Signal linearity of an extended range pulse counting detector: Applications to accurate and precise U-Pb dating of zircon by laser ablation quadrupole ICP-MS

Abstract

Element concentration and isotope ratio measurements by single-collector mass spectrometry often require the detection system to handle ion beams with very large intensity ratios. In order to obtain accurate and reproducible element concentration and isotope ratio data, the detection system must have a linear response with respect to the intensity of the incident ion beam. An extended range scaling pulse counting detector equipped on a Varian 810 quadrupole inductively coupled plasma-mass spectrometer (ICP-MS) was tested for linearity across count rates of ∼2000 to 110,000,000 cps with different concentrations of natural U solutions. We also tested detector linearity by the laser ablation analysis of 206 Pb/ 238 U, 207 Pb/ 235 U, and 207 Pb/ 206 Pb ratios in well-characterized 416-1565 Ma zircon standards. Results indicate that there is no correlation between the measured isotope ratio and ion intensity for the solution tests or the tests of natural zircon standards. The results of these tests confirm the suitability of this instrument for isotope ratio measurements that require a substantial dynamic range without having to switch between pulse counting and analog modes on electron multipliers or switching between electron multiplier to Faraday detectors. Copyright 2010 by the American Geophysical Union.