Analytical methods in geochemistry

Neutron activation analysis is a sensitive and versatile method of rock analysis, chiefly applicable to trace elements and capable of determining a large number of elements simultaneously without necessarily destroying the sample. There are two approaches. Instrumental neutron activation analysis (INAA) employs a powdered rock or mineral sample radiochemical neutron activation analysis (RNAA) involves the chemical separation of selected elements. The range of elements analysed is given in Table 1.5 and the methods are described in detail by Muecke (1980). 

When elemental concentrations are below about 2 ppm, a chemical separation may be employed foUowing die irradiadon of the sample, but prior to counting. This approach, known as radiochemical neutron acdvadon analysis (RNAA), clearly has the advwtage of increased senadvity. 

AAS cannot compete with more rapid methods of silicate analysis such as XRF and ICP. Nevertheless, because AAS is comparatively cheap both in the capita quday and in running costs, it most frequently finds its use in one of three specific applications. 

Isotope dilution mass spectrometry is the most accurate and most sensitive of all trace element analytical techniques and is particularly suited to measuring very low concentrations. The method is described in some detail by Henderson and Pankhurst (1984) and depends upon the addition of an isotopic tracer or spike to the sample. The spike contains a known concentration of a particular element whose isotopic composition is also known. If a known amount of spike and a known amount of sample are mixed, and the isotope ratio of the mixture determined, the concentration of the element in the sample can be calculated. 

The method is particularly useful in determinmg the abundances of REE at low concentrations, although four of the REE (Pr, Tb, Ho and Tm) are mono-isotopic and cannot be analysed by this method. The main disadvantage is that even with automated mass spectrometry the method is time-consuming and expensive and so is normally reserved for measurements which can be used to calibrate other more rapid methods. 

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In this chapter we discuss improvements documented in the literature over the past decade in these areas and others. Chemical procedures, decay-counting spectroscopy, and mass spectrometric techniques published prior to 1992 were previously discussed by Lally (1992), Ivanovich and Murray (1992), and Chen et al. (1992). Because ICPMS methods were not discussed in preceding reviews and have become more commonly used in the past decade, we also include some theoretical discussion of ICPMS techniques and their variants. We also primarily focus our discussion of analytical developments on the longer-lived isotopes of uranium, thorium, protactinium, and radium in the uranium and thorium decay series, as these have been more widely applied in geochemistry and geochronology. 

Smith, D.B., Woodruff, L.G., O Leary, R.M., Cannon, W.F., Garrett, R.G., Kilburn, J.E., Goldhaber, M. B. 2009. Pilot studies for the North American Soil Geochemical Landscapes Project - site selection, sampling protocols, analytical methods, and quality control. Applied Geochemistry, in press. 

Application of analytical techniques from molecular geochemistry can be used to study reactions at the molecular level. Such studies can elucidate the partitioning and interactions of contaminant species in aqueous, solid, and gas phases. While spectroscopic methods provide information on chanical reactions on the contaminant-solid interface, other techniques may provide additional spatial information at an atomic level. In an extensive review on molecular geochemistry. O Day (1999) sununarizes common analytical methods (Table 5.2) and discusses their benefits in understanding contaminant-solid interactions at the molecular level. 

Before describing the Archaeometry Lab at MURR s involvement in research on obsidian sources and artifacts from South America, some background information on obsidian geochemistry is helpful. In addition, a description of advantages and disadvantages of various analytical methods employed to characterize obsidian is presented. 

In conjunction with monitoring herbicide and degradation product occurrence, USGS research during the past decade has emphasized the development of methods for the analysis of triazine herbicides and their degradation products. To research and develop analytical methods for pesticides, a laboratory for organic geochemistry was established by... 

Albarede, F. Beard, B. 2004. Analytical methods for non-traditional isotopes. Reviews in Mineralogy Geochemistry, 55, 113-152. 

Barter, S.R. and Horsfield, B., 1993. Determination of structural components of kerogens by the use of analytical pyrolysis methods. In Engel, M.H. and Macko, S.A. (eds). Organic geochemistry. Principles and applications. Plenum Press, NY, pp. 271-287. 

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