Mass spectrometry (MS) is often the most suitable analytical method for determining isotope ratios. However, due to intrinsic and unavoidable effects during MS measurements, isotope ratios traceable to the International System of Units (SI) are not directly available. These effects, known as Instrumental Isotopic Fractionation (IFF) [1], necessitate corrections usually using a reference material [2]. To characterize reference materials, a primary method is required; in isotope analysis, this approach is called Full Gravimetric Isotope Mixture (FGIM) [2][3]. In this method, several mixtures from isotopically enriched/depleted parent materials are prepared, and a set of linear equations describing the isotope ratios in these mixtures is established. Solving these linear equations yields the desired correction factors (also known as K-factors), allowing to correct for IFF [4].

This approach requires fulfilling two conditions: first, for each isotope, there must be one spike parent material (e.g., for Li, there must be two, and for Hg, there must be seven); second, the purity (mass fraction of the analyte element) of these parent materials must be known. The first condition can be avoided by preparing additional mixtures or by considering the exponential law. The second condition cannot be easily avoided, as purity directly enters the aforementioned set of equations and must therefore be known or determined.

Isotope Dilution Mass Spectrometry (IDMS) [5]–[7] is the fundamental primary method to determine the mass fraction of an analyte element in a substance. However, successful application of the IDMS approach requires the knowledge of absolute isotope ratios, leading to a classic Catch-22 situation: determining absolute isotope ratios requires pre-existing knowledge of the purity, and vice versa.

In this talk, we present how we can escape this Catch-22 situation by combining mass spectrometry and ion chromatography [8]. This advancement of the gravimetric isotope mixture method, which we call Ion Chromatography enhanced Gravimetric Isotope Mixtures (ICeGIM) method, overcomes the limitations of both the IDMS and FGIM methods. We will present the mathematical framework comprehensively and show the first application of ICeGIM in the re-characterization of LSVEC, the internationally agreed-upon isotope reference material for lithium. Furthermore, we will demonstrate the performance of ICeGIM in terms of achievable uncertainties and discuss other applications.

References

[1] J. Irrgeher, T. Prohaska, in Sector Field Mass Spectrometry for Elementa and Isotopic Analysis (Ed: N.J. Johanna Irrgeher Andreas Zitek Thomas Prohaska), The Royal Society Of Chemistry, Cambridge 2015, 107.

[2] L. Yang, S. Tong, L. Zhou, Z. Hu, Z. Mester, J. Meija, J.Anal .At. Spectrom. 2018, 33, 1849.

[3] A.O. Nier, Phys .Rev. 1950, 77, 789.

[4] A. Stoll-Werian, L. Flierl, O. Rienitz, J. Noordmann, R. Kessel, A. Pramann, Spectrochim .Acta B 2019, 157, 76.

[5] K.G. Heumann, Mass Spectrom. Rev. 1992, 11, 41.

[6] L. Ouerdane, Z. Mester, J. Meija, Anal. Chem. 2009, 81, 5075.

[7] J. Vogl, W. Pritzkow, MAPAN 2010, 25, 135.

[8] L. Flierl, O. Rienitz, J. Vogl, A. Pramann, Analytical Bioanalytical Chemistry 2024, accepted.