In situ207Pb/206Pb isotope ratio measurements using two Daly detectors equipped on an ICP-mass spectrometer

Abstract

The simultaneous detection of ²⁰⁶Pb and ²⁰⁷Pb ions has been made by multiple-ion counting ICP-mass spectrometry using two Daly detectors (MC-ICPMS). To evaluate the long-term gain stability of the detectors, ¹³⁵Ba/¹³⁸Ba and ¹³⁶Ba/¹³⁸Ba ratios have been measured by a combination of Daly and Faraday detectors (¹³⁵Ba(D)/¹³⁸Ba(F)) and an electron multiplier and Faraday detectors (¹³⁶Ba(EM)/¹³⁸Ba(F)). The measured ¹³⁶Ba(EM)/¹³⁸Ba(F) ratio changed 2% through the 10-hour analysis, whereas the ¹³⁵Ba(D)/¹³⁸Ba(F) showed smaller changes (<0.5%) over the 10-hour period, demonstrating that the Daly detector could provide better gain stability against conventional electron multipliers. After the correction for the counting loss due to dead time, the Daly detector is capable of accepting signal intensities as high as 10⁷ cps. This indicates that the overlap of the analysis range, between the Daly detector (10⁰ to 10⁷ cps) and the Faraday detector (10⁴ to 10¹⁰ cps), would be at least two orders of magnitude, suggestive of easier cross calibration of the collector gain between the detectors. With the present two Daly detectors, in situ²⁰⁷Pb/²⁰⁶Pb ratio measurements have been made on the Nancy 91500 zircon standard through the sample introduction technique using laser ablation. The overall analytical precision and the relative deviation from the literature values were 5.1% and 0.04%, respectively. The data obtained here clearly demonstrate that the LA-MC-ICPMS technique equipped with the Daly detectors would become a major analytical tool for in situ U–Pb geochronology.