A new concept for correction of instrumental isotopic fractionation in MC-ICP-MS using a pair of monoisotopic elements: a critical evaluation

In this research work, we investigated the possibility of using mixtures of monoisotopic elements ( 93 Nb/ 89 Y, 165 Ho/ 159 Tb and 209 Bi/ 197 Au) as calibrators for accurate determination of isotope ratios by multicollector mass spectrometry (MC-ICP-MS). The potential advantage of using monoisotopic elements for calibration lies in their significantly lower uncertainty in atomic masses. This would certainly affect the uncertainty of a measured isotope ratio after data processing, particularly if absolute isotope ratios are being evaluated. Model isotopic systems ( 87 Sr/ 86 Sr- 93 Nb/ 89 Y, 92 Zr/ 90 Zr- 93 Nb/ 89 Y, 208 Pb/ 207 Pb- 209 Bi/ 197 Au and 167 Er/ 166 Er- 165 Ho/ 159 Tb) were selected and the Internal Standard method (Russell's law) and optimized regression model (ORM) for isotope measurements, using the mentioned mixtures of monoisotopic elements as calibrators, were applied. The precision and trueness of the obtained results were assessed. For example, the 167 Er/ 166 Er isotope ratio in a certified reference material of erbium was measured using a gravimetric mixture of Ho and Tb as well as a 160 Gd/ 158 Gd pair (NRC GADS-1) as instrumental isotopic fractionation (IIF) calibrators in ORM. Although the average result for both approaches was in the range of instrumental error (0.68223 ± 0.00351 with 165 Ho/ 159 Tb and 0.68236 ± 0.00034 with 160 Gd/ 158 Gd), the precision was ten times lower when a mixture of monoisotopic elements was used for correction. Based on the results obtained, it was assessed that Russell's law does not provide a correct description for systems with a mixture of two monoisotopic elements, probably due to the assumption of equal IIF for the analyte and internal calibrator. In our opinion, such an assumption is not true in a system when the internal calibrator is a pair of two different elements. A new mathematical description of such a system could enable the introduction of a suitable correction. The ORM method is free from assumptions about the same IIF for individual chemical entities and for this reason it looks more encouraging and shows the potential to be useable even with a mixture of monoisotopic elements as calibrators. In this paper we present experimental results confirming this assumption, although the current challenge is to ensure a sufficiently high precision of calibrator ratio measurement and, consequently, better regression linearity and higher precision of the obtained results. In this research wor