Systematic comparison of delta S-34 measurements by multicollector inductively coupled plasma mass spectrometry and evaluation of full uncertainty budget using two different metrological approaches

摘要

A systematic comparison of delta S-34 measurements by multicollector inductively coupled plasma mass spectrometry (MCICPMS) using two different standardisation approaches and two different reference materials as working standards is reported for the first time. Full uncertainty budgets have been calculated using the Kragten approach. Internal standardisation by measuring Si ratios and external standardisation by bracketing samples with either NIST RM8553 or NIST RM8554 have been compared to correct for the effects of mass bias. The delta S-34 value and the associated uncertainty No) were found to be slightly different when using different approaches. Corrected S-34:S-32 ratios for NIST RM8553 and NIST RM8554 differed from those previously reported by 0.1 to 0.5% when using external standardisation. S-34:S-32 ratios for NIST RM8553 and NIST RM8554 differed from those previously reported by 0.06 to 0.15% when using Si internal standardisation. This indicated that in order to minimise deviation from true ratio values, Si internal standardisation would be a more appropriate option when using a Neptune MCICPMS instrument. delta S-34 values were obtained for four different methionine samples and expanded uncertainties (k = 2) expressed in delta S-34 No) ranged from 0.7 parts per thousand to 1.6 parts per thousand. Regardless of the approach used for the mass bias correction, three parameters provided the major contributions to the standard uncertainty of the delta(SV)-S-34-CDT value. These were the measured S-34:S-32 in the sample, the measured S-34:S-32 for the working standard and the known delta(SV)-S-34-CDT value of the working standard. Results using both approaches are compared and the Si internal standardisation approach is used to provide results for an inter-laboratory comparison. Copyright (c) 2008 John Wiley & Sons, Ltd.