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a 3507 Middleton Avenue, Cincinnati, OH 45220
b USEPA, ORD, NRMRL, 5995 Center Hill Avenue, Cincinnati, OH 45224
* Corresponding author (Ryan.Jim{at}epa.gov)
Received for publication January 14, 2004.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONCHEMICAL EXTRACTION METHODSSOLID PHASE SPECIATION:...TECHNIQUES USED IN SURFACE...SUMMARY, CONCLUSIONS, AND...REFERENCES |
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONCHEMICAL EXTRACTION METHODSSOLID PHASE SPECIATION:...TECHNIQUES USED IN SURFACE...SUMMARY, CONCLUSIONS, AND...REFERENCES |
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Man's perturbation of nature's slowly occurring life cycle of metals includes (i) the extraction, smelting, and processing of metal bearing ores into products, (ii) the distribution and use of these products by industry and consumers, and (iii) the return of these metals in a concentrated form to the natural environment through disposal of processing wastes and the discard of spent products. The metal or metals then become contaminants in the receiving environments. Part of the reason they become contaminants is seen within the description of the man-made life cycles above. They include (i) the rapidity of the man-made cycles relative to natural ones, (ii) the transfer of the metals from mines to random environmental locations where higher potentials of direct exposure occur, (iii) the relatively high concentrations of the metals in discarded products compared to those in the receiving environment, and (iv) the chemical form, or species, in which a metal is found in the receiving environmental system. The latter issue is the central concern of this paper.
The complexity of metal contaminated sites has and continues to be simplified to a measure of the total metal content. While total metal content is a critical measure in assessing risk of a contaminated site, total metal content alone does not provide predictive insights on the bioavailability, mobility, and fate of the metal contaminant. From the perspective of risk assessment the following example illustrates the importance of speciation. Using total metal concentration as the index of risk it follows that a site with a concentration of 5000 mg Pb kg–1 is significantly more toxic than one which is 500 mg Pb kg–1. However, suppose that the only species of Pb in the 5000 mg Pb kg–1 soil was galena (lead sulfide, PbS; Ksp = 10–28.1), while the only species of Pb in the 500 mg Pb kg–1 soil was cerussite (lead carbonate, PbCO3; Ksp = 10–13.13). In absolute terms, from simple thermodynamic calculations alone, a greater amount of Pb is likely to be both mobile under water transport and bioavailable in soluble form to organisms in the soil with the lower Pb concentration, because cerussite is vastly more soluble than galena. Mobility is clearly dependent on chemical form. And although bioavailability and toxicity are complex processes and are biological species dependent as well as even genetically dependent within species, this example indicates that the chemical form of a metal contaminant is an important factor in assessing human health or ecological risks.
Why should speciation analyses be performed? The simple answer is that it is part of an overall approach to understand the complex chemistry and behavior of contaminants in environmental and biological systems. However, there are other more practical reasons. One is that improved knowledge and understanding of the chemistry of metals and their interaction with environmental system chemistries would enable fundamental progress in a variety of areas, for instance environmental, geological, agricultural, and economic. In the environmental field, an ultimate goal might be to use speciation analyses to accurately determine the human health or ecological risks posed by the metal species discovered and quantitated at a site and redirect this understanding into the design, selection, optimization, and monitoring of remediation strategies applied to clean up the site, if necessary. Today, with the advent of in situ, submolecular research tools to probe the local environment of metals, that ultimate goal may be within our grasp.
The primary objective of this manuscript is to offer to the geoscience research community a detailed outline of a number of possible techniques (old, new, and improved) that could be used for speciation analyses of metals in the natural environment, especially soils. It describes the physical processes exploited by the techniques and the types of information that can be obtained from them. It would be impossible to describe every analytical method (see Table 2 for a list of 283 techniques); therefore, we review the most promising ones from several diverse fields that could be employed in a research program. It is hoped that this manuscript will encourage those who have not yet attempted speciation analyses in their research to do so, while also enticing those already utilizing such techniques to employ a different method that complements or enhances their current capabilities. The scope of this review is broad, ranging from the simplistic (perhaps controversial) chemical extraction methods to sophisticated microfocused X-ray absorption spectroscopic techniques available only at synchrotron radiation facilities. The authors realize the abundance and significance of recent literature; however, it is academic folly to believe that the most relevant research has been accomplished in only the past five years. Therefore, well-established, less-glamorous methods developed decades ago, have been included as they still have a valuable place as primary and secondary tools in the determination of metal species.
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CHEMICAL EXTRACTION METHODS |
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TOPNOTESABSTRACTINTRODUCTIONCHEMICAL EXTRACTION METHODSSOLID PHASE SPECIATION:...TECHNIQUES USED IN SURFACE...SUMMARY, CONCLUSIONS, AND...REFERENCES |
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The idea behind these extraction methods is based on the assumption that a particular extractant is phase or retention mode specific in its chemical attack on a mixture of forms. For instance, it assumes that water will remove only easily soluble forms, that buffered acetic acid will attack and dissolve only carbonates, and that NH4OAc (or MgCl2) at pH 7 will liberate only adsorbates. Sequential application of a series of specific reagents coupled with elemental determination for each yields a fractionation pattern or a relative partitioning of metal forms into operationally defined "species." In sequential extraction schemes, reagents should be of an increasingly drastic chemical action, or of a totally different specificity, thus providing clear distinctions among sources of the extracts obtained. In practice, selectivity for a single phase or mode in a thermodynamic sense does not occur. Nor is there consistency of fractionation results between the various extractants of a given class (e.g., electrolytes or chelating agents) and different application methods used. This has prompted Kersten and Forstner (1986) to suggest that the operational species extracted be defined in terms of the reagent used rather than the phase or retention mode assumed attacked in the extraction.
In addition to the "fundamental" limitations cited above, there are two major factors which play a role in determining the success of a given extraction procedure. These are: (i) the chemical properties of the extractant and (ii) experimental parameter effects (e.g., length of time the extractant and sample are in contact, temperature, concentration, and solid to solution ratio). Further considerations apply if the extraction procedure is a sequential scheme for determining a fractionation pattern for a given soil or sediment sample. These include: (i) the sequence of steps, (ii) matrix effects (cross contamination and re-adsorption), and (iii) heterogeneity and physical associations of the solid components. Lastly, sample procurement, handling, storage, and preparation all affect the results in a crucial way. Contamination at any step, length of time, and temperature of storage, whether the sample is dried or stored moist, and whether oxygen is allowed to enter the system, can induce large methodological errors in the results of such extractions. For a more vigorous discussion of these see Batley (1989). In addition, there is a wealth of literature on specific research topics, as well as two detailed reviews specifically directed to the field of soils (Pickering, 1986) and sewage sludge–amended soils (Lake et al., 1984). The pitfalls and criticisms of these methods are recounted in the literature and are repeated here to illustrate the need to have independent methods available to perform speciation of metals in solid matrices.
Shortcomings of Chemical Extraction Methods
There are several reviews that report on the most significant criticisms of the extraction methods of speciation (Pilkington and Warren, 1979; Nirel et al., 1985; Nirel and Morel, 1990; Scheckel et al., 2003). Those criticisms will be covered below. However, one criticism that seems important but not recognized is the origin of these methods. Since they were meant for trace metals in sediment materials, their application to heavily contaminated soils may be suspect when concentrations of the "trace" metals are no longer trace but major constituents. In this situation, the metal chemistry is no longer dominated by the other major components of the system, but is itself controlling the chemistry of other elements.
The criticism above points out that the basic chemical assumptions behind the method may be suspect. In soil deposition, the concentration of the metal may be much higher. In addition, many source metal forms are artificial minerals, often unlike any natural species that are normally found. For example, relatively insoluble lead chromate salts have been used as pigments in yellow traffic paints. However, if this compound becomes distributed in soil and is stable under the soil conditions, what would a sequential extraction say about the form, which by assumption of this method, should not exist naturally? In addition, there are many sites contaminated with many different metals as well as man-made organic petrochemical wastes. How are such "unnatural" or unexpected species (and there may be many such examples in polluted soils) accounted for in schemes that have broad classes but a sediment chemistry model to guide interpretation?
In their critique, Nirel and Morel (1990) state that the operational definitions classifying extracts is a tautology and prevents the possibility of critically examining the logic of the schemes. Most critics claim validation of extraction schemes and the classes of "species" extracted have not been done. That is, comparisons of sequential extractions with thermodynamic models and direct instrumental analyses are lacking. The reagents used are also not as selective as many advocates assert. Salomons and Forstner (1984) point out extraction efficiencies vary according to the solid to reagent ratio and the length of contact time between the two. Drying, the particle size, and whether a sample has been ground or left intact also influence the results. In addition, the effect of equilibrium shifts as the extracts are removed or introduced from contact with the solid matrices is unknown. Re-adsorption of the trace element into other compartments can take place before solid–extract separation, while precipitation can occur during extraction or storage of the extract before elemental analysis. Recent research has demonstrated the pitfalls of sequential extraction procedures on examination of non-equilibrated systems. Scheckel et al. (2003) confirmed via X-ray diffraction (XRD) and X-ray absorption (XAS) spectroscopies that application of sequential extraction methods to Pb-contaminated, P-amended samples results in the formation of pyromorphite [Pb4(PO4)3Cl] during the extraction steps. The over- and underestimation of metal concentrations or "speciation" in particular steps of an extraction method could pose serious consequences in addressing risk assessment based solely on extraction results.
These major criticisms leave much to be desired about extraction methods. However, in spite of these limitations, simple extractions do exhibit connection with empirical models of bioavailability and absorption (Ruby et al., 1996). Thus, they are being used and can mislead an investigator into unfounded confidence in interpretation of the results. So, with an appreciation for the pitfalls and limitations of these techniques and the operational character of the species found, extraction schemes can be a useful tool in metal partitioning but should always be confirmed by other methods. As the discussion above implies, independent and more precise methods for species determination are badly needed. Such methods, of the spectroscopic nature, are now coming of age and are the focus of this manuscript.
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SOLID PHASE SPECIATION: INSTRUMENTAL TECHNIQUES |
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TOPNOTESABSTRACTINTRODUCTIONCHEMICAL EXTRACTION METHODSSOLID PHASE SPECIATION:...TECHNIQUES USED IN SURFACE...SUMMARY, CONCLUSIONS, AND...REFERENCES |
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When energy probes interact with matter, they give rise to signals telling a story about their interaction. In some cases, little or no energy is exchanged, in others a variable amount is exchanged between probe and sample. Diffraction is an example of the first case, while inelastic scattering is an example of a continuously varying amount of energy exchanged. A histogram or plot of the intensities of the energy produced, within a sequence of narrow energy bands, by a sample under excitation by the chosen probe, is called a spectrum. In the instances in which discrete amounts of energy are exchanged between probe and sample, spectral lines are produced. The probes are giving up or, more rarely, absorbing fixed amounts of energy from energy levels of nuclei, atoms, molecules, and macroscopic lattice structures in solids and liquids. There are a great variety of spectroscopies available to obtain information about the structure of matter at the atomic scale to determine speciation. Figure 1 shows the electromagnetic (em) spectrum, the wavelength and frequency of various types of em radiation, their names, what types of transitions in matter give rise to them, and the type of spectroscopy that is involved in analysis with particular em radiations.
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For the time being, it seems clear that a battery of analytical techniques is necessary to achieve true species determination. It is also apparent that some amount of information desired will be subjected to tradeoff. The limitations of a technique might preclude simultaneous acquisition of two pieces of information. For example, mass spectrometry has high sensitivity and ability to handle complex mixtures, and can yield structural information about discrete compounds and forms desorbed from surfaces. However, quantification is next to impossible and details of surface structure are lost in the destructive nature of the technique. Molecular fluorescence spectroscopy, IR, and XRD can unambiguously identify known compounds in simple mixtures and are nondestructive. However, the first two methods are limited to those compounds that fluoresce or have IR active bonds, while the XRD is limited to crystalline forms. Both IR and XRD have low sensitivity and difficulty with very complex mixtures and unknown forms. Physicochemical separation techniques could make samples accessible to the less sensitive methods via preconcentration. Nevertheless, all forms might not be separated, nor would they necessarily remain in the same proportions as in the original sample. The scope of a speciation approach will be defined by what type and by how much information is desired. The ultimate goal is to be able to do the former projects quickly and efficiently, and to acquire enough information to enable new theoretical techniques and predictive models to be developed. That is the challenge awaiting our discipline.
Sources for many of the details about the types of techniques which have been in general use for a number of years in analytical chemistry for qualitative and quantitative determination of elements and molecules as well as molecular structure elucidation can be obtained from general books on instrumental methods in analytical chemistry (Winefordner, 1976; Natusch and Hopke, 1983; Onori and Tabet, 1985; Skoog, 1985; Christian and Callis, 1986; Willard, 1988), methods for metal speciation (Bernhard et al., 1986; Patterson and Passino, 1987, 1990; Kramer and Allen, 1988, Batley, 1989; Broekaert and Güçer, 1989), and methods for surface analysis (Hochella and White, 1990; Bertsch and Hunter, 1996; Scheidegger and Sparks, 1996a). In addition, reviews and reports from symposia on specific methods, as well as their applications to soils, minerals, geologic materials, and environmental samples are common in the literature, for example: vibrational spectroscopies (IR, Raman spectroscopy) (Jackson, 1973; Gieseking, 1975; Nicol, 1975; Beutelspacher and Van Der Marel, 1976; Zussman, 1977; Bell and Hair, 1980; Klute, 1986; Hawthorne, 1988; Perry, 1990), mass spectrometry (MS) and secondary ion MS (SIMS) (Jackson, 1973; Nicol, 1975; Natusch and Hopke, 1983; Karasek et al., 1985; Bernhard et al., 1986; Perry, 1990), nuclear magnetic resonance (NMR) (Jackson, 1973; Klute, 1986; Wershaw and Mikita, 1987; Hawthorne, 1988; Perry, 1990), proton induced X-ray emission (PIXE) (Johansson and Campbell, 1988), X-ray methods (Walsh, 1971; Jackson, 1973; Nicol, 1975; Zussman, 1977; Stucki and Banwart, 1980; Page et al., 1982; Natusch and Hopke, 1983; Bernhard et al., 1986; Klute, 1986; Hawthorne, 1988; Bish and Post, 1989; Brown and Parks, 1989; Valkovic, 1989; Hochella and White, 1990; Perry, 1990; Brown, 1990; Bertsch and Hunter, 1996; Fendorf and Sparks, 1996; Scheidegger and Sparks, 1996a; Brown et al., 1998), and chromatographic techniques (Vickery, 1983). Table 1 shows various instrumental methods of chemical analysis, most for solid phases, some useful for solution species. Table 1 is extensive and covers most of what one would like to know about a given technique.
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The best scenarios are those of pure macroscopic single crystals. Such forms can be determined by X-ray crystallographic techniques. These analyses give bond angles and bond lengths to high precision (0.01 Å or better). Even so, mixtures of different phases and isomers, or oxygen, light, or X-ray sensitivity can make the structure determination difficult. These techniques depend on the symmetries of the molecular forms and their regular spatial arrangement to diffract the X-rays into a symmetric pattern of peaks (Bragg peaks) in two dimensions. The analysis of that symmetry pattern yields the desired information.
If one cannot obtain large single crystals, a collection of the crystallites (essentially a powder), is examined by XPD. As this is the X-ray diffraction pattern of a large number of tiny crystals immersed in the beam, it appears as the sum of many single crystal patterns oriented in numerous random directions. The result (on a photographic plate) is a series of concentric diffraction rings, symmetric in the azimuthal angle. The intensity distribution for fixed azimuth yields a one-dimensional pattern of peaks. A probable structure can be obtained from information acquired from the distribution and number of peaks, while refinement consists of a comparison of this peak pattern with those of compounds with similar chemistry and known structure. There are also techniques in which neutrons (ND) (Stucki and Banwart, 1980; Hawthorne, 1988) or electrons (ED) are used instead of X-rays as probes to produce the diffraction pattern (see ED below).
For noncrystalline materials the Bragg peak patterns of diffraction disappear. Thus other techniques must be employed to determine molecular structure. NMR can be used to yield structural information about compounds, although less refined than X-ray diffraction, for both crystalline and amorphous substances. NMR originates from the precession of the spin magnetic moment of a nucleus, if not exactly zero, in an externally generated macroscopic magnetic field. The frequency of this precession is proportional to the external field strength and has values in the radio frequency (RF) range (i.e., megahertz, MHz). Nuclei with a spin quantum number of 1/2 are the most advantageous to exploit, as they exhibit only two spin energy states or levels, whose energy difference is also proportional to the external field strength. Other nuclei of greater spin values also process, but have a larger number of energy levels. Under absorption of energy, these nuclei can flip their magnetic moment direction, thus changing energy levels. This causes the absorption spectra of spin 1/2 nuclei to be simpler than others.
It is through these RF absorption spectra that the chemical information is obtained. This is accomplished through an analysis of the chemical shifts and coupling constants observed in the NMR spectrum for a given element. Large sets of data on chemical shifts and coupling constants collected for the element of interest within a number of different structures provide the basis for empirical tables that correlate the parameters with structural features. The chemical shifts are changes in the energy of the nuclear magnetic transitions involved, due to the effect of the electronic configuration of the molecule (which shields the nucleus from the externally applied fixed field) near the absorbing nuclei. Nuclear spin–spin coupling provides the origin of the splittings observed (the source of the derived coupling constants) and these too are dependent on molecular geometry and the types and spins of the nuclei of neighboring atoms. Coordination and analysis of each of the pieces of information above allows deduction of structural features. Large uncertainties in the structures obtained are a consequence of the complexity of analysis and deviations from the expected correlations. In terms of quantitative analysis, the areas under the absorption bands are proportional to the number of nuclei absorbing. Integration over the absorption region produces a "step" whose height is proportional to the number of nuclei. Comparison of a given peak of the unknown with that of a standard yields the concentration of the unknown.
The excitation of vibrational or rotational transitions in molecules yields infrared absorption bands (wavelength approximately 1–1000 µm) characteristic of the types of atoms and their bonds. However, only those bonds that have an overall dipole moment and vibrational modes yielding a change in that moment give rise to infrared absorption and emission. A complementary effect that yields information about changes in polarizability from vibrational excitation, rather than dipole moment as in IR, is the Raman effect. It is seen in the non-elastic scattering of photons, usually from a laser source, by molecules in the sample. A fraction of the energy of a certain number of photons in the incident beam is taken up by molecular vibrational excitations. The spectrum is obtained by observing the relative frequency shift of peaks, created by the detection of non-elastically scattered photons, from the elastically scattered peak. Both IR and Raman spectroscopies can be performed using optically focused beams, thus permitting analysis of microsamples. However, greater beam intensities and finer resolution are achievable using visible light lasers, thus giving Raman a distinct advantage in that area although synchrotron IR is many orders of magnitude brighter in intensity than traditional laboratory IR equipment.
Both IR and Raman spectra, coming from molecular transitions, yield information about molecular structure, as certain spectral features are characteristic of pairs (or groups) of atoms and bond types. The presence or absence of particular bands will help narrow the identification of a species, given its molecular weight. It will, at the very least, yield partial structural features associated with given "functional groups," such as commonly found ring or chain structures and anionic groups. While NMR and IR–Raman spectra give structural information that can help narrow possibilities, the method of obtaining structure is a deductive art relying on experience, intuition, and large computer accessible databases or handbooks of known spectral features.
Mass spectrometry can also be a valuable tool to elucidate the structure of unknown molecular forms. With proper volatilization and ionization, ions representing both the parent molecule and a fragmentation pattern (unique, given the ionization energy) are recorded. Certain mass analyzers are often paired with specific ionization methods due to mass range anticipated, resolution desired, and duty cycle (pulse or continuous) of the ion beam. Some general ionization methods are field ionization (FI), chemical ionization (CI), and electron impact (EI) in increasing order of hardness (degree of ionization).
Softer ionization yields more parent molecular ions for identification by accurate molecular weight determination, while harder ionization gives a richer spectrum of fragments for structural studies. Other techniques, particularly for solids, are the thermal desorption (TD), field desorption (FD), Cf-252 plasma desorption (PD), laser desorption (LD), secondary ion MS (SIMS), and fast atom bombardment (FAB) techniques. The last three techniques present the possibility of focused microprobe beams (1 µm for LD, 0.02 µm for SIMS). These methods attempt to dislodge atoms or molecules from the surfaces of samples. Provided the mass analyzer has sufficient resolution, assessment of the mass of the parent molecule can sometimes yield at least the proper chemical formula. Mass fragmentation patterns give information about functional groups present in the molecule. As with other spectral analyses, experience and database accessibility are extremely important.
Although the sharp diffraction peaks of crystals do not exist for amorphous materials, some X-ray techniques still have a role in their structure determination. Assuming that amorphous phases of solids have little long range order but still possess molecular structures with fixed atomic arrangement, X-ray absorption spectroscopy (XAS) and X-ray scattering (XRS) provide structural information about the local environment of the element whose absorption edge is being exploited. XAS research is typically limited to synchrotron radiation sources scattered throughout the world. The various XAS techniques include extended X-ray absorption fine structure (EXAFS) spectroscopy, a surface variant (SEXAFS), X-ray absorption near edge structure spectroscopy (XANES, also called NEXAFS), and small and wide (or large) angle X-ray scattering (SAXS and WAXS or LAXS), the last two being coupled with anomalous scattering analysis techniques.
EXAFS originates in the modulation of the X-ray absorption spectrum as a function of energy (the defined fine structure). This is due to the local chemical environment around the absorbing atom affecting the outgoing photoelectron wave produced under X-ray excitation. The strength of that wave backscattered from near neighbors modulates the absorber atom's electronic wave function and thus its ability to absorb further X-ray photons. XANES provides information about the oxidation state of the atom whose absorption edge is probed. WAXS exploits the modulation of the amorphous scattered X-ray intensity as a function of angle. It too is dependent on the local chemical environment around a scattering atom through the interference due to neighboring atoms. In EXAFS and WAXS–ASA, intensity patterns are collected, processed, and Fourier transformed to produce radial distribution functions (RDFs), giving one-dimensional information about the distances to atomic shells. The identity of the atoms and their number in a shell are also available from the data, but with less certainty, relying on other information and chemical intuition to aid determination.
Generally speaking, each of the techniques discussed can require several grams of the material (with MS being the notable exception) to successfully determine the molecular form or structure. As mentioned before, a priori information and chemical intuition are extremely important in the process. The techniques above have relatively high detection limits, except for mass spectrometry. This implies that impurities, which are always present, usually do not contribute to the spectra and can be ignored.
Chemical Analysis of Mixtures
The discussion above focused on deriving the structure of pure compounds (which is the identification of the species). In the case of mixtures, "speciation" could take on a different meaning. It could merely imply identifying the presence of known forms within the mixture and their relative abundance. Or, it could imply isolation of unknown forms for a full or partial chemical characterization. In general, binary mixtures are simpler to handle than ternary mixtures and so on. Absolute amounts of the forms and their abundance ratios are additional important aspects of mixture determination. For instance, if you have a few picograms of material with five different components, the problem is very different than if you have a few grams with two or three components. Similarly, if you have a mixture of three components with two of them being of the "trace" variety, it is very different again from an ABC mixture composed of 20% A, 30% B, and 50% C. A definition of concentration ranges adapted from Davies (1980) is shown below:
When looking for trace or ultra-trace constituents, "uninteresting" major and minor components are generally referred to as the matrix in which the former are found. In addition, when looking at very small samples (<10–2 g), and/or small spatial dimensions (say <0.5 mm), one performs a microanalysis.
There are roughly three ways to proceed with mixture analyses. The first is to attempt to separate the components of the mixture in a straightforward fashion. This depends on what is known beforehand about the mixture. General methods of obtaining particular fractions of a mixture by physicochemical separations are well documented (see below). Separations could be by mineral type, particle size, density, magnetic properties, solubility, or volatility, and accomplished through sieving, filtration, density gradients, flotation, or the application of electric or magnetic fields, solvents, or heat.
The second general procedure for determination of mixtures is to subject it to some spectroscopic method that yields molecular identification as in the pure compound speciation section above. The result is a superposition of the spectra of the different components of the mixture. Depending on the complexity of the spectra of the individual components and the number of these components, this superposition can be relatively simple or exceedingly complex. The method is to attempt to iteratively determine the weights necessary to achieve the best fit of a superposition of pure component spectra (previously determined) to the complex spectrum of the mixture. This can be used both qualitatively and in quantitative analysis. Alternatively, techniques involving subtraction of the major component spectra may allow the appearance of the minor or trace component spectra.
The last procedure is the microanalysis and/or microprobe methods. The use of focused beams implies that small areas or volumes, and thus smaller amounts of the materials investigated can be sampled or viewed, either destructively or nondestructively. The ability to produce finer focused beams of electrons has steadily improved over the last few decades (now down around approximately 1 nm), while the focusing of beams of ions or atoms is now approaching the practical limits (approximately 0.02 µm on the University of Chicago's SIM–SIMS device). With the improvements in materials and the advent of synchrotron sources, focused X-ray beams are becoming a plausible reality (range 10 µm to 50 nm). Lastly, optical microscopes can produce UV, visible (often lasers), and IR beams in the range of 0.2 to 10 µm in diameter. This latter range of diameters is at the diffraction limit for these radiations but near-field optics utilizing very fine pointed optical fibers might enable this limit to be sidestepped, at least with monochromatic sources. In this arena, the host of spectroscopies developed and exploited with these beams allows speciation of mixtures at the microscale.
Physicochemical separation and spectroscopic methods are applicable to gases and liquids, while solids, particles, powders, colloids, and the like can be treated by them as well as microanalysis and/or microprobe methods. However, if desorption processes for solids in mass spectrometry could be understood well enough or perhaps designed to allow the emission of whole parent molecules or complexes it would be most advantageous. This method coupled with a high-resolution tandem mass spectrometer might provide the separation and sensitivity for detection and identification of species from a mixture of solid phase forms. This last possibility might well lie in the distant future, so for now the methods discussed above must suffice. Table 1 displays the various types of analyses one could perform, in terms of information and physical distribution, and the various spectroscopic and nonspectroscopic methods available to permit access to that information. Table 1 was compiled, augmented, and updated from a number of sources (Nicol, 1975; Winefordner, 1976; Natusch and Hopke, 1983; Christian and Callis, 1986; Kelley, 1987; Adams et al., 1988; Kiss, 1988; Willard, 1988; Eighmy et al., 1994; Bertsch and Hunter, 1996; Scheidegger and Sparks, 1996a).
Physicochemical Separation Methods
Individual particles in a typical environmental or industrial sample may range from submicrometer to millimeter dimensions, from clays and silts at the lower end (<50 µm) to sands and gravels. Total metal concentrations tend to vary with particle size; higher concentrations (µg g–1) being found in the smaller particles (Forstner and Wittmann, 1981; Salomons and Forstner, 1984) due to available surface area (m2 g–1).
The usual procedure employed for the physical separation of different size fractions involves sieving (for particles >20 µm in effective diameter), gravity sedimentation, and differential centrifugation (Rose et al., 1979). The size fractions thus obtained can then be analyzed directly, or subjected to sequential chemical extractions (Gibbs, 1977; Tessier et al., 1979; Forstner and Patchineelam, 1980). If chemical extractions are planned, the initial sieving should be performed on the fresh sample, that is, without any preliminary drying (see below).
An alternative procedure, as pioneered by prospectors panning for gold (Theobald, 1957), involves separation of the particles according to their density. Such methods, often involving the use of heavy organic liquids having specific gravities in the range 1.5 to 3.3, are used in the mineral processing industry and for characterizing clays, but environmental applications have been slow to appear. Warren and coworkers (Pilkington and Warren, 1979; Dossis and Warren, 1980) subjected several near-shore marine sediment samples to density gradient centrifugation, using mixtures of acetone and tetrabromoethane to give a series of liquids of specific gravities 2.2 to 2.75. The sediment components were separated into organic debris, conglomerates, quartz + calcite shells, magnesium calcite shells, aragonite shells, and heavy minerals. Cadmium, lead, and zinc were preferentially concentrated in the organic subfraction, but the Cd to Pb to Zn ratios were similar in each subfraction regardless of mineralogy. Galena (PbS), sphalerite (ZnS), hematite (Fe2O3) and magnetite (Fe3O4) were identified in various density subfractions by X-ray diffraction techniques.
A final approach, of more limited application, involves the separation of particles according to their magnetic properties. It is often combined with other separation methods, such as density and/or size fractionation, in a compound scheme. There have been many published reports of the application of this method to separate Fe oxides and chloritic clays from other constituents in soils (Schulze, 1981), before trace metal analysis of airborne particulates (Hopke et al., 1980; Hulett et al., 1980; Cabaniss and Linton, 1984), of roadside soils (Olson and Skogerboe, 1975) and urban dusts (Linton et al., 1980), and of sewage sludge and sludge amended soil (Essington and Mattigod, 1991), and in at least one report (Hulett et al., 1980) the magnetic particles were found to be enriched in the trace metals Cd, Co, Cr, Cu, Mn, Ni, and Zn.
Chemical Analysis of Surfaces, Sorbates, and Microsamples
A surface interface can be defined as a boundary physically separating two substances. These could be different solid phases of the same material, immiscible liquids (liquid–liquid interface), or interfaces between vacuum–solid, vacuum–liquid, liquid–solid, and gas–solid or gas–liquid mixtures. Although from a physical and macroscopic point of view, a surface is a boundary, at the atomic scale, a clear-cut surface is not easily discerned. There are many chemical processes going on at surfaces: sorption, desorption, evaporation, condensation, ionic exchange processes, dissolution and precipitation, inner and outer sphere complexation, diffusive migration, catalysis, etc. (Sparks, 1995; Trivedi and Axe, 2001). This section will refer to gas–solid or liquid–solid interfaces and the chemistry of sorbed species interacting at those interfaces. In particular we will be concentrating on the solid surfaces of soil particulates in contact with gases and liquids. Figure 2 shows the regimes of bulk, thin film, and surface analysis, in a rough conceptual schematic (Briggs and Seah, 1990).
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With the complexity of heterogeneous environmental systems and materials, the inability of any single technique to obtain all of the desired information or for it to be acquired simultaneously is an impossible fact that must be faced. Because of this, it is clear that a combination of techniques, each providing a piece to the puzzle, will be necessary for a clear picture. Direct answers to questions must give way to inference and intuition. Much use must be made of a priori information about what species are possible, through what is known about the chemistry of the element considered and the natural system into which it is introduced (Porter et al., 2004). Knowledge of the form in which the metal contaminant originally appeared (or into which it evolves before reaching its destination) is obviously an important piece of information. In addition, although some techniques lack sensitivity or do not provide direct molecular information, this does not preclude employment of them in speciation analysis.
A review by Brown (1990) and a book by Ferguson (1989) each present a lexicon of different methods that have been used in the physicochemical analysis of particulates, surfaces, and species sorbed on them. Table 2 is a lexicon that was compiled and augmented from these two sources (Ferguson, 1989; Hochella and White, 1990). Some of the methods are the same spectroscopies mentioned before or are enhanced versions of them. Others are recently developed techniques exploiting the interactions of photons, electrons, ions, atoms, and static fields with the surfaces and detecting particles emitted, scattered, and reflected from those surfaces or, alternatively to measure the loss in beam intensity due to absorption by the sample. Figure 3 shows schematically how many of the surface analysis techniques are designed around beams of the aforementioned probes.
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Since there are so many techniques, what will follow will not be an exhaustive representation of the possible techniques. For more details than is given below, an excellent introductory review of surface analysis is given by Kelley (1987), while exhaustive coverage can be found in Briggs and Seah (1990) and Kiss (1988). Both, however, are geared toward manufactured materials research, rather than geochemical or environmental applications; for the latter consult Hawthorne (1988), Brown (1990), Perry (1990), Bertsch and Hunter (1996), and Scheidegger and Sparks (1996a).
Special Method: Chromatography Coupled with Plasma Spectroscopic Techniques (e.g., ICP–MS, ICP–AAS or OES, MIP–MS, and Direct Solids Analysis MS)
Each of the techniques listed above uses plasma to vaporize and/or ionize a sample exposed to it or injected into it. This is not a surface technique, but is a useful speciation tool, especially for isotopic studies. An ICP–MS uses a quadruple mass analyzer for detection, while ICP–AAS and OES utilize atomic absorption or emission spectrophotometers as the detectors. Instead of an ICP, using Ar gas plasma, one can have a He microwave induced plasma (MIP) source. Due to the high temperatures of the plasma, all molecular information is destroyed, and thus only elemental and/or isotopic species can be determined. Excellent reviews of the application of ICP to examine metals from soil environments can be found in the literature (Bacon et al., 1997; Barefoot, 1998; de la Guardia and Garrigues, 1998; Yamasaki, 2000).
Each of these methods suffers from "spectral interferences" due to matrix effects (AAS–AES), or from the formation of molecular ions with oxygen or carrier gases (in MS). All require prior separation of complex mixtures, either via wet chemistry or a GC or LC system, on which they rely heavily for true speciation. Therefore, generally speaking, only volatile or solution complexes or species can be studied. In the case of solids, either laser ablation (LA) or a glow discharge (GD) plasma yield ions for direct mass analysis or for passage into a plasma torch. This avoids errors due to sample preparation (i.e., incomplete digestions, transfer losses, and improper dilutions). Both LA and GD suffer, however, from difficulties in quantification and reproducibility because of lack of standards and of course, permit only elemental and isotopic determination. Alternatively, using the isotope dilution technique, high precision elemental determinations to very low detection limits are possible (Denoyer et al., 1982; Lee and Kittrick, 1984; Nirel and Morel, 1990).
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TECHNIQUES USED IN SURFACE CHEMISTRY |
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TOPNOTESABSTRACTINTRODUCTIONCHEMICAL EXTRACTION METHODSSOLID PHASE SPECIATION:...TECHNIQUES USED IN SURFACE...SUMMARY, CONCLUSIONS, AND...REFERENCES |
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There are several drawbacks to MS techniques (Winefordner, 1976). The most obvious is that during removal of surface material, the technique destroys the sample surface. Systematic errors can at times be very serious. Learning to optimize instrumental conditions, to obtain good mass spectra, and to achieve quantitative analysis for a particular experiment may require some months to years of experience with an instrument. A complex mix of metal species on soil particles may prevent acquisition of an analyzable spectrum even for experienced personnel. Solids, in general, pose great difficulties for mass spectrometric analysis. Most of the MS techniques below require the samples to be under vacuum. Since gases, moisture, and other volatiles are part of a soil matrix, outgassing will be a limitation. Lastly, since the method is a destructive one, it does not yield information about how sorbed species are bound to the surfaces of interest.
Many groups develop new methods themselves by customizing commercial instruments or constructing them from scratch (Olson et al., 1992; Budde, 2001). Certainly, such options always exist and perhaps a combination of some of the methods below might provide the maximum amount of information needed for speciation in soil or other environmental matrices. Listed below are some of the MS methodologies used to analyze organic materials, solids, and their surfaces. A number have been applied to soils or their components.
Ionization Techniques
Secondary Ion Mass Spectroscopy (SIMS).
This technique utilizes ion beams created from gas (Ar) or liquid metal sources, such as gallium. It is, of course, destructive of the sample as it causes sputtering of ions from the surface examined (Becker and Dietze, 1998). Mass spectra reveal the presence of the elements liberated by the beam and are similar in appearance to the LAMMA spectrum (Fig. 4)
, discussed below. It has been used for polymers, large biomolecules, surfaces, and particulates. It has also been utilized in the study of surface predominance of elements in airborne particles and for observation of phosphate precipitation onto goethite surfaces (Keyser et al., 1978; Farmer and Linton, 1984; Grasserbauer, 1986; Martin et al., 1988; Willard, 1988). It has also been used to generate depth profile maps of leached vitrification glasses for nuclear waste disposal (Lodding et al., 1992). Additionally, SIMS has been employed to observe Pb and Zn adsorption on Al- and Fe-coated sand (Coston et al., 1995) and to examine heavy metal distribution in maize roots grown in highly contaminated greenhouse soils (Kaldorf et al., 1999).
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Ion Microprobe Mass Analyzer (IMMA).
This is the microprobe version of SIMS (0.2-µm beam diameter) with a sophisticated high-resolution two stage mass analyzer (Ferrer and Thurman, 2003). Can be used for study of surfaces, particulates, and thin sections (Willard, 1988).
Laser Microprobe Mass Analyzer (LAMMA).
This technique gives molecular ions at low laser intensity and fragments at higher intensity. The sputtered ions are drawn into a time of flight (TOF) or Fourier transform (FT) mass analyzer. It has been applied to organics, organometallics, soil particles and thin soil sections. Figure 4 depicts LAMMA mass spectra of two different points in a thin soil section (Henstra et al., 1981), showing a different elemental composition at the two points. It is also referred to as surface analysis by laser ionization (SALI) (Nitsche et al., 1978; Henstra et al., 1981; Muller et al., 1981; Denoyer et al., 1982; Hercules et al., 1982).
Californium-252 Plasma Desorption (PD).
In this technique, a sample coated onto a thin foil is exposed to a Cf nucleus fission fragment which forms microplasma, sputtering molecular ions from the surface. Its utility for soil particles would be very limited, if at all. It could be useful for organic and organometallics that could be isolated from soils (Willard, 1988).
Field Desorption (FD).
This is a method in which a sample is placed on an electrode, heated, and negatively charged, from which the molecular ions are desorbed. Samples are usually solutions applied to a carbon whisker electrode from which the solvent has been evaporated. It can be applied to solids if they can be fashioned into fine point ("whiskered") (Lehmann and Schulten, 1977; Schulten, 1979, 1982; Grasserbauer, 1986; Willard, 1988).
Thermal Desorption (TD).
In TD, samples are heated on a tungsten filament or in a sample cup to 2300 K. Species boiled off of the surface have a probability to be ionized; other techniques utilize an electron impact ionizer. This could be useful for soil sample surface studies or isotope ratio studies (Willard, 1988; Bacon et al., 1992). It has been used to determine organic matter and metal carbonate contents of soils (Gaal et al., 1994) and to distinguish between organomercurials and metallic Hg in contaminated sites (Hempel et al., 1995).
Specialized Mass Analyzer and Separation Techniques
Mass Spectrometer–Mass Spectrometer (MS–MS).
This is also referred to as a tandem mass spectrometer (TMS). This utilizes two mass spectrometers (say MS1 and MS2) with different mass range, mass resolution, and ionization technique. MS1 might have a low ionization, high mass resolution, and large mass range capability. This gives large numbers of whole molecular ions viewed over a very broad mass range. Then, in one particular mode of study, a single ion spectral line from MS1 will be passed to MS2. MS2 will have a high ionization, low mass resolution, smaller mass range capability. This will be utilized to elucidate the structure of the (fixed mass) molecular ion line generated by MS1 using an ion fragmentation pattern. This technique coupled with prior chromatographic separations and/or proper ionization of solids could be an extremely powerful tool for a speciation project. It offers the separating ability needed for very complex mixtures. It has been used in the study of large biomolecules (Bernhard et al., 1986; Willard, 1988), explosives (Casetta and Garofolo, 1994), and chemical warfare components (Black et al., 1994).
Electron Techniques: Imaging, Spectroscopy and Diffraction
Electrons behave as both particles and waves. These aspects of electron behavior have allowed the development of methods for obtaining a great deal of information from solids, surfaces, and particulates. This is accomplished by looking at the scattering or diffraction of the electron beams (SEM, TEM, EELS, ED) and X-rays or electrons produced by excitation of atoms (SEM–EDX or WDX, EPXMA, AES, XPS) for both imaging and spectroscopy. Figure 5
depicts the many forms of radiation and other effects that occur when an electron beam impinges on a solid material. It also shows the atomic processes involved in the production of these information signals. The ability to focus electron beams to very small spot size (approximately 20 Å) and to raster them, as well as their inherently short wavelength (approximately 0.1 Å) yields excellent spatial resolution for elemental mapping and for topographical imaging of surface variation and structure. Electron spectroscopies give us much more information about matter on surfaces or at the submicron scale. EELS yields molecular information from primary electron energy losses to atoms and molecules, while AES and XPS do so via shifts in energies of electrons ejected from atoms bound in molecular associations, relative to the energies expected from those same atoms in a free state. Note that although XPS is an electron spectroscopy, it can only be explored through X-ray excitation and is discussed in a subsequent section. Electron diffraction is yet one more way to identify and to characterize microchemical species through their crystal structure, if crystalline.
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(2.307 keV) and the Pb M lines (2.393 keV) are within the energy resolution of the EDX detectors (approximately 0.15 keV), thus they are often difficult to separate or identify. This method has been used in petrology (Welton, 1984) and to study sediment and suspended trace metal contaminants in rivers, bays, and marine sites (Ramamoorthy and Massalski, 1978; Jedwab, 1979; Luther et al., 1980; Norrish et al., 1986), aquatic colloids (Leppard, 1992), sewage sludge and sludge-amended soil, solid metal phases (Essington and Mattigod, 1985, 1991), airborne particulates and fly ash (Linton et al., 1976; Keyser et al., 1978; Farmer and Linton, 1984; Hansen et al., 1984), clay minerals (Beutelspacher and Van Der Marel, 1968; Jaynes and Bigham, 1986), Fe and Al sesquioxide coatings on mineral–soil particle surfaces (Hendershot and Lavkulich, 1983), humic matter (Tan, 1985), Pb (and other metals) in house and urban dusts (Linton et al., 1980; Hunt et al., 1992) and in soils (Smart and Tovey, 1982; Whalley, 1985; Bain et al., 1986; Mattigod et al., 1986; Rybicka et al., 1994; Yarlagadda et al., 1995; Adamo et al., 1996; Garcia-Rizo et al., 1999; Welter et al., 1999), metals in solidified matrices (Neuwirth et al., 1989; McWhinney et al., 1990; Cocke et al., 1992; Roy et al., 1992; Cotter-Howells and Caporn, 1996), ferric oxides (Landa and Gast, 1973), and as a tool to determine trace metals by electrodeposition (Chong et al., 1990). The technique is limited by low sensitivity with EDX, but can be improved by using wavelength dispersive X-ray analysis (WDX). Although WDX increases sensitivity, it can add greatly to the time required for the X-ray analysis (Kiss, 1988). Another limitation is spatial resolution of the elemental mapping for this and EPXMA. This is due to the scattering of primary and secondary electrons deep within the material yielding X-rays from a large fixed volume about 3 µm across, irrespective of the beam spot size (Fig. 7). Most samples must be dried and coated with a conductive material and placed in a high vacuum, which is a limitation for most charged beam techniques. However, resolution continues to improve, and recent designs have given genesis to environmental SEM. ESEM is able to tolerate the presence of up to 10 torr of gases in the sample chamber. They use large differential pumping, place samples very close to the beam and secondary electron detector, and lastly, take advantage of the gas to both eliminate sample charging (thus coating) and to act as a preamplifier. These considerations permit 50 Å resolution at 10 torr and virtual in situ analyses of moist or unprepared samples (no drying or coating).
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Transmission Electron Microscopy and Scanning TEM (TEM–STEM)
This is an electron microscopy that is analogous to optical microscopy in which illumination is from below (transmission). That is, the imaging optics (magnetic coils) is on the opposite side of the sample from the beam source. The sample, in general, is milled or sectioned to provide electron transparent foils for TEM analysis. If the sample is particulate or a powder, a number of methods have been put forth in the literature to prepare them for TEM. The particles could be embedded in an epoxy before ultramicrotomy, or ground and polished on diamond lap polishers. Other methods call for a suspension of them to be deposited onto a thin film (graphite or colloidion), or centrifuged and fixed onto a grid with a water-soluble melamine resin. Several references (Spurr, 1969; Barber, 1971; Ogura, 1981; Bachhuber and Frosch, 1983; Kushida et al., 1983; Nomizu and Mizuike, 1986; Nomizu et al., 1987, 1988; Frosch and Westphal, 1989; Perret et al., 1991) describe the details. The lower the accelerating voltage of the primary beam, the thinner the samples must be to achieve transparency. Selected area electron diffraction (SAED) is usually available on TEM–STEM units, EDX is easy to add, and EELS is another important tool to use with TEM for gathering microchemical information. TEM–EDX has both advantages and drawbacks with respect to SEM–EDX analyses. One advantage is that the resolution for X-ray mapping is significantly better for TEM than SEM due to the simple fact that the volume excited is much smaller (from the thinness of sections viewed in TEM, Fig. 8)
. TEM also has much greater spatial resolution for imaging, with 2 to 5 Å features often visible. A drawback is that with the smaller volume fewer X-rays are produced and detected. Therefore, the very small volume examined is a drawback (statistically) relative to the enhancement of resolution. The question that arises is, What can be inferred about macroscopic samples in bulk, much less the site from which they were gathered, from the tiny amount of each sample that is probed? Some of the drawbacks with respect to SEM can be lessened with the use of a STEM. STEMs combine the best of both SEM and TEM, in that each mode of operation is possible with these instruments. This technique has been used in conjunction with the microchemical tools mentioned above to study characterization of ultra-small particles (Iijima, 1985) and iron-rich clay minerals (Ghabru et al., 1990), crystallinity of ferric oxides (Landa and Gast, 1973), precipitation of phosphate on goethite (Martin et al., 1988), effects of tannic acid on Cu and Zn retention on Al oxides (Goh et al., 1986), nanometer size Au inclusions in pyrite grains (Bakken et al., 1989), characterization of submicron aquatic colloids (Nomizu and Mizuike, 1986; Nomizu et al., 1987, 1988; Perret et al., 1991; Leppard, 1992), study of bacteria as nucleation sites for inorganic minerals in lake sediments (Ferris et al., 1987
