Abstract

Collisional damping of ion density fluctuations of an ion beam extracted from an ICP in a linear rf quadrupole cell [dynamic reaction cell (DRC)] is studied. Target gas pressure, parameter q, entrance and exit lens potentials and DRC rod offset were optimized to broaden the ¹⁰⁷Ag⁺ ion packets of 0.2 ms duration to 2–4 ms (FWHM). It is shown that operating the DRC under such optimized conditions reduces the RSD of isotope ratio measurements obtained with a downstream quadrupole mass analyzer to the theoretical limit defined by the counting statistics error. For Ag and Pb samples at 40 ng ml⁻¹, measured with the cell pressurized by Ne, isotope ratio RSDs of 0.02–0.03% at a counting statistics error of 0.01–0.02% are shown. All 4 isotopes of Pb were measured in the same acquisition. For NIST 982 Isotope Standard at a concentration of 10 ng ml^-1, ²⁰⁴Pb/²⁰⁶Pb, ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb isotope ratios were determined with an external precision (n = 7) of 0.056–0.117%, 0.022–0.034% and 0.014–0.028% RSD, and with an average inaccuracy of 340 ppm, 781 ppm and 180 ppm, respectively. NIST 981 at 10 ng ml⁻¹ was used as an external standard. Reactive elimination of isobaric interferences in the DRC is shown to allow precise isotope ratio measurement of normally interfered isotopes of Fe by a quadrupole ICP-MS. ArN⁺ and ArO⁺ interferences on Fe⁺ were removed by ion–molecule reactions with NH₃. Collisional damping of the ion beam fluctuations was sustained and internal precision is shown to be limited only by counting statistics. For Fe at 50 ng ml⁻¹, the average internal precision for 19 samples (9 replicates per sample) of ⁵⁴Fe/⁵⁶Fe and ⁵⁷Fe/⁵⁶Fe ratios was 0.068% and 0.1% RSD, with a corresponding average counting statistics error of 0.053% and 0.082% and external precision of 0.055% and 0.056% RSD, respectively.