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 (⁹³Nb/⁸⁹Y, ¹⁶⁵Ho/¹⁵⁹Tb and ²⁰⁹Bi/¹⁹⁷Au) 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 (⁸⁷Sr/⁸⁶Sr–⁹³Nb/⁸⁹Y, ⁹²Zr/⁹⁰Zr–⁹³Nb/⁸⁹Y, ²⁰⁸Pb/²⁰⁷Pb–²⁰⁹Bi/¹⁹⁷Au and ¹⁶⁷Er/¹⁶⁶Er–¹⁶⁵Ho/¹⁵⁹Tb) 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 ¹⁶⁷Er/¹⁶⁶Er isotope ratio in a certified reference material of erbium was measured using a gravimetric mixture of Ho and Tb as well as a ¹⁶⁰Gd/¹⁵⁸Gd 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 ¹⁶⁵Ho/¹⁵⁹Tb and 0.68236 ± 0.00034 with ¹⁶⁰Gd/¹⁵⁸Gd), 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.