E Laser Microprobe Mass Analysis

Laser microprobe mass analyzers permit mass spectrometric analysis of very small volumes (0.01-1 pm3) of thin Sections. The method is based on laser induced ion production from a microvolume and analysis of the evaporated ions in a time-of-flight mass-spectrometer. The technique allows detection of all elements and isotopes with a sensitivity approaching the ppm range and an extremely low limit of detection 10 15 to 10-20 g. Transmission type instruments such as the LAMMA 500 are designed for the analysis of particles of 3 pm in diam. The lateral resolution is about 0.5-1 pm. Because the area to be analyzed is selected by an optical microscope, distribution of chemical constituents can be precisely correlated with morphologic structures (Hillenkamp et al., 1982 39), Simons, 198440), Kaufmann, 1984)41 . 

Thin Sections or pieces of the rock about 1 mm in size or less can be also analyzed with this instrument at glancing incident irradiation, if the site to be analyzed is close to the surface of the section or the edge of a fraction. However in this mode of operation, mass resolution is severely reduced. Nevertheless ions can be identified because of their low mass numbers and the very low background in the spectra, by known cluster patterns or by comparison with reference spectra obtained with reduced laser irradiation. The spectra have shown to be highly reproducible. For more precise bulk. sample analysis, other instruments such as the LAMMA 1000 are more suited. Their lateral resolution is 1-3 pm in diameter and the depth of analysis is typically 0.1 pm. 

Fossil organic particles in situ usually release positive ions at masses 23,24, 39, 41, 54, 56 thus indicating Na, Mg, K and Fe. But other signals also occur (Fig. 19). The negative ion mass spectra are dominated by carbon clusters with zero, one or two hydrogen atoms attached. They resemble spectra obtained from polymer foils (Gardella et al., 1980)42 rather than those from coals and carbon films (Fiirstenau et al., 1979)43). Consequently, the material contains more long hydrocarbon chains rather than aromatic constituents. Peaks at m/e = 79 (benzylium) and 90 (tropylium) indicate aromatic constituents. Unspecific ions like CN, CNO, and Cl are commonly present (Fig. 11). 

In the fossil materials studied, the CnHm clusters exhibit high intensities around n = 3, 4, 5 or 6 and then decline in intensity (Fig. 11). The highest negative mass ion detected was at 156 m/e. Small peaks produced at higher laser energies, could result from rearrangements during the ionization process, when a laser 

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Verbueken, A. H. Bruynseels,E J. Vangrieken, R.E. Laser microprobe mass analysis—A review of applications in the life sciences. Biomed. Mass Spectrom. 1985, 12, 438-463. 

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