Microanalysis of minerals by an Excimer UV-LA-ICP-MS system

This paper is a performance evaluation of a prototype laser-ablation microanalytical system composed of an UV Excimer laser and a high-sensitivity inductively coupled plasma mass spectrometer (XLA-ICP-MS). The laser was optimized for trace-element microanalysis of silicate minerals, and the effects of different parameters (laser power, focus, carrier gas flow, etc.) on the performance characteristics were studied. The crater size and shape produced by the XLA system were compared to a solid state IR LA system (Nd:YAG) and proved to be superior. Crater sizes achievable in thin sections vary from 1000 to 10 mu m, although the best analytical results for this prototype were achieved at crater diameters of similar to 40-60 mu m and depths of similar to 30-40 mu m. Detection limits vary from similar to 1 ppb for 1000-mu m craters to similar to 1000 ppb for 10-mu m craters. The precision obtained for the measurements depends on both crater size and concentration in the ablated mineral. Typical RSD for five replicate analyses of the above-mentioned minerals were +/- 4% and +/- 10% for concentrations > 10 and 1-10 ppm, respectively. Comparison of XLA with solution nebulization analysis shows an excellent agreement for most elements. The XLA source, in contrast with other laser sources currently available, produced no noticeable chemical fractionation during ablation for all elements except radiogenic Pb-207 and, to a lesser extent, Pb-206. This effect is a serious limitation for using LA-ICP-MS for in situ lead geochronology.