The triple-isotope calibration approach: a universal and standard-free calibration approach for obtaining absolute isotope ratios of multi-isotopic elements

Jochen Vogl

doi:10.1007/s00216-020-03050-4

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.Graphical abstractThe triple-isotope calibration approach: the intersection of the triple isotope fractionation lines of an element recorded by two individual mass spectometers yields the absolute isotope ratios of this element

Highlights

It took some decades to realize that the 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
For all isotope ratio measurements within this study, the isotopic certified reference material (iCRM) Bundesanstalt für Materialforschung und -prüfung (BAM)-I012 [11] and NIST SRM 981 [12] were used as samples
The above outlined model assumption was tested by applying MC-thermal ionization mass spectrometry (TIMS) and MC-ICP-MS

Summary

Introduction

Immediately after the invention of the first mass spectrograph, the isotopic composition of neon was investigated [1]. 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]. When elements undergo a chemical or a physical process leading from a starting material (A) to a product (B), most likely their isotopes are being treated disproportionally, which results in the so-called isotope fractionation. The starting material will be located somewhere on the regression line and the slope provides the apparent θ value This is applicable to natural processes, but as well to technical ones, such as the isotope fractionation in a mass spectrometer. The mathematical background is provided in the Electronic Supplementary Material (ESM_1), section 1

Materials and methods

Results and discussion

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