Editors' Column
Analysis of Micrometer-Sized Samples Microanalysis is currently an active area of research, with reports of new applications of neutron activation analysis and of the laser-Raman mi croprobe, among other developments. R. Ganapathy of J. T. Baker Chemi cal Co. and D. E. Brownlee of the Uni versity of Washington (Science, 1979, 206, 1075-77) report the first use of highly sensitive neutron activation analysis techniques to determine trace element abundances of individual micrometeorites. Micrometeorites are micrometer-sized interplanetary dust particles that enter the earth's atmo sphere. They are collected by U-2 air craft and are being studied because many of them represent comet parti cles. T h e two particles analyzed by the researchers were only 70 X 35 and 42 X 28 μηι. Abundances of trace ele ments in the two particles had to be reported in mass units rather than as concentrations because of the diffi culty in accurately determining the weight of the micrometeorites them selves. At the low end, 34 femtograms of Ir were found in one of the parti cles. Jerrold L. Abraham of the Universi ty of California at San Diego and Edgar S. Etz of the National Bureau of Standards, {Science, 1979,206, 716-18), describe the in situ micro analysis of a biological sample with the laser-Raman microprobe. T h e mi croprobe, according to the authors, "offers exciting new prospects for bio logical studies by providing nonde structive compound-specific molecu lar microanalysis with good spatial resolution and high sensitivity to prin cipal molecular components." They point out that a major weakness of current techniques involving electron, proton, and ion beams with X-ray or secondary ion analysis has been their general limitation to inorganic and el emental rather than organic and com pound identification. The microprobe is basically a con ventional Raman spectrometer opti mized to permit the acquisition of spectra from single microparticles or samples of micrometer dimensions. In
practice, the light beam from a laser is focused to a small spot on the sample, typically 2-20 μηι in diameter, and the scattered radiation is detected by a cooled photomultiplier tube and pro cessed by photon counting electronics. For measurements to be successful, the sample cannot be highly absorbing at the exciting laser wavelength, or it will burn up under the intense beam, which may reach several thousand watts per square centimeter in power density. T h e investigators have found, however, that many sections of tissue
are stable enough under the beam to yield satisfactory Raman spectra. Abraham and Etz point out that in situ microanalysis is much more valu able than bulk analysis, which de stroys topographic relationships. "We believe," they write, " t h a t this laserRaman microprobe system, by extend ing microanalytical techniques from the elemental to the molecular level, has a great potential for use in many applications in pathology, toxicology, forensics, and environmental studies." Stuart A. Borman
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980 · 295 A
