Capabilities of an Argon Fluoride 193 nm Excimer Laser for Laser Ablation Inductively Coupled Plasma Mass Spectometry Microanalysis of Geological Materials

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Abstract
Recent developments in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) have demonstrated its potential for in situ microanalysis for major, minor and trace elements in solids, such as minerals. With the low backgrounds and high sensitivity of new ICP-MS instruments, limits of detection of 1–10 ng g -1 in a 40 µm ablation pit for many elements can be reached. Fractionation effects due to different ablation rates of various elements have prevented quantification without matrix-matched standards with 1064 nm Nd:YAG lasers. These effects have been reduced but not eliminated using shorter UV wavelengths ( e.g . a quadrupled Nd:YAG 266 nm). Excimer lasers with wavelengths below 200 nm are expected to reduce fractionation effects further, but they present a serious challenge to the design of optical systems, especially if high-resolution UV ablation needs to be combined with high quality visual observation, which is essential for the study of complex materials, such as geological samples. An LA system was developed using an homogenized UV laser beam (193 nm, Argon Fluoride excimer) with a common UV–visual objective on a modified petrographic microscope with reflected and transmitted light illumination, in combination with a Perkin-Elmer Elan 6000 ICP-MS instrument. The optical system allows imaging of both visible and UV laser light onto the sample surface at the same time. Laser operating parameters and their influence on the ablation process were investigated using NIST SRM 612/610. Fractionation effects due to differential ablation of various elements as a function of time can be reduced to interelement correlation coefficients of r =0.9 or better and have become insignificant within the precision of quadrupole ICP-MS using this new optical design. Energy densities and repetition rates need to be kept within limited ranges for accurate and reproducible determinations of trace elements such as Zn, U and Pb, which have previously presented strong fractionation problems. LA-ICP-MS determinations on natural hornblende, augite, and garnet, calibrated against NIST SRM 612 using any major element as an internal standard, agree well with independent literature data. These experiments with the Argon Fluoride 193 nm excimer system demonstrate a greatly reduced matrix dependence of the ablation process, which facilitates in situ analysis of unknown samples.