Effect of Atmosphere on Spectral Emission from Plasmas Generated by the Laser Microprobe

The effects of various atmospheres on laser-induced optical emission of plasmas from solid samples have been investigated. Atmospheres of argon, air, oxygen, nitrogen, helium, and also vacuum, were used. Samples of iron in steel and in iron oxide-coated tape, and of magnesium in aluminum foil, human serum, and liver were employed. Laser energies of approximately 1.2, 3.6, and 8.0 mJ were used. Signal-to-background ratios were not found to vary systematically with atmosphere and laser energy, but significantly larger values occurred in vacuum at 1.6-mJ laser energy. Signal intensities were greater in denser atmospheres at 3.6 and 8 mJ, but approximately the same at 1.2 mJ. Signal intensities varied directly with these laser energies, except in vacuum where the signal was independent of laser energy. Metallic and nonmetallic targets behaved similarly.