Several X-ray spectroscopies are of interest to geochemistry and environmental soil science.  In the Environmental and Soil Geochemistry group at Dartmouth, we principally use three of these techniques, X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and synchrotron X-ray diffraction (S-XRD).  The information each of these techniques is unique, but all of them use high energy X-ray sources to identify and  characterize a wide variety of geological and environmental materials.  Each of these techniques also takes advantage of the intense and continuously tunable X-rays produced at synchrotrons, large particle accelerators designed to produce a wide spectrum of light energies. 

A Brief Overview of X-ray Absorption Spectroscopy

X-ray absorption spectroscopy is an element-specific probe of the local structure (short range) of elements in a sample.  Interpretation of XAS spectra commonly uses standards with known structures, but can also be accomplished using theory to derive the structure of a material.  In either case, the species of the material is determined based on its unique local structure.  An important advantage of this technique is its utility for heterogeneous sample, a wide variety of solid and liquids, including whole soils and liquids, can all be examined directly and non-destructively.  Additionally, Since the local structure does not depend on long-range crystalline order, the structure of amorphous phases (and that of dissolved species) is easily achieved.

XAS is useful for concentrations from about 10 ppm to major elements.  As such, it is useful to speciate trace elements such as contaminants adsorbed to pure minerals,  soils and sediments, and is also a valuable tool for studying the mineralogical composition of the soil or sediment (especially when used in conjunction with other techniques such as X-ray diffraction).

X-ray absorption spectroscopy results form the absorption of a high energy X-ray by an atom in a sample.  This absorption occurs at a defined energy corresponding to the binding energy of the electron in the material.  The ejected electron interacts with the surrounding atoms to produce the spectrum that is observed.  Occasionally, the electron can be excited into vacant bound electronic states (unoccupied molecular orbitals) near the valence band.  As a result, distinct absorptions will result at these energies.  Often these features are diagnostic of coordination and are of use for geochemistry.   For example, the toxic chromate anion is tetrahedral and as a result has a large absorption feature just below the absorption edge (the so-called pre-edge) that is not present in the more benign Cr(III).  As a result, the presence of this feature is diagnostic for the more toxic form of chromium.

Since the electron excited is usually the 1s or 2p electrons, these energies are usually quite high (thousands of electron volts), this technique demands high energy (and tunable) X-ray excitation.  As a result, it is done at synchrotron radiation facilities.  There are 4 major sychrotron facilities in the US, and others in Europe, Asia and elsewhere.  These facilitie provide a bright source that is required for these experiments to be useful at the concentrations needed for geochemical applications. 

X-ray absorption spectroscopy is commonly divided into two spectral regions, the first is the X-ray absorption near edge structure (XANES) spectral region.  XANES spectra are unique to the oxidation state and speciation of the element of interest, and consequently is often used as a methods to determine the oxidation state and coordination environment of materials.  XANES spectra are commonly compared to standards to determine which species are present in an unknown sample.  Once species are identified, Their relative abundance is quantified using linear-combination fitting (or other curve-fitting algorithms) using XANES standards to reconstruct the experimental data.  It is important to note that XANES is sensitive to bonding environment as well as oxidation state.  Consequently, XANES is capable of discriminating species of similar formal oxidation state but different coordination.  For example, Arsenic(III) oxide is easily differentiated from Arsenic(III) sulfide, and octahedral Mo(VI) can be differentiated from tetrahedral Mo(VI).

The more distant region of the X-ray absorption spectrum is termed the extended X-ray absorption fine structure (EXAFS) region.  EXAFS spectra are best described as a series of periodic sine waves that decay in intensity as the incident energy increases from the absorption edge.  These sine waves result from the interaction of the elected photoelectron with the surrounding atomic environment.  As such, their amplitude and phase depend on the local structure of excited atom.  Since this interaction is well understood, theory is sufficiently advanced that the local structure of the excited atom can be determined by matching a theoretical spectrum to the experimental spectrum.   This fitting yields many types of information, including the identity of neighboring atoms, their distance from the excited atom, the number of atoms in the shell, and the degree of disorder in the particular atomic shell (as expressed by the Debye-Waller factor).  These distances and coordination numbers are diagnostic of a specific mineral or adsorbate-mineral interaction; consequently, the data are useful to identify and quantify major mineral phases, adsorption complexes, and crystallinity.  Linear combination of EXAFS spectra using standards is also commonly used for quantitation for samples containing many species since it is quite difficult in practice to separate many species into their component shells.

back to top

A Brief Overview of X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy also uses X-rays to excite electrons from molecular orbitals into the continuum.  However, conventional XPS does not measure absorption while scanning through the absorption edge as is done for XAS.  Rather, XPS is conducted by by using a fixed energy source to excite electrons from the sample and then measuring their kinetic energy.  Since their kinetic energy is dependent on their binding energy (and, strictly speaking, a correction factor called the work function), different chemical species can be identified based on their distinct binding energies.  These experiments are commonly carried out with Al or Mg anodes in the laboratory; these metals release X-rays when a high voltage is applied that is monochromatic and useful for XPS.  However, synchrotron-based XPS (S-XPS) offers many benefits.  Most importantly, since the excitation energy is variable, the kinetic energy of photons can be varied continuously.  This is useful because the distance that an electron can travel though the solid (called the escape depth) varies with kinetic energy.  Synchrotron-based XPS permits the kinetic energy for a given orbital to be tuned to a value (typically about 50-100 eV) where the escape depth reaches a minimum (about 1 monolayer or a few Ångstroms).  Consequently, S-XPS is highly sensitive for surfaces.  The incident energy can then be tuned to determine the thickness of any surface layers.  Synchrotron-based XPS has other advantages over other methods; it affords superior resolution to conventional instruments, permitting the differentiation of similar chemical species.  S-XPS also is useful for light elements (those with atomic number of Ca and lower), because they are difficult to analyze by XAS since their X-rays are too low in energy to pass though air as well.  Therefore, elements such as Al, Si, N, and C usually are analyzed by XPS.  Fortunately, complementary XAS information can usually be obtained for these light elements concurrently to XPS in synchrotron-based XPS systems. 

It should be mentioned that there are limitations associated with this technique because it detects electrons.  First, since electrons do not pass through air, these experiments must be run in vacuum.  This also means that samples must be dried (water evaporates in a vacuum); this drying may impact the system or result in many chemical transformations.  Also, since this technique is highly surface sensitive, it is less useful to analyze bulk properties; in fact, it may make it impossible to identify anything but interferences on a dirty-surface.  Consequently, XPS is often done with clean, freshly-cleaved single crystals.

back to top

A Brief Overview of Synchrotron-based X-ray Diffraction

Synchrotron X-ray Diffraction (S-XRD) is similar in design to conventional XRD.  In it, X-rays are diffracted through a crystalline material, revealing the interatomic spacing, atomic identities, and positions of atoms within the crystal.  Diffraction is the most fundamental means of determining structure of minerals and other even proteins, and it is especially useful to characterize environmental samples.  Conventionally, XRD has several limitations.  A key disadvantage of XRD is that it is limited to crystalline materials (since amorphous materials do not diffract).   XRD also is time-consuming and uses a large volume of sample.  Fortunately,  synchrotron-based XRD is useful to circumvent these limitations.  They offer exceptional resolution, even on very small samples containing only a few grains of a particular mineral.  This resolution and the excellent detectors for S-XRD permit the identification and quantification of trace phases not possible using other means.  Amorphous materials and thin films can also be analyzed thanks in part to the instrumental configuration of these instruments. Finally, S-XRD can be collected over a large range of angles, impossible with conventional instruments and invaluable when performing Rietveld refinements.  Rietveld refinements are used to determine the unit cell parameters and site occupancy for a powder XRD pattern given a crystal lattice.  Such methods have proven invaluable for the study of things ranging from locating waters of hydration in zeolite, to arsenate substitution in sulfate minerals.