The intersection of the fractionation lines of two mass spectrometer provide the absolute isotope ratios of a sample
Source: BAM
Since the discovery of instrumental isotope fractionation (IIF) in mass spectrometry (MS), the calibration of isotope ratio measurements has been an ongoing challenge. Some approaches for measuring the isotope ratio simply bypass the correction of the IIF, such as delta measurements, or calibration materials are used, shifting the problem of calibration to the producer of the reference material. For certifying isotope RMs with absolute isotope ratios only a few approaches are available, namely the isotope mixture approach, the double spike approach, the mass bias regression model and total evaporation in thermal ionization mass spectrometry (TIMS). All of them require either enriched isotopes, isotope RMs of another element or an RM for correcting residual error. The enriched isotopes which are required for the isotope mixture and the double spike approach need to be fully characterized beforehand, and therefore can be considered as RMs as well. So, all mentioned calibration approaches require a RM for performing absolute isotope ratio measurements.
Isotope RMs, however, are lacking and are not available for all chemical elements. Therefore, a new standard-free calibration approach for obtaining absolute isotope ratios of multi-isotopic elements has been developed. The underlying principle is that each MS suffers from IIF. This results in a specific isotope fractionation line in a three-isotope diagram, with the absolute isotope ratios ranging somewhere on this line. When applying a second MS featuring a different ionization mechanism, we obtain a second isotope fractionation line with a different slope in the same three-isotope diagram and with the absolute isotope ratios ranging somewhere on this line as well. Consequentially, the intersection of both lines yields the absolute isotope ratios of the measured sample. This theory has been tested for Cd and Pb isotope RMs with a TIMS and an inductively coupled plasma mass spectrometer. During the measurements the ionization conditions were changed such that a different extent of the isotope fractionation has been achieved. With the resulting data set the theory described above could be verified. The obtained absolute isotope ratios were metrologically compatible with the certified isotope ratios with an average bias of -5 ‰. This calibration approach, the so-called Triple Isotope Calibration Approach, is universal and can be applied to any multi-isotopic element. It is not limited by the type of the mass spectrometer.
The triple-isotope calibration approach: a universal and standard-free calibration approach for obtaining absolute isotope ratios of multi-isotopic elements
Jochen Vogl
published in Analytical and Bioanalytical Chemistry, Vol. 413, issue 3, pages 821 - 826, 2020
BAM Inorganic Trace Analysis division