Determination of absolute 13C/12C isotope amount ratios by MC-ICPMS using calibration with synthetic isotope mixtures

Abstract

This paper demonstrates that the combination of simultaneous measurements of ¹²C⁺ and ¹³C⁺ ion currents by MC-ICPMS and calibration based on the use of gravimetrically prepared synthetic isotope mixtures provides a unique possibility for accurate determination of absolute ¹³C/¹²C isotope amount ratios which are metrologically traceable to the SI units. We have applied this approach to the determination of ¹³C/¹²C isotope amount ratios in glycine – an amino acid of importance in a variety of applications, including paleoecological studies and food security analysis. Eleven synthetic isotope mixtures with different n(¹³C)/n(¹²C) isotope amount ratios were prepared by gravimetric weighing and mixing of two well-characterized parent glycines, each highly enriched in either ¹²C or ¹³C isotopes. The parent materials were dissolved in dilute nitric acid and mixed as solutions. In measurements of the ¹³C/¹²C isotope ratio in aqueous solutions it was found essential to account for the relatively high procedural blank and for spectral interference of ¹²C¹H⁺ ions in ¹³C⁺ ion current. The former is due to dissolution of atmospheric CO₂ gas and presence of non-ionic organic contaminants even in high purity water. An improved measurement protocol was used in this study to address the above features. It involved performing ¹³C/¹²C isotope ratio measurements in high resolution mode of MC-ICPMS to completely resolve the spectral interference in ¹³C⁺ ion current and keeping elevated concentrations of carbon (∼0.5%) in the measured samples to minimize contribution of the blank. A value of the ¹³C/¹²C isotope amount ratio of 0.010648 ± 0.000042 (expanded uncertainty, k = 2) was obtained for the glycine candidate reference material in 12 measurement sessions with independently prepared calibration mixtures. This work shows excellent agreement between the iteration procedure and analytical solution which were used for the calculation of the calibration factor K. It also demonstrates that possible non-linearity between measured ion currents and isotope amount fractions has an insignificant effect on the accuracy of ¹³C/¹²C isotope amount ratio measurements of natural glycine.