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

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 <1‰ level when a newly developed correction technique for slow response of a Faraday preamplifier (0.2∼1 s) was applied. The resulting precision or repeatability of the isotopic ratio measurements was improved to the 0.7‰ level, which was almost half the level achieved by the conventional measurements. The validity of this correction technique was also demonstrated by the 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.