Pb isotope analysis of ng size samples by TIMS equipped with a 1013 Ω resistor using a 207Pb–204Pb double spike

Abstract

The use of the double spike technique to correct for instrumental mass fractionation has yielded high precision results for lead isotope measurements by thermal ionisation mass spectrometry (TIMS), but the applicability to ng size Pb samples is hampered by the small size of the ²⁰⁴Pb ion beam in the natural isotope composition analysis. To overcome this limitation, we successfully demonstrate the application of a 10¹³ Ω resistor in the Faraday cup amplifier feedback loop used for the collection of ²⁰⁴Pb in combination with a newly produced ²⁰⁷Pb–²⁰⁴Pb double spike to correct for instrumental mass fractionation. The use of a 10¹³ Ω resistor for the collection of the small ²⁰⁴Pb ion beam leads to a tenfold improvement in the signal-to-noise ratio, but necessitates an external gain correction using a secondary standard and careful monitoring of the ion beam stability. SRM 981 aliquots of 5 ng display unparalleled reproducibility of 90–125 ppm (2 SD) and are in excellent agreement with recommended values: ²⁰⁶Pb/²⁰⁴Pb = 16.9404 ± 0.0016, ²⁰⁷Pb/²⁰⁴Pb = 15.4977 ± 0.0019 and ²⁰⁶Pb/²⁰⁴Pb = 36.7193 ± 0.0042 (2 SD, n = 22). Comparable high quality data have been obtained for 5 ng aliquots of geological reference materials AGV-1 and BCR-1 (e.g.²⁰⁶Pb/²⁰⁴Pb = 18.9399 ± 0.0011 and 18.8208 ± 0.0011 respectively), indicating that analytical blank contribution is negligible for sample sizes down to 5 ng. With further reduction in, or precise correction for, analytical blanks, the combination of the double spike technique with high-resistance amplifiers has the potential to produce highly accurate and precise (<200 ppm) Pb isotope data for samples below the ng level. This methodology will open a range of research possibilities presently impeded by the low amount of Pb available for analysis.