An uncertainty budget for trace analysis by isotope-dilution ICP-MS with proper consideration of correlation

Abstract

Isotope-dilution mass spectrometry (IDMS) is considered to be a method without significant correction factors. It is also believed that this method is well understood. But unfortunately a large number of different uncertainty budgets have been published that consider different correction factors. These differences lead to conflicting combined uncertainties especially in trace analysis. It is described how the known correction factors must be considered in the uncertainty budget of values determined by IDMS combined with ICP-MS (ICP-IDMS). The corrections applied are dead time, background, interference, mass discrimination, blank correction and air buoyancy.IDMS measurements consist always of a series of isotope abundance ratio measurements and can be done according to different measurement protocols. Because the measurement protocols of IDMS are often rather sophisticated, correlations of influence quantities are difficult to identify. Therefore the measurement protocol has to be carefully considered in the specification of the measurand and a strategy is presented to properly account for these correlations. This will be exemplified for the estimation of mass fractions of platinum group elements (PGEs) and Re in the geological reference material UB-N (from CRPG-CNRS, Nancy in France) with ICP-IDMS. The PGEs with more than one isotope and the element Re are measured with on-line cation-exchange chromatography coupled to a quadrupole ICP-MS. All contents are below 10 microg kg(-1). Only osmium is separated from the matrix by direct sparging of OsO(4) into the plasma. This leads to transient signals for all PGEs and Re. It is possible to estimate the combined uncertainties and keep them favourably small despite the low contents, the transient signals and the sophisticated correction model.

Platinum‐Group Element and Rhenium Concentrations in Low Abundance Reference Materials

“…Moser et al (2003) applied this, together with analysis of variance (ANOVA) and demonstrated that the uncertainty budgets of the PGE concentrations in UB-N were dominated by sample inhomogeneity. It is more complicated to apply the Ingamells approach to analyte concentrations at trace levels.…”

Signal correlation in isotope ratio measurements with mass spectrometry: Effects on uncertainty propagation

“…Several authors have discussed the effects of correlation among the variables in IDMS uncertainty propagation [9,16]. In these accounts, the correlation of isotope ratios was noted.…”

“…In 2003, Hibbert discussed the effects of signal correlation in analytical chemistry ratio measurements which share components in the numerator and denominator raising the question of whether or not correlation effects appear in isotope-dilution mass spectrometry [17]. The same year Moser et al outlined uncertainty propagation in isotope dilution where a correlation between the isotope ratios was considered [9]. However, these are exceptions to the common error analysis that treats all isotopic intensities as independent variables for error propagation purposes [18][19][20][21][22][23][24].…”

“…Similarly, it is known that isotopic signals are not statistically independent variables. This arises from the fact that the isotopic composition of elements is constant and analytical mass spectrometry measurements are not always dominated by random noise [9]. The correlation of isotopic signals is, after all, the whole premise of isotope ratio mass spectrometry.…”

Isotope dilution mass spectrometry — A primary method of measurement and its role for RM certification

“…In 2001 Nelms et al critically reviewed the most commonly used methods in ICPMS [80]. Moser et al analysed the influence of dead time on the IDMS result and its uncertainty [81]. They found the coherence unexpectedly difficult to understand as several minima for the uncertainty contribution occurred depending on the chosen scenario.…”