioned 510 ANALYTICAL SCIENCES APRIL 2006, VOL. 22

Various anionic metal complexes including halo complexes are widely used in LLE for metal separation and preconcentration. Different ion-pairing reagents, most often alkylammonium salts, are applied in LLE, and can be used in SPE. Studies on ion-pair SPE of anionic chloride complexes of Ir4+, Pt4+, and Pd2+ with tetrabutylammonium (TBA) have shown that these elements are extracted over a wide acidity range, from 3 M HCl to pH 5. A good recovery can be achieved at high

flow rates through a column packed with C16-bonded silica gel not only for platinum but even for iridium (Fig. 7).57 2+ 2– To preconcentrate Pb , SPE of ion-pair formed by PbI4 and tetrabutylammonium cation was applied.99 Preimmobilized sorbents can also be used for the recovery of ion-pair

compounds. A C18 sorbent premodified with Aliquat 336 was used for preconcentrating Zn2+ and Cd2+ from HCl solutions.100,101 Cationic surfactants, especially dimethyllaurylbenzylammonium bromide (Sterinol), was shown to be suitable for the 4+ 2– preconcentration of Pt as PtCl6 from 0.1 mol/l HCl on a Separon SGX C18 column with subsequent elution with ethanol and determination by atomic emission spectrometry.100 A similar 3+ – method was developed for the preconcetration of Tl as TlCl4 for its final determination in plants by atomic emission or absorption spectrometry.103 In addition to Sterinol, cetyltrimethyl- ammonium bromide, N-(1-carbaethoxypentadecyl)trimethyl- ammonium bromide (Septonex, Chech Rep.), and benzyldimethyl- tetradecylammonium chloride (Zephiramin, Japan) were used as ion-pairing reagents.95

6 Application of SPE to Metal Preconcentration

Fig. 6 Dependence of the solid-phase extraction percentage of Eu3+ Alkyl-bonded silica C18 and C16 have received most 3+ and Am on a Diapak C16 cartridge on nitric acid concentration in considerable attention in solid-phase metal extraction. The high aqueous solution in the presence of diphenyl(dialkylcarbamoylmethyl)- alkyl group density at the surface (e.g., about 3.3 mmol/m2 for 104 phosphine oxide (M) bearing different substituents to phosphorus (a) C16-bonded silica gel) provides sufficiently effective shielding and nitrogen (b) atoms. a: (1) Eu; alkyl = Tol2Et2, reagent of the initial silica surface. High adsorption and elution rates × –3 × –3 × concentration 6 10 , (2) Am; Tol2Et2, 6 10 , (3) Eu; Ph2Et2, 1 are important advantages of such adsorbents, which makes it –2 × –2 × –2 10 , (4) Am; Ph2Et2, 1 10 , (5) Eu; i-Bu2Et2, 1 10 , (6) Eu; possible to pass rapidly the solutions under study. The –2 –2 PhEtEt2, 1 × 10 , (7) Eu; Bu2Et2, 2 × 10 , (8) Eu; (BuO)2Et2, 1 × –2 –2 –2 constancy of the alkylated phase volume in contact with various 10 , (9) Eu; An2Et2, 1 × 10 , (10) Am; An2Et2, 1 × 10 . b: (1) Eu; –2 organic and aqueous solutions is another significant feature. alkyl = Ph2Et2, reagent concentration 1 × 10 , (2) Eu; Ph2Pip, 1 × –2 –2 –3 The adsorbents are prepared for application after a short 10 , (3) Eu; Ph2M, 1 × 10 , (4) Eu; Ph2Bu2, 3.6 × 10 , (5) Eu; Ph2(i- –4 –4 Amyl)2, 5 × 10 , (6) Eu; Ph2Oct2, 2 × 10 . preconditioning, e.g., after passing methanol through the cartridge. A limitation in their use consists in a rather low 105 capacity. According to work, a typical C18 material contains only 1 – 5% w/w octadecyl groups. For a Diapak C16 cartridge before, the distribution between the aqueous and the alkyl- packed with 1 cm3 of the adsorbent, the capacity with respect to bonded solid phases is a basis of ion-pair reversed-phase HPLC most of inorganic compounds is about 5 – 20 mg.106 This is as well, and the results of chromatographic studies of such enough for preconcentrating trace elements, but breakthrough complexes, together with liquid–liquid extraction data, can be experiments are necessary before the use of a cartridge for useful for selecting the systems and conditions of SPE. purification purposes. However, one should remember that the capacity of an alkyl- A variety of SPE units are manufactured for dynamic separation bonded phase, unlike that of a bulk organic solvent, is much and preconcentration, for example, Sep-Pak cartridges of Waters smaller. Therefore, the domain of linearity of the distribution Division of Millipore Corp., Merck, Baker, Varian cartridges, isotherm in SPE and HPLC is, apparently, much narrower that Diapak cartridges (BioChimMak, Moscow).63,66,75,107–109 in LLE. Exractions are also performed with membrane disks containing

1,10-Phenanthroline (Phen) forms cationic complexes of C18-bonded silica (8-µm particles, 6-nm pore size) on many metal ions.95–97 The possibility of the solid-phase polytetrafluoroethylene (PTFE) or glass fiber supports (J. T. extraction of such compounds has been demonstrated, taking a Baker, Varian). The typical composition of the disks is 80 – 2+ 95 Fe -Phen complexes as an example. Sometimes there is no 90% w/w C18 or C8-bonded silica and 10 – 20% support, the disk distinct border between complexes of different types, when dimensions are 47 mm in diameter and 0.5 mm thick.75–81 mixed-ligand compounds are formed and extracted. For When the complexing reagent is added to the test aqueous example, a cationic complex of Sn4+ with oxine in the presence sample as a solution prior to metal extraction, the following of trichloroacetate ions is extracted in the form of Sn(Ox)2 procedure is usually used. An aliquot of the reagent solution in 55 (CCl3COO)2. water or in a mixture of water and a water-miscible organic ANALYTICAL SCIENCES APRIL 2006, VOL. 22 511 solvent is added to the sample after adjusting its acidity. The resulting solution is pumped through a cartridge with the flow rate being from fractions of ml to 50 ml/min if the solution volume is as large as several hundred ml or more. Methanol, ethanol, or mineral acid solutions are used as eluents; their typical volumes are from fractions of ml to 50 ml. In the dynamic mode, non-covalent modification can be used to prepare sorbents containing an individual complexing reagent or a mixture of reagents. Water–alcohol (methanol, ethanol) solutions of reagents are often applied to modify the reversed- phase adsorbent by passing the reagent solution through the cartridge directly in the flow system up to the appearance of the reagent or up to its constant concentration in the eluate. A further preconcentration procedure is similar to that described above with two exceptions: there is no need to add the reagent to the sample solution, and the reagent must be retained on the cartridge (column, disk) during the extraction/elution runs, if reusable adsorption units are employed. The number of runs on Fig. 7 Dependence of the solid-phase extraction percentage of Pd2+, 4+ 3+ 3+ one premodified adsorbent can be as high as several Pt , Ir and Au on a Diapak C16 cartridge on the acidity of aqueous × –3 hundreds.61,110 chloride solution in the presence of 1 10 M tetrabutylammonium chloride. Different automatic on-line separation and preconcentration systems were developed for the determination of trace metals in water samples and sample solutions by spectrophotometry, atomic and mass spectrometry. Such flow systems have the lesser extent, dithiophosphate groups, although a variety of following advantages over off-line modes: high sampling other chelating as well as neutral and ion-pairing reagents can frequency and throughput of analysis (one analytical be applied to metal separation and preconcentration. separation/determination run takes 10 – 200 s), low consumption Combinations of SPE with any determination techniques of reagents and sample (0.1 – 2 ml), decreased risk of sample suitable for the analysis of solutions are feasible. Various water contamination due to the use of totally closed systems, etc.111–121 samples and other environmental samples, food stuffs, For example, combining a micro-scale flow-injection (FI) biological samples, chemical reagents, high-purity substances preconcentration system on-line with electrothermal atomic have been analyzed using SPE. Not only the total amounts of absorption spectrometry (ET AAS) resulted in a powerful elements, but also different metal species as well, can be integrated system that permitted fully automatic operation, determined after their preseparation on reversed-phase avoiding time-consuming manual work and improving the adsorbents. accuracy and precision.112–114 Different microcolumns packed with as small amounts of C18-boned silica gel as 15 µl and eluate volumes of 40 µl were used in such systems.111,115,116 7 Solid-Phase Extraction in Element Speciation Methods have been developed for the determination of a Analysis number of trace metals in water samples, as well as digested biological and other samples. FI-SPE-FAAS systems are less Solid-phase extraction is used not only for the preconcentration sensitive than ET AAS-based systems, but they are cheaper, and separation of the total amounts of elements, but also in the easier to perform, and have some other advantages.112–116 The determination of their species in various matrices. Among the 119 C18 mini-columns were used for more than 300 cycles. methods for speciation analysis reported so far, those devoted to Most sensitive are combinations of FI-SPE with ICP-MS, the determination of different oxidation states predominate, which were successfully applied to the determination of trace while a smaller number of papers have been published on the and ultratrace heavy and noble metals, rare earth and other degree of metal complexation or the determination of elements.122–124 The performance of two FI systems for on-line organometallic compounds. Most of the works reported during separation and preconcentration of Cu2+, Cd2+, Pb2+, Bi3+, Au3+, the last decade deal with the on-line element speciation analysis Ag+, Se4+ in seawater and determination by ICP-MS were of water.126 compared.124 One system was coupled to the nebulizer of the For the chromium speciation analysis of natural water 127,128 6+ spectrometer while the other one was coupled to an samples, C18-bonded silica colums were used. Cr and electrothermal vaporizer (ETV) also combined with the same Cr3+ were on-line preconcentrated in the presence of spectrometer. The matrix separation and analyte tetrabutylammonium bromide and potassium hydrogen preconcentration were accomplished by ammonium DEDTP phthalate, respectively, eluted with methanol–water mixtures, immobilized on C18 silica in a column coupled directly to the and introduced into an atomic emission spectrometer using pneumatic nebulizer, or the concentrate was vaporized after hydraulic high-pressure nebulization. Microcolumns with the being automatically injected into a graphite tube, prior to the same SPE material were applied in speciation studies of vaporization of the analytes. As expected, the limits of chromium in seawater samples129 using DEDTC complexes detection are significantly better with the FI-ETV-ICP-MS selectively formed with Cr6+ in the 1 – 2 pH range and with Cr3+ systems (in the range 0.15 – 5 ng/l for most of the elements in the 4 – 9 pH range in the presence of Mn2+. The studied) in the use of methanol as an eluent and a sampling preconcentration and following flame AAS determinations were frequency of 21 – 22 h–1. carried out in a FIA system after elution with methanol.

The applications of SPE on alkylated silica adsorbents are Disposal C18-bonded silica cartridges pretreated with tributyltin summarized in Table 3. The most widely used metal chloride were utilized for the selective preconcentration of Cr6+ complexing reagents are based on dithiocarbamate and, to a from aqueous solutions followed by ET AAS determination. As 512 ANALYTICAL SCIENCES APRIL 2006, VOL. 22

Table 3 Applications of solid-phase extraction Determination Determination Sample Metal ion Reagent Ref. Sample Metal ion Reagent Ref. method method

2+ 2+ a e 2+ 2+ e Cu-based Ni , Zn TAN SPS 74 Drinking water Cu , Cd , NH4DEADEDTC FAAS 158 alloys Pb2+ Alloy samples Bi3+ Cyanex 301a ET AAS 68 As3+, As5+ NaDEDTC ET AAS 159 2+ a Cu-Be alloys Be GA FAAS 67 6+ e 2+ e Estuarine Cr NaDEDTC ET AAS 156 Alloy steel Co NN FAAS 147 water Co2+ NaDEDTC ET AASe 115 Biological Fe3+, Cu2+, QADEAP RP-HPLC 185 Ni2+, Cu2+, NaDEDTC ET AASe 54 samples Ni2+, Zn2+, Cd2+, Pb2+ 2+ 2+ 2+ e Mn Cu , Cd NH4PyDTC ET AAS 113, 2+ 2+ e Cu , Cd , NH4DEDTP ET AAS 121 114 2+ 2+ 2+ e Pb Cu , Cd , NH4DEDTP ET AAS 121 2+ e 2+ Zn NH4DEDTP FAAS 41 Pb 3+ 2+ e 2+ 2+ e As , Cu , NH4DEDTP ICP-MS 148 Cu , Cd , NH4DEDTP ET AAS 120 Se4+, Cd2+, Pb2+ 2+ 3+ 2+ e Hg , Tl , Zn NH4DEDTP ET AAS 41 2+ 3+ 2+ e Pb , Bi Pb NH4PyDTC ET AAS 112 1+ 4+ e Apple leaves, Ag , Te , NH4DEDTP ICP-MS 149 2+ 2+ e 3+ 6+ Geological Cu , Cd Phen FAAS 97 milk powder, Au , U samples urine 2+ 2+ e 2+ Cu , Cd , NH4DEDTP ET AAS 121 Dried marine Be AA ET AAS 183 Pb2+ organism – 2+ 2+ Green algae, TlCl4 Sterinol, FAAS, FES, 101 High-purity Cd , Pb NH4DEADEDTC ET AAS 160 hay, luzerne Septonex, CPBr ICP-ES Na, K, Mg, Ca 4+ 2+ Horse kidney, Se , Hg NH4PyDTC NAA 150 chlorides, human diet nitrates, Human hair, Co2+ Nitroso-R-salt, FAASe 98 sulfates 3+ 2+ e tea, peach TBABr High-purity Al Fe , Co , NH4TMDTC HPF-FAAS 155 leaves Ni2+, Cu2+, Mussel Co2+ NN FAASe 147 Zn2+, Cd2+, Cu2+, Cd2+ Phen FAASe 97 Au, Tl, 2– e 2+ 3+ Peach leaves, PbI4 KI, FAAS 99 Pb , Bi powdered hair, TBABr Bi3+ Oxine DP-ASV 161 powdered rice Lake water Ag1+ QADEAP SP 182 2– a Plant materials PtCl4 Sterinol , ES 100 2+ e CPBr Loam soil Zn NH4DBDP FAAS 41 6+ e Planting Co2+, Ni2+, Tm-APP RP-HPLC 184 Natural water Cr NaDEDTC ET AAS 156 2+ 2+ e effluents Cu2+, Ag1+, Mn , Co , NH4DEADEDTC ICP-MS , 162 2+ 2+ Cd2+, Hg2+, Ni , Cu , FAAS 2+ 2+ Pb2+ Zn , Cd , 2+ 2+ 2+ Pb Plant Cu , Cd NH4DEADEDTC FAAS 151 2+ 2+ materials, Ni , Cu , CMDTC CZEP 185 2+ 2+ food stuffs Cd , Hg , 2+ Spinach, As3+, Sb3+ KDPyDTPc ET AAS, 152 Pb 3+ d cabbage ICP-MS Fe XO SP 85 2+ 2+ e Tomato, leaves Co2+ NN FAASe 147 Cu , Cd , NH4DEADEDTC FAAS 158 Pb2+ 3+ 3+ c Common salt Sb , As KDPyDTP ET AAS, 152 As3+, Sb3+ KDPrDTPc ET AAS 152 ICP-MS ICP-MS Deionized Co2+, Ni2+, NaDEDTC ET AASe 153 Sr2+ DCH18C6c RD 92 2+ 6+ water Cu2+, Cd2+, Pd , W , NH4PCDT INAA 163 Pb2+ Pt4+, Au3+, Ni2+, Cu2+, NaDEDTC ET AASe 116 U6+ Cd2+, Pb2+ U6+ TOPOb SP 78 Distilled water Cu2+ NCb SP 154 Pharmaceutical Zn2+ TAN a SPSe 70 Ag1+ TMTTFb ET AAS 81 preparations

3+ 6+ 2+ b Drinking water Al , Cr , NH4TMDTC HPF-FAAS 155 Pure water Co QADEAB , SPS 164 Mn2+, Fe3+, CTMAB Ni2+, Ag1+, Co2+ QADMAB, SPS 157 Cd2+, Pb2+ CTMAB 6+ e Cr NaDEDTC ET AAS 156 Rain water Co2+, Ni2+, Tm-APP RP-HPLC 184 2+ Co QADMeAB, SPS 157 Cu2+, Ag1+, CTMeAB Cd2+, Hg2+, 2+ 2+ e Ni , Cu , NaDEDTC FAAS 116 Pb2+ Cd2+, Pb2+ ANALYTICAL SCIENCES APRIL 2006, VOL. 22 513

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Rain water V5+, Cr6+, NaDBzDTC TXRF 165 Seawater Co2+ NaDEDTC ET LEAFSe 111 Mn2+, Fe3+, Co2+, Ni2+, NaDEDTC ET AASe 153 Ni2+, Cu2+, Cu2+, Cd2+, Zn2+, As3+, Pb2+ 2+ 4+ 2+ Pb , Se , Ni NH4DPeDTP ET AAS 40 Mo5+, Cd2+, Ni2+, Cu2+, NaDEDTC ET AASe 54 Tl3+ Cd2+, Pb2+ 2+ b 2+ 2+ e Cu SB AAS 76 Ni , Cu , NaNH4DTC ET AAS 116 2+ 2+ 2+ 2+ Cd , Pb River water Co , Ni , Tm-APP RP-HPLC 184 2+ b 2+ 1+ Cu NaDEDTC AAS 76 Cu , Ag , 2+ 3+ e 2+ 2+ Cu , As , NH4DEDTP ICP-MS , 125 Cd , Hg , 4+ 2+ 2+ Se , Ag , ETV-ICP-MS Pb 2+ 3+ 6+ Cd , Au , Cr NaDEDTC ET AAS 156 2+ 3+ 2+ 2+ e Pb , Bi Mn , Co , NH4DEADEDTC ICP-MS , 162 2+ 2+ 2+ 2+ Cu , Cd Phen FAAS 97 Ni , Cu , FAAS 2+ 2+ e Zn2+, Cd2+, Cu , Cd NH4PyDC ET AAS 113, 2+ 114 Pb 2+ 2+ a e 3+ 2+ d Cu , Cd NaDEDTC ET AAS 65 Fe , Co XO SP 62 2+ 2+ e 2 2+ e Cu , Cd , NH4DEADEDTC FAAS 158 Ni , Cu , NaDEDTC ET AAS 116 2+ 2+ 2+ Pb Cd , Pb 2+ 2+ e 2+ b Cu , Cd , NH4DEDTP ET AAS 121 Cu HNQ FAAS 77 2+ 2+ b Pb Cu HPAG FAAS 181 2+ e 2+ 3+ Zn NH4DBDP FAAS 41 Cu , As , NH4DEDTC ICP-MS 148 2+ a 4+ 2+ Zn Aliquat 336 ET AAS 102 Se , Cd , 3+ 5+ 3+ 3+ As , As NaDEDTC ET AAS 159 In , Tl , 6+ e 2+ 2+ Mo Oxine FAAS 174 Hg , Pb , 1+ 4+ e 3+ Ag , Te , NH4DEDTP ICP-MS 149 Bi 3+ 6+ 2+ a Au , U Zn Aliquat 336 ET AAS 102 2+ a e 1+ b 81 Cd Aliquat 336 ET AAS 103 Ag TMTTF ET AAS 2+ e 1+ Cd NH4DEDTP ET AAS 175 Ag QADEAP SP 182 a 1+ 4+ Rare earths HDEHP , ICP-MS 66 Ag , Te , NH4DEDTP ICP-MS 149 3+ 6+ H2MEHP Au , U 2+ e 2+ a Pb NH4DEADEDTC ET AAS 168 Cd Aliquat 336 ET AAS 103 2+ e 2+ b Pb NH4PyDTC ET AAS 112 Ba DB18C6 AAS 166 2+ 2+ b Pb NaDEDTC ET AAS 166 Hg H18C6TO CV AAS 75 6+ Hg2+, NaDEDTCa ICP-MS 137 U Oxine DPP 176 + CH3Hg Sediments: Pb2+ NaDEDTC ET AAS 167 Drainage Co2+, Ni2+, NaDEDTC ET AAS 177 Ni2+, Cu2+, NaDEDTC ET AAS 54 Pb2+ 2+ 2+ 2+ 2+ e Cd , Pb Estuarine Cu , Cd , NH4DEDTP FAAS 118 2+ 2+ Pb NH4DEADEDTC ET AAS 168 Pb 2+ b 2+ 2+ e Pb HaAnMeS FAAS 169 Lakes Cu , Cd , NH4DEDTP FAAS 118 2+ 2+ a Pb Seawater Be GA FAAS 67 2+ 2+ e 6+ e Rivers Cu , Cd , NH4DEDTP FAAS 118 Cr NaDEDC ET AAS 156 2+ 2+ 2+ Pb Co , Ni , Tm-APP RP-HPLC 184 2+ e 2+ 1+ Zn NH4DBDTP FAAS 41 Cu , Ag , 2+ 2+ e 2+ 2+ Sea Cu , Cd , Dz FAAS 178 Cd , Hg , 2+ Pb2+ Pb Mn2+, Fe3+, 5-Br-PAPS ICP-MS 170 Snow water Cu2+ SBb AAS 76 Ni2+, Cu2+, 2+ 2+ Sodium salt: Zn , Cd , 3+ 2+ 2+ Acetate, Fe , Ni , 5-Br-PAPSP ICP-AES, 179 Pb 2+ 2+ 2+ 3+ perchlorate, Cu , Zn FAAS Mn , Fe , Oxine ET AAS 171 phosphates Co2+, Ni2+, Cu2+, Zn2+, Soil Sr2+ DCH18C6 RD 92 Cd2+ Spring water Pb2+ HAnMSb FAAS 169 Mn2+, Fe3+, Oxine ICP-AES 172 Cu2+ HNQb FAAS 77 Ni2+, Cu2+, Cu2+ NCb SP 154 Zn2+, Cd2+, Ag1+ HT18C6a AAS 80 Pb2+ Hg2+ HT18C6TOb CV AAS 75 2+ 2+ e 2+ b Mn , Co , NH4DEADEDTC ICP-MS , 162 Pb DHPEA FAAS 180 Ni2+, Cu2+, FAAS Pb2+ SB-Sb FAAS 83 Zn2+, Cd2+, Bi3+ Cyanex 301a ET AAS 68 2+ Pb Standard Pd2+ DEBTUra ET AASe 71 3+ 2+ e Fe , Co , NH4PyDTC ET AAS 173 solution of Ru, 2+ 2+ Ni , Cu , Rh, Os, Ir, Pt, 2+ 2+ Cd , Pb Ag, Au 514 ANALYTICAL SCIENCES APRIL 2006, VOL. 22

(Continued)

Steel carbon Co2+, Ni2+, NaDEDTCa ET AAS 177 Tap water Hg2+ HT18C6TOb CV AAS 75 Pb2+ U6+ TOPOb SP 78 Tap water Be2+ GAa FAAS 67 Waste water Cr3+, Cr6+ TBAA HP FAAS 128 Co2+, Ni2+, Tm-APP RP-HPLC 184 Co2+, Ni2+, Tm-APP RP-HPLC 184 Cu2+, Ag1+, Cu2+, Ag1+, Cd2+, Hg2+, Cd2+, Hg2+, Pb2+ Pb2+ Co2+ QADMAB, SPS 157 Cu2+ HPAGb FAAS 181 CTMAB Cu2+, Co2+ PAN d LCD 64 Co2+ QADEABb, SPS 164 Cd2+, Zn2+, XOd FAAS 85 CTMAB Pb2+ Cu2+ HNQb FAAS 77 Well water Be2+ GAa FAAS 67 Cu2+ SBb AAS 76 Co2+ QADEABb, SPS 164 Cu2+ NCb SP 154 CTMAB Cu2+ HPAGb FAAS 181 Co2+ QADMAB, SPS 157 Ag1+ HT18C6b AAS 80 CTMAB Ag1+ TMTTFa ET AAS 81 Cu2+ HNQb FAAS 77 Ag1+ QADEAP SP 182 Ba2+ DB18C6b AAS 166 Ba2+ DB18C6b AAS 166 Hg2+ HT18C6TOb CV AAS 75

Otherwise indicated, the reagent is added to the aqueous sample solution before SPE recovery. The adsorbent is C18-bonded silica (C18) packed in a column (cartridge). a. Reagent premodified on adsorbent. b. Membrane disk, C18. c. C16-bonded silica. d. Phenyl-bonded silica. e. On-line determination. in the previous cases, the preconcentration factors of more than determined by ICP-MS. The mercury species can be also 100-fold were readily achieved. Di- and trialkyltin salts, which determined by cold vapor AAS.138 are specific reagents for oxyanions can be recommended for the Trace metals tend to be complexed by organic and inorganic separation and preconcentration of different anions of ligands in natural water. An estimation of the degree of phosphorus, arsenic, and selenium as well.130 complexation and, therefore, the free metal ion concentrations A flow-injection procedure for the separation and mainly responsible for their chemical reactivity and biological preconcentration of inorganic arsenic species based on the availability is a difficult and an important problem of speciation 3+ 2+ complexation of As with NH4DEDTP and sorption on a C18- analysis. A method was described for measuring the free Cu bonded silica gel minicolumn was proposed.131 After reduction concentration and copper complexation in seawater, based on 2+ to the trivalent state by using potassium iodide and ascorbic the sorption of copper onto C18 cartridges and internal Cu acid, the total arsenic was also retained on the column, eluted calibration.139 The calibration was achieved by adding with ethanol and determined by ET AAS in spiked tap water Cu–EDTA (ethylenediaminetetraacetic acid) buffer to seawater and reference water samples. samples. It was shown from the experimental and computed Ammonium pyrrolidine dithiocarbamate and alkyl-bonded values for inorganic complexation that only up to 2% of the silica gels (C16 and C18) were used in the simultaneous dissolved copper can be accounted for as inorganic species; the determination of Sb3+ and total inorganic Sb132 as well as Se4+ remainder appeared to be organically complexed. The Cu2+- 6+ 133 and Se in different natural water samples. bathocuproine complex was separated by SPE on C18 cartridges Flow-injection flame AAS with SPE preconcentration was or disks followed by elution and spectrophotometric detection applied to the simultaneous determination of Fe2+ and Fe3+. and applied in the speciation studies of copper in fresh water 140 Upon injection of a sample containing iron in both oxidation samples. The use of C18 cartridges was tested as a tool to states, Fe3+ was directly carried to the spectrometer, whereas separate complexed from free copper when the metal is released 2+ Fe -ferrozine complex was in-line retained on a C18-bonded by olive mill waste water from polluted river sediments and silica column placed between the injection valve and soil.141 spectrometer and was subsequently eluted with methanol.134 A method was developed to distinguish between labile The same reagent and a similar column were used for the complexes, non-polar organic adsorbable matter and ion spectrophotometric determination of Fe2+ in seawater.135 exchangable substances in fresh water using ion-exchange,

Organomercury compounds are generally more toxic than reversed-phase (C18) and comlexing adsorbent columns inorganic mercury salts, therefore their differential combined with ICP-MS measurements.142 Similar three-column determination is an important problem. A simple and rapid systems, including a C18 cartridge, were applied in the method for in situ preconcentration of inorganic and organic speciation analyses of Cu2+, Cd2+, Pb2+, and Mn2+ in fresh- and mercury compounds in water samples, based on SPE using seawater samples.143,144 Using a mixed column or sequential dithizone preimmobilized on a C18-bonded silica column, was columns of silica gel and C18-bonded silica, cationic and neutral developed.136 A speciation analysis of methylmercury (MeHg), Cu2+ and Pb2+ species can be adsorbed, followed by sequential 2+ phenylmercury (PhHg) and Hg was performed by reversed- elution with methanol and then by 0.1 M HNO3 with subsequent phase HPLC. A field sampling strategy for the determination of automatic delivery of eluate to a heated ET AAS atomizer.145 MeHg and inorganic mercury was also described in a paper.137 A flow-through sensor for aluminium based on the use of a

River-water samples were in situ passed through a minicolumn C18 column and salicylaldehyde picolinoylhydrazone as a 146 packed with C18-bonded silica gel pretreated with NaDEDTC. fluorometric reagent was described. Acid reactive, total The preconcentrated mercury species were eluted with a monomeric, and non-labile monomeric aluminium can be thiourea solution, separated by liquid chromatography and determined and two other forms (acid soluble and labile ANALYTICAL SCIENCES APRIL 2006, VOL. 22 515 monomeric) can be studied using an on-line ion-exchange 8. G. V. Myasoedova and S. B. Savvin, Crit. Rev. Anal. column. The labile Fe2+ and Fe3+ species were complexes Chem., 1986, 17, 1. directly with Phen and Ferron (8-hydroxy-7-iodoquinoline-5- 9. R. M. Izatt, J. S. Bradshaw, R. L. Bruening, B. J. Tarbet, sulfonic acid), respectively, separated in a system of a C18 and and M. L. Bruening, Pure Appl. Chem., 1995, 67, 1069. an anion-exchange phases, and determined by FAAS.147 The 10. G. V. Lisichkin and F. Yu. Fadeev, Russ. Chem. J., 1996, method was applied to the iron speciation analysis in wine 40, 65. samples. 11. E. V. Thurman and M. S. Mills, “Solid-Phase Extraction. Principles and Practice”, 1998, Wiley-Interscience, New York. 8 Conclusions 12. J. Pawliszyn, “Solid-Phase Microextraction. Theory and Practice”, 1997, Wiley-VCH, New York. Alkyl-bonded silica gels compose a unique class of adsorbents 13. “Extraction Chromatograpy”, ed. T. Braun and G. that can be applied to the enrichment and separation of a great Ghersini, 1975, Akademiai Kiado, Budapest. variety of substances, ranging from biomolecules and organic 14. L. R. Snyder and J. J. Kirkland, “An Introduction to compounds to inorganic ions and their species. Although Modern Liquid Chromatography”, 2nd ed., 1978, New numerous effective adsorption materials are known, solid-phase York. extraction on alkyl silicas is currently the leader among the 15. P. J. Schoenmakers, “Optimization of Chromatographic techniques for the preconcentration of organic substances. This Selectivity. A Guide to Method Development”, 1986, review shows that the described method is versatile and useful Elsevier, Amsterdam. for inorganic analysis as well. SPE on alkyl-bonded silica gels 16. L. P. Hammett, “Physical Organic Chemistry Reaction should be considered as one of adsorption techniques, but the Rates, Equilibria and Mechanisms”, 2nd ed., 1970, method has also much in common with liquid–liquid extraction McGraw-Hill Book Comp., New York. and the wealth of information available for classical solvent 17. V. A. Palm, “Fundamentals of Quantitative Theory of extraction is largely applicable to SPE. The use of such Organic Reactions (in Russian)”, 1977, Khimiya, information facilitates the selection of reagents and the Leningrad. conditions for SPE of inorganic ions in the form of various 18. R. Kaliszan, “Quantitative Structure—Chromatographic compounds. Retention Relationships”, 1987, J. Wiley, New York. Alkyl silicas are mechanically and chemically stable materials 19. C. Hansch and A. Leo, “Substituent Constants for within a wide acidity range, although it should be mentioned Correlation Analysis in Chemistry and Biology”, 1979, that their chemical stability is lower at pH higher than 9 because Wiley-Interscience, New York. of hydrolysis. These materials are not caked in time, and they 20. C. Hansch and A. Leo, “Exploring QSAR: Fundamentals can be used as columns or thin layers. The reversed-phase and Applications in Chemistry and Biology”, 1995, ACS, materials themselves are not selective, but the selectivity of Washington. extraction can be achieved by use of proper reagent and 21. J. Sangster, “Octanol–Water Partition Coefficients: composition of the aqueous phase. Further improvements of the Fundamentals and Physical Chemistry”, 1997, Wiley- selectivity and the performance characteristics of the whole Interscience, New York. analysis are provided by using on-line combinations of SPE and 22. A. R. Timerbaev, I. G. Tsoi, and O. M. Petrukhin, J. selective instrumental determinations. Although a number of Chromatogr., 1991, 555, 163. such combinations are known and described in this paper, much 23. A. R. Timerbaev, O. S. Semenova, I. G. Tsoi, and O. M. more should be done for wider applications of SPE. When Petrukhin, J. Chromatogr., 1993, 648, 307. developing such hyphenated techniques, one should bear in 24. A. R. Timerbaev, O. M. Petrukhin, and Yu. A. Zolotov, mind a risk of washing out the reagent from a premodified Fresenius J. Anal. Chem., 1987, 327, 87. phase if a large volume of test solution, not containing added 25. A. R. Timerbaev and O. M. Petrukhin, “Liquid Adsorbtion reagent, is passed through the preconcentration unit. 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Sterinol N-dimethyllaurylbenzylammonium bromide TAN 1-(2-Thiazolyazo)-2-naphthol TBAA Tetrabutylammonium acetate Abbreviations Tm-APP Tetra(m-aminophenyl)porphyrin TPMDPD Tetraphenyl methylenediphosphine dioxide TBABr Tetrabutylammonium bromide AA Acetylacetone TBP Tri-n-butylphosphate Aliquat 336 Methyltricaprylammonium chloride TMTTF Tetramethyltetrathiafulvalen BHAMS B i s ( 1 - hydroxy-9,10-anthraquinone-2- TOPO Tri-n-octhylphosphine oxide methyl)sulfide XO Xylenol Orange 5-Br-PAPSP 2-(5-Bromo-2-pyridylazo)-5-(N-propyl-N- AES Atomic emission spectrometry sulfopropylamino)phenol ASV Anodic stripping voltammetry CMDTC Bis(carboxymethyl)dithiocarbamate CV AAS Cold vapor atomic absorption spectrometry CPBr Cetylpyridinium bromide DPP Differential pulse polarography CTMABr Cetyltrimethylammonium bromide DPASV Differential pulse anodic stripping voltammetry Cupferron Ammonium N-nitroso-N-phenylhydroxylamine CZEP Capillary zone electrophoresis Cyanex 301 Bis(2,4,4-trimethylpentyl)dithiophosphinic acid ET AAS Electrothermal atomic absorption spectrometry DB18C6 Dibenzo-18-crown-6 ET LEAFS Laser-excited atomic fluorescence spectrometry in DCH18C6 Dicyclohexano-18-crown-6 an electrothermal atomizer DEBT N,N-Diethyl-N′-benzoylthiourea ETV-ICP-MS Electrothermal vaporization coupled to inductively ANALYTICAL SCIENCES APRIL 2006, VOL. 22 519

coupled plasma mass spectrometry INAA Instrumental neutron activation analysis FAAS Flame atomic absorption spectrometry LCD Linear-colorimetric determination FAES Flame atomic emission spectrometry LLE Liquid–liquid extraction HPF-FAAS High performance flow flame atomic spectrometry NAA Neutron activation analysis HPLC High performance liquid chromatography RA Radiometric analysis ICP-AES Inductively coupled plasma atomic emission SP Spectrophotometry spectrometry SPSP Solid-phase spectrophotometry ICP-MS Inductively coupled plasma mass spectrometry TXRF Total X-ray reflection fluorescence