Precise Pb isotope ratio determination of picogram-size samples: A comparison between multiple Faraday collectors equipped with 1012 Ω amplifiers and multiple ion counters

Abstract

While typical ion-counting peak hopping methods using a single Daly detector or a single secondary electron multiplier have proven very effective in analyzing small Pb samples by TIMS, the measurements often require long acquisition times, typically of several hours. Here we compare and evaluate static multi-collector determination of Pb isotope ratios in samples containing between 100 and 1pg total Pb using Faraday detectors, equipped with 1012Ω feedback resistors in the current amplifier system, versus a multiple ion counting (MIC) system, installed in a Triton Plus TIMS. Faraday cup measurements of pg size Pb samples using 1012Ω amplifiers require precise measurement of long baselines for optimal repeatability (defined as internal precision or the precision of a single run) and intermediate precision (defined as the closeness of the replicated measurements over an extended period of time). In our system, using a 20min baseline before and after the measurement, we achieved a repeatability of 0.02–0.03% (2SE) and an intermediate precision of 0.05% (2SD) on 207Pb/206Pb and 208Pb/206Pb in as little as 1h for samples containing between 80 and 10pg Pb. This is equivalent to the performance obtained from a 4 to 5hour-long single SEM peak hopping analysis of larger (10.0–0.5ng) samples. For measurements in MIC mode, the main analytical uncertainty is the relative instability of the detector yields, which is 0.1–0.3% (2SD; following a 1hour detector stabilization period). Using a correction based on the average yield measured before and after sample analysis, we achieved a repeatability of 0.03%–0.06% (2SE) and an intermediate precision of 0.18%–0.23% (2SD) on 207Pb/206Pb for 80 to 10pg loads. For sample sizes between 10 and 1pg, which were only measured in MIC mode because the 204Pb intensity is below detection limit in Faraday mode, the main source of uncertainty is the variable loading blank contribution (5–15%). We conclude that a Faraday array equipped with 1012Ω amplifiers performs very well for Pb sample load sizes down to 10pg, allowing much more rapid data acquisition than peak-hoping SEM measurements and producing data that is 4–5 times more precise than static MIC measurements. In contrast, the MIC system offers considerable promise for tracer Pb work in the sub-10pg Pb analyte range where data with a few % level of accuracy and precision are useful, providing that acceptable total procedural blanks (below 0.1pg) can be achieved.