Ablation Technique for Laser Ablation–Inductively Coupled Plasma Mass Spectrometry

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Abstract
A laser ablation technique has been developed for elemental and isotopic ratio analyses of solid materials using inductively coupled plasma mass spectrometry. Using a soft ablation technique developed in this study, reproducibility of the signal intensity and precision of the elemental and isotopic ratio measurement were successfully improved. The technique involves beginning the ablation at low laser power (lower than 1 mJ) and gradually increasing power to achieve a constant and stable signal. Using this method minimises the release of large fragments and removes the spiky and high intensity signal which is characteristic of conventional ablation techniques. The method eliminates the requirement for a pre-ablation signal prior to data acquisition and consequently analysis time and sampling depth can be minimised. In order to evaluate the versatility of the soft ablation technique, Pb:U isotopic and elemental data for zircon samples have been determined. Conventional laser ablation techniques for Pb:U abundance ratios show a serious fractionation effect because of the greater volatility of Pb. Using the soft ablation technique developed here, this elemental fractionation can be effectively minimised and the resultant Pb:U age data for zircon samples show excellent agreement with the U–Pb data obtained by thermal ionisation mass spectrometry or secondary ion mass spectrometry. The data presented here demonstrate clearly that careful control of the ablation process can produce accurate and precise elemental and isotopic analysis from 10–12µm crater pits.

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