Isotope-ratio measurements of lead in NIST standard reference materials by multiple-collector inductively coupled plasma mass spectrometry

Abstract

The capability of a second-generation Nu Instruments multiple collector inductively coupled plasma mass spectrometer (MC-ICP–MS) has been evaluated for precise and accurate isotope-ratio determinations of lead. Essentially the mass spectrometer is a double-focusing instrument of Nier–Johnson analyzer geometry equipped with a newly designed variable-dispersion ion optical device, enabling the measured ion beams to be focused into a fixed array of Faraday collectors and an ion-counting assembly. NIST SRM Pb 981, 982, and 983 isotopic standards were used. Addition of thallium to the lead standards and subsequent simultaneous measurement of the thallium and lead isotopes enabled correction for mass discrimination, by use of the exponential correction law and ²⁰⁵Tl/²⁰³Tl = 2.3875. Six measurements of SRM Pb-982 furnished the results ²⁰⁶Pb/²⁰⁴Pb = 36.7326(68), ²⁰⁷Pb/²⁰⁴Pb = 17.1543(30), ²⁰⁸Pb/²⁰⁴Pb = 36.7249(69), ²⁰⁷Pb/²⁰⁶Pb = 0.46700(1), and ²⁰⁸Pb/²⁰⁶Pb = 0.99979(2); the NIST-certified values were 36.738(37), 17.159(25), 36.744(50), 0.46707(20), and 1.00016(36), respectively. Direct isotope lead analysis in silicates can be performed without any chemical separation. NIST SRM 610 glass was dissolved and introduced into the MC-ICP–MS by means of a micro concentric nebulizer. The ratios observed were in excellent agreement with previously reported data obtained by TIMS and laser ablation MC-ICP–MS, despite the high Ca/Pb concentration ratio (200/1) and the presence of many other elements at levels comparable with that of lead. Approximately 0.2 μg lead are sufficient for isotope analysis with ratio uncertainties between 240 and 530 ppm.