Ion microprobe mass spectrometry

The electron probe microanalyzer (EM or EPMA) nses a beam of high-energy electrons to bombard the surface of a solid sample. This results, as we have already seen, in the removal of an inner shell electron. As discussed in Section 14.2, this can result in the ejection of a photoelectron (the basis of XPS) and the emission of an X-ray photon. The X-ray photons emitted have wavelengths characteristic of the elements present. The EPMA uses either a wavelength dispersive (WD) or energy dispersive (ED) X-ray spectrometer to detect and identify the emitted X-rays. This is very much analogous to XRF spectrometry (Chapter 8), where the primary beam was of X-rays, not electrons. This instrument was discussed at the end of Chapter 8, which should be reviewed if necessary. 

The instrumentation for EM uses the same type of X-ray spectrometers discussed in detail in Chapter 8, with an electron beam as the source and a UHV system that includes the sample compartment. An ED X-ray spectrometer allows the simultaneous collection and display of the X-ray spectrum of all elements from boron to uranium. The ED spectrometer is used for rapid qualitative survey scans of sample surfaces. The WD spectrometer has much better resolution and is used for quantitative analysis of elements. The WD spectrometer is usually equipped with several diffracting crystals to optimize resolution and to cover the entire spectral range. The electron beam, sample stage, spectrometer, data collection, and processing are all under computer control. 

1 The wavelength of XRF radiation of an element is virtually independent of the chemical form of the element, but in ESCA, the photoelectron energy is not independent of the chemical environment of the element. Explain. 

2 Why is an electron flood gun used in ESCA, Auger, and other surface analysis instruments  

3 ESCA and Auger instruments are useful for the analysis of elements present in concentrations greater than 1% although the absolute sensitivity of the method is about 10 g. Explain. 

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Linton, R. W., Williams, P., Evans, C. A. Natusch, D. R. S. 1977. Determination of the surface predominance of toxic trace elements in airborne particles by ion microprobe mass spectrometry and Auger electron spectroscopy. Analytical Chemistry, 49, 1514-1521. 

Fig. 2. Carbon isotopic values (with error bars) of individual Precambrian microfossils from single stratigraphic horizons of three geologic units measured by ion microprobe mass spectrometry compared with those of the carbonate and total organic carbon measured in bulk samples of the same geologic units. Values plotted for carbonate and total organic carbon are from Strauss and Moore (1992) for microfossils from the Bitter Springs and Gunflint Formations, from House et al (2000) and those for microfossils from the Dresser Formation, from Ueno et al. (2001).

Bakale, D.K., Colby, B.N., Evans, C.A. (1975) High mass resolution ion microprobe mass spectrometry to complex matrices. Ana/. Chem., 47,1532-1536. 

The isotopic anomalies detected in the iron peak elements are extremely small (Figure 4), since the anomalous material is diluted with material with a terrestrial isotopic composition. This contamination results from the fact that the meteorite inclusion is taken into solution before chemically extracting the relevant element in a form suitable for conventional TIMS analysis. However, ion microprobe mass spectrometry can be used to analyse small meteoritic inclusions in situ without the need of chemical processing. This enables single inclusions to be analysed for a variety of elements, whilst maintaining the petrographic context of the sample. The carbonaceous chondrites Murchison and Murray also contain refractory inclusions such as corundum and hibonite, but they are invariably small and difficult... 

Ireland TR (1995) Ion microprobe mass spectrometry. In Hyman M and Rowe M (eds) Techniques and Applications in Cosmochemistry, Geochemistry and Geochronology in Advances in Analytical Chemistry, Volume II, pp 51-118. Greenwich (UK) JAI Press. Krankowsky D and Eberhardt P (1990). In Mason JW and Horwood E (eds) Comet Halley Investigations, Results and Interpretations, Volume 1, pp. 273-296. New York Simon and Schuster. 

J. A. McHugh and J. F. Stevens, Elemental analysis of single micrometer-size airborne particulates by ion microprobe mass spectrometry. Anal. Chem. 44, 2187-2192 (1972). 

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