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

摘要

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 (10(0)-10(9)), 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., B-11(/10),Cd-114/110, and (208/206)pb) 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 B-11(/10), 0.17-0.22% for Cd-114(/110), and 0.16-0.22% for Pb-208/206) or A mode (0.13 to 0.10% for (11)(/)(10)(B), from 0.16 to 0.12% for(114/110)Cd and from 0.16 to 0.14% for (208/206)pb) 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 (T-f, which applied to convert the analog to pulse signal outputs). When the T-f was employed as a constant by the instrumental operating software rather than a dynamic correction value, a random error was occurred. This constant T-f 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 B-11(,) 1.0-1.2 M CPS for Cd-114 and Pb-208) was recommended to obtain the best IRs performance in ICP-QMS analysis.