geochemical analysis

geochemical analysis Geochemistry is the study of the composition of geological materials and the behaviour of individual elements during geological processes. Geochemical analysis is now a vital tool in most fields of geological and environmental research. It is used, for example, in studies of water, soil, and air quality, of formation of rocks and minerals, of fossilization mechanisms, of metal accumulation in organisms from contaminated water and soil, and in determining the suitability of materials for civil engineering purposes. Some of the most commonly available geochemical techniques are described below.

Electron probe microanalysis (EPMA)

EPMA is a non-destructive technique for the analysis of polished and carbon-coated specimens of minerals, glasses, and synthetic materials. It can also be used to give an indication of the elements present in organic or unpolished or uncoated materials. Analyses can be made at individual points (usually 1 to 40 μm (micrometres) in diameter) or over small areas (usually less than 1 mm²) of the sample surface to produce geochemical maps. Almost every element of the periodic table can be determined, with the exception of those with low atomic numbers, mainly hydrogen, lithium, and beryllium, and for many instruments boron, carbon, nitrogen, and oxygen. Specimens are introduced into the sample vacuum chamber and viewed under high magnification to select areas of interest. A beam of electrons is fired at the surface of the sample, producing X-rays with energies and wavelengths specific to the elements present. The instrument is calibrated by analysing standard samples with known compositions. Among the most useful applications of EPMA is in the study of subtle chemical zoning in minerals by element mapping. Zoning of this kind, which is often invisible using light microscopy, can yield information on crystallization conditions and changes in the chemistry of a magma or hydrothermal fluid.

X-ray fluorescence spectrometry (XRF)

XRF is a routine technique for the determination of major elements and many trace elements in rocks and minerals, at concentrations from 1 or 2 ppm (parts per million) to 100 per cent. Samples are usually prepared as glass discs for major element analyses, by fusing the sample powder with a known proportion of a commercially available flux, or as pressed powder pellets for trace-element analyses, made by mixing the sample powder with a binding agent, then pressing the mixture into a compact disc with a smooth upper surface. The sample surface is irradiated with primary X-rays, producing secondary X-rays with energies and wavelengths characteristic of the elements present. The concentration of the elements is determined by comparing the intensity of the various energy or wavelength peaks with those produced by standard samples of known composition. One of the most common uses of XRF is in the geochemical analysis of suites of igneous, metamorphic, and sedimentary rocks in studies of crustal and mantle evolution.

Inductively coupled plasma–atomic emission spectrometry (ICP–AES)

ICP–AES is used for the rapid measurement of major elements and a wide variety of trace elements in waters and sample solutions. Solid materials are powdered and homogenized, and a known amount is dissolved, using a combination of high-purity acids, and diluted to produce a sample solution. This is nebulized (sprayed) into a high-temperature argon plasma, where it is vaporized and ionized to yield radiation characteristic of the elements contained in the sample and with intensities proportional to their concentration. The intensity of the signal is converted into element concentration by comparison with a calibration curve. Detection limits are often less than 0.005 wt % (weight per cent) in the sample. Almost all elements of the periodic table can be measured, although for some elements, or those at low concentrations, other techniques may be more appropriate. Some of the most useful applications of ICP–AES are in the analysis of natural waters for transition metals or other potentially contaminating elements, not ideally determined by ICP–MS (see below).

Inductively coupled plasma–mass spectrometry (ICP–MS)

ICP–MS is used for the rapid, quantitative determination of a variety of elements at low concentrations. As for ICP-AES, sample solutions are introduced into an argon plasma, although ions are measured rather than photons. For the analysis of small volumes of solid materials, it is also possible to use a high-intensity laser beam to ablate holes 10 to 100 μm diameter (by variable depth) in the surface of a polished sample. For the analysis of solutions, detection limits are very low (below ppb (parts per billion) in solution) but are higher for laser ablation techniques (between 10 ppb and 1 ppm in the solid). Most elements in the periodic table can be determined although the best results are obtained for elements with an atomic mass greater than 80, and the method is not generally appropriate for major elements. Examples of the use of ICP–MS are the determination of rare-earth elements in magmatic rocks for studies of their origin and evolution and the analysis of road dusts, vegetation, waters, soils, and animal tissues for potentially harmful elements such as lead.

The interpretation of geochemical data

Data for major elements are generally reported in the form of oxides as weight per cent (e.g. silica, SiO₂ wt. per cent) and trace elements in parts per million (ppm) or micrograms per gram (μg g⁻¹). An important consideration in the interpretation of data is analytical uncertainty, particularly when evaluating the existence or significance of small variations in composition. Some of the most common sources of uncertainty are sample contamination, incomplete sample digestion (some minerals are extremely resistant to acids), interferences between elements (for example in EPMA, where X-ray energies for two elements may overlap), and poor instrument calibration. The extent of these problems is best limited at the time of analysis by taking necessary precautions and through discussion with experienced laboratory staff. They are, however, impossible to eliminate completely and an indication of analytical uncertainty should therefore be reported with the data, ideally as ‘error bars’ on geochemical diagrams. Uncertainties can be estimated from the regular analysis of reference materials of known composition and from the analysis of two or more samples. To help in interpretation, data are often imported into graphing and statistical computer programmes. A wide variety of diagrams can be produced to compare the data with those from previous studies and to demonstrate trends or new interpretations.

Ben J. Williamson and and Tim P. Jones

Bibliography

Gill, R. (1997) Modern analytical geochemistry: an introduction to quantitative chemical analysis techniques for Earth, environmental and materials scientists. Longman, Harlow.
Rollinson, H. (1993) Using geochemical data: evaluation, presentation, interpretation. Longman, Harlow.