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Into the depths
(UBS). Few papers have reported quantitative depth-profile data from LIBS. The researchers used a pulsed Nd:YAG laser that wasfiredrepeatedly at a single site to ablate the sample. The laser shots were monitored by a gated chargecoupled device detector to yield data of emission wavelengths versus time. The laser shots were fired at electrolytically deposited brass samples that were analyzed for chromium, nickel, copper, and zinc. .n addition, zinc-coated steel was studied. The scientists report ablation depths of 6.5 ng per pulse, which imply absolute detection limits on the order of femtograms per pulse for an element present in ppm concentrations. In addition LIBS was able to pinpoint the location of the Zn-Fe interface in the steel sample However, differences in the ablation depth caused by matrix effects are visible. Moreover, although there is good agreement between LIBS and glow discharge analyses of the same samples, the latter method has better resolution at the interface between elemental layers. Nevertheless, the authors believe that further work on the focusing step and more precise control of the laser beam energy distribution could significantly improve the ability of LIBS to depth Depth emission profiles for Cr, Ni, Cu, and Zn. profile. (J, Anal. At. Spectrom. (Adapted with permission. Copyright 1997 The 1997, 12, 859-62) Royal Society of Chemistry.)
Depth profiles—spatially resolved elemental analyses—can be collected by techniques such as secondary-ion MS and Xray photoelectron spectroscopy. However, these approaches typically require high vacuums and expensive instruments and are therefore limited to applications in fields such as semiionductors. Yet, a ssew of potential industrial applications would benefit from quantitative depth profiling if it ran quickly, required less expensive instrumentation and W3.s mcHSured ' ^ "- *>J ov^v
eral orders of magnitude larger than the dissymmetry ratio. They use camphorquinone as a model because its AS/S approaches 2 at the 266-nm excitation wavelength used in the study By adjusting the polarization of the laser pump beams with a photoelastic
Analytical Chemistry News & Features, November 1, 1997 6 5 1 A
