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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
Preconcentration and separation processes are often needed for the determination of trace elements in environmental samples, high-purity materials and other matrices.1–4 Solid-phase extraction (SPE) with the use of alkyl-bonded and other surface-modified silica gels has been used in the last 15 years as an alternative to liquid–liquid extraction in the analysis of various samples. The method is simple; it enables the rapid and complete isolation of the analytes of interest from a complex matrix with a preconcentration factor of several orders of magnitude to be achieved. A surface-modified adsorbent is a support bearing at the surface of a chemically bonded “monomolecular” layer of adsorbing groups, which provides fast kinetics of the recovery and elution of the molecules and ions under study.5–9 SPE by use of alkyl-bonded silica gels has been widely applied in the environmental analysis of polyaromatic hydrocarbons, pesticides, substances of biochemical origin and other organic compounds. However, its application in metal extraction and preconcentration has been much less extensive, although the capabilities of the technique in inorganic analysis also appear to be very promising.
To avoid confusion, it should be noted that different adsorption methods are named solid-phase extraction.6 First of all, these are techniques based on the use of chemically bonded comlexing groupings.10 Commercially available Kelex-100, containing iminodiacetate functions is one of best known adsorbents of this kind.7,8 A number of adsorption materials with various classical chelating groups as well as with macrocycles attached to silica gel by a chemical bond are applied.9,10 There are some new technologies that have appeared in SPE and SPE-related products. Solid-phase microextraction (SPME) involves the sorption of analytes onto a microfiber, which is made of a fused-silica fiber coated with a hydrophobic polymer. SPME is usually followed by direct desorption in the inlet of a gas chromatograph.11,12 This method can be considered as a microvariant of extraction chromatography.13 There are some new phases for SPE including highly cross-linked copolymers, graphitized carbon for adsorbing polar organic compounds, internal reversed-phase sorbents for clean-up of complex biological materials, etc.6
Alkyl-bonded sorbents have been mainly studied for SPE in inorganic analysis; our attention in this paper will be focused on such materials. The most simple mode of metal preconcentration by SPE is based on the addition of a soluble complexing reagent to the test solution, followed by the adsorption of metal complexes formed on an alkyl-bonded material. Another mode in SPE involves a preliminary dynamic modification of a similar adsorbent, e.g. by a solution of a chelating agent, which is retained at the material surface during the following separation process due to non-covalent interactions. Thus-prepared sorbents possess the properties of materials bearing covalently-bonded chelating functions. Both modes of SPE are examined in this paper.

2 Theory of Solid-Phase Extraction and Related Techniques
Like most of separation methods, SPE is based on the distribution of a solute between two phases. Both batch and column variants of SPE can be used for preconcentration purposes. The column mode can be considered as a version of reversed phase liquid–solid chromatography (RP LSC).13 In fact, alkyl-bonded stationary phases with the alkyl length nc = 8, 16, or 18 and different grain sizes are most often used. Chromatographic retention is conveniently described by the capacity factor, k, which is defined as ki= qi,s/qi,m, where qi,s and qi,m denote the total quantity of a solute (i) to be present in the stationary phase (s) and the mobile phase (m), respectively.14,15 The higher is the k value, the better is the solute retained, and the later is eluted from the column. Correspondingly, the separation selectivity can be defined as the relative retention of two solutes, βi,j, which is sometimes called the separation factor, expressed in terms of the capacity factors, βi,j = kj/ki. If the preconcentration of a group of dissolved ions or molecules is required, the k values for all of the solutes should be as close as possible.
Two approaches can be applied to the treatment of SPE experimental data. The first one may be based on the Snyder “competition” model, which describes the distribution of a solute between liquid and solid phases.14,15 In this model it is assumed that the solid surface is covered with mobile-phase molecules, and that solute molecules have to compete with the solvent molecules in this adsorption layer to occupy an adsorption site. It is the difference between the affinity of the mobile phase and that of solute for the stationary phase that determines the retention in LSC and, therefore, in SPE according to the competition model. Snyder14 formulated the following equation that interrelates the distribution coefficient KD= ci,s/ci,m (c denotes concentration in one phase) with the adsorption area of the solute molecule Ai and the adsorption energy of the solute on a standard adsorbent Si0:
log KD = log Va+ α(Si0 – AiE0), (1)
where Va is the volume of the adsorbed solvent per gram of the stationary phase and α is the adsorbent activity.
In the frames of the second model, which is more practical for our further speculations, the distribution of a solute in SPE can be considered as a partition between two liquid phases. By definition, the capacity factor is a dimensionless quantity, which is in this case described by
k = KDVs/Vm, (2)
where Vs and Vm are the volumes of the stationary and liquid phases, respectively. It is assumed in this model that the analyte-containing phase is a homogeneous solution. Because relative values are used in the calculations, in both cases the experimental values of the capacity factors allow us to discuss the dependence of the chromatographic efficiency on both the properties of the stationary phase and the solutes to be separated and the experimental conditions.
Methods of correlation analysis are used for this purpose, which may be used if the principle of the linearity of free energies (PLFE) is valid, i.e., if the free energy of a chemical or a physicochemical process can be expressed as additive partial 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 now a great number of data are available that make it possible to discuss and moreover to predict the chromatographic behavior of compounds, depending on their properties and on the chromatographic system used.18 Reversed-phase high-performance liquid chromatography (RP HPLC) has been mainly used for organic separations, and the great majority of data has been obtained for organic substances. We will mention here only some results, which may have interest concerning this work.
The retention (k) first increases exponentially with the hydrocarbon chain length; that is, log k enhances with increasing nc. However, if the chain lengthens further, the retention rise becomes less pronounced and, to a first approximation, one may suppose that the capacity factor tends to be independent of the chain length at nc between 14 and 22. A bend in the log k – nc dependence or the boundary value of nc is dependent on the test sample. It has been shown experimentally that this value increases with the molecular mass of sample molecules. Such a dependence violates simple considerations of the interaction between the sample molecules and an alkyl-bonded surface. One could expect a linear increase of k with nc if such a surface would behave like a liquid. Also on the contrary, the k value should not depend on nc in case of a solid adsorption surface.18

The partition of substances between two liquid phases is important for various fields of science and technology. For example, a priori estimation of the possibility of substance permeation into cells is of importance for biological studies. The distribution between water and n-octanol describes quite adequately the membrane permeability. A database including several thousand partition constants is now available for this standard two-phase system.19–21 A statistical analysis of the experimental data has made it possible to estimate the hydrophobicity (or hydrophilicity) factors for discrete fragments of organic molecules. The correlations between capacity factors and the hydrophobic/hydrophilic balance for organic substances verify an assumption that the capacity factor is, to a first approximation, proportional to the molecular volume of the compound. A more detailed examination shows that the capacity factor is a function of the hydrophilic/lipophilic balance for the compound. A number of equations are known that enable the capacity factors as functions of various parameters characterizing different compound properties to be evaluated.17,18
The solvent effect in Snyder’s equation is described by the adsorption energy of the solvent per unit area, usually referred to as the solvent strength.13,14,18 Obviously, the adsorption energy depends not only on the solvent, but also on the adsorbent. For a hydrophobic adsorbent, e.g. carbon, alkanes have the highest and methanol has the lowest values of the solvent strength. This is why high distribution coefficients in SPE are achieved in the use of alkyl-bonded adsorbents as a stationary phase and an aqueous solution as a mobile phase that makes it possible to apply short columns, and even filters for preconcentration of organic compounds. It is also worth noting that such materials possess rather low volume capacities with respect to solutes in RP HPLC, where a solid or pseudosolid surface works as the stationary phase. Ion-pair RP HPLC is used for the separation of substances in the form of ion pairs. In this case, the hydrophobic/hydrophilic balance is still of importance, but the dependence on the solvent and experimental
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
The alkylated phases belong to so-called boundary phases. Their properties are determined by the matrix (silica gel in our 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 hydroxylated silica gel have been thoroughly studied. This adsorbent is widely used, e.g., for the separation of metal chelates by normal-phase chromatography.26–28 It is worth noting that silica gel is a hard reagent in terms of the principle of hard and soft acids and bases (HSAB).29,30 Thus, alkyl-bonded silica gels possess the properties of both a hydrophobic adsorbent and a hydroxylated silica gel.
With increasing the alkyl length, the hydroxyl groups become less available sterically. Nowadays, only commercial phases with a hydrocarbon chain length of C16 – C18 are practically used. The reactivity of residual hydroxyl groups is reduced to an acceptable level in adsorbents with such alkyl groups. It should be taken into account that complete alkylation is impossible, and the conventional adsorbents have 50% “free”, although difficultly availably hydroxyl groups.10
Liquid–liquid (solvent) extraction (LLE) is very widely applied to metal separation and enrichment. The technique is based on the formation of metal complexes and their distribution between an aqueous phase and an organic solvent. LLE depends on the stability and partition constants of metal complexes. One of the essential differences of the LLE methods from chromatographic ones, particularly from those based on the use of surface-modified adsorbents, consists in the possibility of the extraction of large metal amounts. This is why LLE is widely used in various technologies, but not only on a laboratory scale, and a huge amount of information on the chemistry of liquid–liquid extraction of various compounds has been accumulated and generalized in a number of reviews, monographs and reference books (e.g., see Refs. 31 – 34). Noticeable similaries of LLE and SPE allowed us to hope that a variety of different metal compounds used in LLE could be employed in SPE. The authors of this paper pay attention not only to the SPE of metal chelates, which have been mainly used, but also to the extraction of metal compounds of other types investigated and applied in conventional LLE.

3 Chelate Compounds
Compounds of different types used in LLE are usually divided into two big groups: neutral compounds and ion associates or pairs.32,33 Against neutral compounds, complexes formed by bidentate monobasic chelating reagents, which belong to cation-exchange extractants, and by neutral extractants, should be first of all mentioned. Neutral metal chelates may also be divided into two types: coordinatively saturated (with respect to Fig. 1 Dependence of the solid-phase extraction percentage of Cu2+ at pH 5.0 on the number of carbon atoms in alkyl groups bonded to the surface of silica gel in the presence of 1 × 10–2 M acetylacetone.extracting ligand) complexes, which do not contain water molecules in the first metal coordination sphere, and coordinatively unsaturated, and therefore hydrated compounds. The behaviors of the complexes of these two types in LLE are in principle different.31–33,35
The extraction constants for any metal chelates and, therefore, the metal recovery and extraction selectivity are functions of stability and partition constants for the chelate compounds.33–35,37 Because similar metal chelates are used in LLE and SPE, it seemed to be obvious that the behavior of metal complexes, depending on their thermodynamic stability, should be common for both techniques, and that the HSAB principle could be applied to the prediction of metal behavior in SPE.35 The partition constants for coordinatively saturated hydrophobic chelates are mainly dependent on the energy of cavity formation and on the formation of hydrogen bonds between electronegative atoms of chelate molecule and water molecules at the cavity surface,36,37 which is on the hydration energy, which is the same in the LLE and SPE systems. An alkyl-bonded surface is hydrophobic, and one can also suppose that the solubility of coordinatively saturated metal chelates in hydrocarbons and the energy of adsorption on alkyl-bonded silica gel are interrelated. The study of the distribution of copper acetylacetonate between an aqueous solution and alkylated silica gel has shown that both the hydrophobicity of silica gel phases and the extraction percentage for Cu2+ increase with the alkyl chain length (Fig. 1).
In fact, the electron paramagnetic resonance spectra of copper diethyldithiocarbamate were shown to be identical for this chelate compound adsorbed at the surface of hexadecyl silica gel and dissolved in an organic solvent.38 This means that non-covalent interactions are responsible for the total interactions between the chelate molecules and the organic solvent molecules or the surface hydrocarbon groups of modified silica gel. The investigation of copper(II) dialkyldithiophosphates adsorption at C16-bonded silica has shown that the area (cm2) occupied by a complex molecule increases with the molecular mass of the reagent alkyl group, and is equal to 9.8 × 10–14 (alkyl = C2H5), 1.2 × 10–13 (C4H9), and 6.7 × 10–13 (C6H13).39 The distribution coefficients of the complexes in LLE and SPE systems follow the same order.
A comparison of some properties of organic solvents (e.g. characterized by solubility parameters) with those of alkyl-silicas and the experimental results mentioned above suggest that the variety of data on liquid–liquid extraction of metal chelates may be used to select the reagents and conditions for the SPE preconcentration of metal ions. This can be illustrated by taking

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