Influence isotope amount ratio determination

Single-collector instruments also prove very usefid for mass content determinations via isotope dilution, as carefiil estimation of all quantities that influence the uncertainty budget demonstrates that the precision on the isotope amount ratio is typically not the dominant factor for high-precision measurements. Often, the accuracy of a mass content measurement will hardly improve through the use of MC-ICP-MS instruments as other influence quantities, such as uncontrolled spectral interferences and sample inhomogeneities, typically deserve more attention. 

The specific incorporation rate (or the dilution of the precursor) gives the amount of product formed from the labeled precursor relative to that formed from the pool of endogenous precursor. Thus, for a specific incorporation rate of 0.1 % (or a dilution of 1 1,000), one product molecule in a thousand is formed from the isotopically labeled precursor. The specific incorporation rate is therefore dependent on the ratio of incorporation of endogenous to administered precursor molecules. It is influenced by the absolute rate of synthesis during the experiment only when this ratio is altered. It is easier to determine the specific activity of the product than its absolute amount since only a small portion of the product has to be isolated. 

Figure 7.7 Cause-effect or fishbone diagram for the isotope dilution determination of the mass fraction of a platinum group element (Os) in geological reference materials [7]. The most important influence quantities are marked with a box. It takes into account natural variations in the Os/ Os ratio, which can contribute significantly to the total uncertainty in the Os concentration. R represents the isotope ratio (the artificially enriched isotope divided by the isotope used for normalization) for the blend (B), the spike (y) and the naturally occurring element (x). Masses of sample and spike are represented by m and are expressed in grams. and ZR,, are the sum of all isotope ratios for the spike and the sample, respectively k represents the amount contents in mol g .

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