Effect of signal acquisition mode on isotope ratio precision and accuracy in ICP-quadrupole-MS analysis

Abstract

Isotope ratios (IRs) precision obtained on ICP-quadrupole MS (ICP-QMS) with ion counting detector (IC) is worse than that of TIMS or MC-ICP-MS with multiple faraday-cup detectors. To collect low and high ion signal (100–109), the IC detector operated in pulse (P), analog (A) or dual (D) acquisition mode has become widespread used in most commercial ICP-QMS instruments since 1990s. However, the analysis performance of IRs measurement by different signal acquisition modes (P, A, or D) were scarcely studied. The precision and accuracy of IRs (i.e., 11/10B, 114/110Cd, and 208/206Pb) obtained in three modes was systematically evaluated in this paper. The internal precision (RSD) of IRs obtained in P mode gradually improved with the increasing of ion counting intensities, which was consistent with the principal of the counting statistical error (SE). However, it suddenly deteriorated on D (0.16–0.24% for 11/10B, 0.17–0.22% for 114/110Cd, and 0.16–0.22% for 208/206Pb) or A mode (0.13 to 0.10% for 11/10B, from 0.16 to 0.12% for 114/110Cd and from 0.16 to 0.14% for 208/206Pb) with a further increase in ion counting. Meanwhile, both accuracy (relative error, RE, %) and external precision (RESD, %) of IRs were better in P mode rather than that of D and A mode. The instability of the A-P correlation (the correlation between pulse and analog signal outputs) was observed by monitoring the variation of signal tuning factor (Tf, which applied to convert the analog to pulse signal outputs). When the Tf was employed as a constant by the instrumental operating software rather than a dynamic correction value, a random error was occurred. This constant Tf value was the main source of the poor precision and accuracy for IRs measurement with A or D mode. Therefore, operating in P acquisition mode with a signal intensity range (0.7–0.9 M CPS for 11B, 1.0–1.2 M CPS for 114Cd and 208Pb) was recommended to obtain the best IRs performance in ICP-QMS analysis. The proposed strategy was used to analyze 208/206Pb in geological standard reference materials (AGV-2, BCR-1, and BCR-2) and obtain prefect accuracy (<0.13%, RE) and precision (0.04–0.07%, RESD).