On the Preparation of Samples for Laser Microprobe Analysis

Abstract

The Q-switched laser microprobe (1, 2, 3, 4, 5) apparently facilitates more sensitive procedures for some elements than are usual in ordinary emission spectroscopy. Several circumstances may be responsible. The strongly self-absorbing lines which characterize plasmas caused by the laser impact invariably disappear when the plasmas are electrically cross-excited. This suggests that most of the elemental plasma reaches full electrical excitation. The nearly adiabatic conversion of solid samples to plasma suggests that little sample is lost in boiling, sputtering or condensation before the spark is applied. We believe the conversion is in fact nearly adiabatic because a well-tuned laser will vaporize a fairly constant mass of matrix from a wide variety of materials (4). Another factor making for efficiency is the fixed position of the electrodes and spark relative to the slit. This is partly due to the high precision with which successive spurts of plasma appear at the same spot. Still another factor is the low black-body and cyanogen band noise, which permits the use of sensitive films such as 103-0 or Royal X Pan. Finally, it may be that the peculiar spectral redistribution of microprobe cross-excited spectra compared to ordinary spark or arc spectra in some way favors greater sensitivity. Whatever the reasons, the increased sensitivity of the laser microprobe (quite apart from its ability to selectively localize an inclusion in a matrix) makes it desirable to have a simple method for preparing divers samples for trace analysis with a microprobe. The following notes outline suitable procedures.