Comparison of the performance of different ICP-MS instruments on the measurement of Cu in a water sample by ICP-IDMS

Three different types of ICP-MS instrument, a quadrupole, a single collector magnetic sector and a multicollector magnetic sector instrument, were compared for the measurement of Cu in a water sample by ICP-IDMS. Identical samples were run on the different instruments and various dilution levels were tested. Criteria for comparison were the total combined uncertainties of the isotope ratio measurements and of the amount content measurements. The influence of the different modes of detection (e.g., pulse counting/analog mode detection and single collection/multiple collection) was evaluated. Sources of uncertainty related to the instrument included the repeatability of the measurements, the repeatability of the correction for mass discrimination, the correction for detector dead time effects, the correction for instrumental background and the correction for isobaric interference. External sources of uncertainty, e. g. the uncertainty of the certified reference material (CRM) for correction of mass discrimination, were evaluated as well. Whereas in many cases sample preparation represents a major source of uncertainty, this did not apply here since the sample did not require particular treatment. Total combined uncertainties were calculated according to the ISO/GUM and EURACHEM guidelines. Excellent agreement was observed between all the measurement results obtained with the different instruments and instrumental settings under investigation. Relative combined uncertainties between 0.4 and 0.07% (k=1) were obtained for "absolute'' isotope ratio measurements (isotope ratios directly SI-traceable via a CRM) on a copper blend used for IDMS. For the copper amount content determined by ICP-IDMS, overall uncertainties between 1.0% and 0.45% (k=2) could be achieved, depending on the type of instrument and the measurement conditions used.