Improvements in precision of isotopic ratio measurements using laser ablation-multiple collector-ICP-mass spectrometry: reduction of changes in measured isotopic ratios

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Abstract
65Cu/63Cu and 56Fe/54Fe isotopic ratios for two metal reference materials (NIST SRM 976 and NIST SRM 665) have been measured by a laser ablation-multiple collector-inductively coupled plasma mass spectrometer (LA-MC-ICPMS). A time resolved analysis (TRA) data acquisition technique was used to monitor the time profile in 65Cu/63Cu and 56Fe/54Fe isotopic ratios. An ArF excimer laser (193 nm) which produces pit sizes of 16–63 µm was used to ablate Cu and Fe from metal samples. Measured 65Cu/63Cu ratios increased systematically (1∼2‰) with prolonged ablation, suggestive of isotopic fractionation during the laser ablation or ionisation process in the ICP. However, systematic increase in the measured 65Cu/63Cu ratio could be minimised down to 56Fe/54Fe isotopic ratio measurement for metal Fe. The systematic increase in the measured 56Fe/54Fe ratio could also be minimised to the <1‰ level after the correction of slow response of the Faraday preamplifier. The data obtained here demonstrated that the time-dependent changes in measured isotopic ratios can be minimised by the careful control of the intensity change and correction of slow response of the Faraday preamplifier.

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