Enhanced sensitivity in laser ablation-ICP mass spectrometry using helium-argon mixtures as aerosol carrier

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Abstract
Laser ablation-ICP-MS is a sensitive and accurate technique for major to trace multi-element analysis at high spatial resolution on the scale of 10 µm. A wide variety of samples can be studied quantitatively, including minerals and their solid, liquid or melt inclusions as required for geochemical studies. As the desired spatial resolution increases, however, detection limits become severely constrained by the total amount of sample material reaching the ICP. Detection limits are therefore determined by the ablation rate and by the efficiency of removal of ablated aerosol particles from the ablation spot and their transport into the plasma. Properties of the carrier gas are known to affect the ablation process and the efficiency of particle transportation. This study explores the effects of different ablation-cell configurations and the use of helium, dry argon and argon moistened with water for the transport of aerosols into an ICP-MS, using a prototype 193 nm ArF excimer laser. Deposition of visible particles deposited around the ablation pit is significantly reduced when helium is used instead of argon. A moderate flux of helium through the chamber, mixed with moistened argon immediately downstream from the ablation chamber, leads to at least a 2-3-fold increase in the signal intensities across the entire mass range when compared with argon gas only. Background intensities above mass 85 are significantly reduced, but polyatomic interferences in the low mass region increase by an order of magnitude, owing to oxide formation caused by the water load. A high flux of helium, mixed just behind the ablation cell with dry argon, yields a 2-3-fold sensitivity enhancement, in addition to greatly reduced background intensity across the entire mass range. This results in one order of magnitude improvement in detection limits for most elements. These modifications permit the routine determination of minor concentrations of chlorine in microscopic fluid inclusions or the analysis of minerals, such as trace element concentrations in quartz (e.g., Na and Li down to 500 ng g –1 , using a 40 µ ablation pit). Furthermore, this improved sensitivity has recently yielded the first quantitative determination of gold concentrations (∼0.1 µg g –1 ) and full rare-earth element patterns in single 25 µm fluid inclusions.

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