The triple-isotope calibration approach: a universal and standard-free calibration approach for obtaining absolute isoto
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The triple-isotope calibration approach: a universal and standard-free calibration approach for obtaining absolute isotope ratios of multi-isotopic elements Jochen Vogl 1 Received: 23 July 2020 / Revised: 30 October 2020 / Accepted: 4 November 2020 # The Author(s) 2020
Abstract The theory of a new calibration approach for obtaining absolute isotope ratios of multi-isotopic elements without the use of any standard has been developed. The calibration approach basically uses the difference in the instrumental isotope fractionation of two different types of mass spectrometers, leading to two different fractionation lines in a three-isotope diagram. When measuring the same sample with both mass spectrometers, the different fractionation lines have one point in common: this is the ‘true’ logarithmized isotope ratio pair of the sample. Thus, the intersection of both fractionation lines provides us with the absolute isotope ratios of the sample. This theory has been tested in practice by measuring Cd and of Pb isotope ratios in the certified reference materials BAM-I012 and NIST SRM 981 by thermal ionization mass spectrometry and by inductively coupled plasma mass spectrometry while varying the ionization conditions for both mass spectrometers. With this experiment, the theory could be verified, and absolute isotope ratios were obtained, which were metrologically compatible with the certified isotope ratios. The so-obtained absolute isotope ratios are biased by − 0.5 % in average, which should be improved with further developments of the method. This calibration approach is universal, as it can be applied to all elements with three or more isotopes and it is not limited to the type of mass spectrometers applied; it can be applied as well to secondary ion mass spectrometry or others. Additionally, this approach provides information on the fractionation process itself via the triple-isotope fractionation exponent θ. Keywords Triple-isotope fractionation . Absolute isotope ratio . Mass spectrometry . Calibration . Uncertainty
Introduction Nearly immediately after the invention of the first mass spectrograph, the isotopic composition of neon was investigated [1]. It took some decades to realize that the meanwhile developed mass spectrometers show a bias in isotope ratio measurements, the instrumental isotope fractionation (IIF, often inaccurately termed ‘mass bias’), and to find a way to correct for. A.O. Nier invented the ‘isotope mixture approach’, where highly enriched and chemically pure isotopes were mixed and the resulting mixtures together with the calculated nominal isotope ratios were used to calibrate the mass spectrometer [2]. This enabled absolute isotope ratio measurements and revolutionized the determination of atomic weights in the * Jochen Vogl [email protected] 1
Bundesanstalt für Materialforschung und -prüfung (BAM), Richard-Willstätter-Straße 11, 12489 Berlin, Germany
following decades. Within the past two decades, the major deficiencies of this approach have been solved, which are the it
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