A novel approach to measure isotope ratios via multi-collector-inductively coupled plasma-mass spectrometry based on sample mixing with a non-enriched standard

In this work, a novel approach to measure isotope ratios via multi-collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) for low amounts of target element is proposed. The methodology is based on mixing of the sample (target element isolate) with a non-enriched in-house standard, previously characterized for its isotopic composition. This methodology has been applied to isotopic analysis of Cu and of Fe in whole blood samples. For this purpose, different mixtures of sample + in-house standard were prepared and adjusted to a final concentration of 500 mu g/L of the target elements for isotopic analysis. delta Cu-65, delta Fe-56, and delta Fe-57 varied linearly as a function of the amount of in-house standard (or of sample) present in the mixture. The isotopic composition of the sample was calculated considering the isotope ratios measured for (i) the mixture and (ii) the in-house standard and (iii) the relative concentrations of target element contributed by the sample and the standard to the mixture, respectively. For validation purposes, the isotopic analysis of whole blood Cu was carried out using both the conventional (using 2 mL of whole blood) and the newly developed approach (using 500 mu L of whole blood). The delta Cu-65 values obtained using mixtures containing 40 % (200 mu g/L) of Cu from the blood samples and 60 % (300 mu g/L) of Cu from the in-house standard were in good agreement with the delta Cu-65 value obtained using the conventional approach (bias a parts per thousand currency sign0.15 aEuro degrees).