Measurement of Isotope Abundance Variations in Nature by Gravimetric Spiking Isotope Dilution Analysis (GS-IDA)

Abstract

Subtle variations in the isotopic composition of elements carry unique information about physical and chemical processes in nature and are now exploited widely in diverse areas of research. Reliable measurement of natural isotope abundance variations is among the biggest challenges in inorganic mass spectrometry as they are highly sensitive to methodological bias. For decades, double spiking of the sample with a mix of two stable isotopes has been considered the reference technique for measuring such variations both by multicollector-inductively coupled plasma mass spectrometry (MC-ICPMS) and multicollector-thermal ionization mass spectrometry (MC-TIMS). However, this technique can only be applied to elements having at least four stable isotopes. Here we present a novel approach that requires measurement of three isotope signals only and which is more robust than the conventional double spiking technique. This became possible by gravimetric mixing of the sample with an isotopic spike in different proportions and by applying principles of isotope dilution for data analysis (GS-IDA). The potential and principle use of the technique is demonstrated for Mg in human urine using MC-TIMS for isotopic analysis. Mg is an element inaccessible to double spiking methods as it consists of three stable isotopes only and shows great potential for metabolically induced isotope effects waiting to be explored.

Atomic spectrometry updates: Review of advances in atomic spectrometry and related techniques

“…Both studies used Nd isotopic standards doped with variable amounts of Sm to replicate the likely natural range of Sm/Nd, and formulated a relationship between the exponential mass bias coefficients to correct the isobaric 144 Sm interference. Reported errors on 143 Nd/ 144 Nd and 147 Sm/ 144 Nd were similar for the two studies, and were roughly equivalent to measurements made by conventional methods. Sanchez-Lorda et al 157 reported that 147 Sm/ 144 Nd reproducibility ranged from 0.2% to 0.7%, which is slightly higher than for IDA measurements.…”

“…De-convolution using a double spike to give bias-free isotope ratios requires an element with a minimum of four accurately measurable isotopes in the natural or the with-spike mixture. However, by using gravimetric mixing of the sample with different proportions of an isotopic spike, Chew and Walczyk 143 have developed a modication of the double spike technique capable of highprecision IR measurements using only three isotope signals. This gravimetric spiking IDA (GS-IDA) approach has the potential to increase the precision of Mg isotope ratios, beneting studies of metabolism in humans and elemental diffusion in mineral species.…”

“…There have also been developments in detection systems, such as a new generation of Faraday detectors equipped with low-resistivity (10 12 U) resistors reported by Koornneef et al 139 These detectors were designed to provide lower baseline noise compared to the conventional 10 11 U ampliers, and were used to measure 87 Sr/ 86 Sr and 143 Nd/ 144 Nd for 1 ng amounts of analyte. Comparison with previous studies indicated that reproducibility was improved by factors of 2 and 1.4, respectively.…”

Thermal Ionisation Mass Spectrometry

“…82 Also linked to the modelling of bone disorders (plus coronary artery disease), Mg isotopes can now be analysed by TIMS using a gravimetric mix of sample and spike. 83 This unusual technique is necessary, since Mg is made up of three stable isotopes only, and is thus unsuitable for the double-spike method. A general review of the TIMS analysis of additional stable isotopes (such as Zn, Cu, Fe, Ca, Mo) in understanding human and animal metabolism is provided by Turnland.…”

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

“…Drawbacks for obtaining absolute isotope ratios are the application of fractionation models and the required accurate quantification of the element amount in the sample and the doublespike. Additionally, this technique can be applied only to elements with at least four isotopes; with the addition of a second blend also elements with three isotopes are accessible [5]. A completely different approach in TIMS is the total evaporation technique, which has been developed for the measurement of U and Pu first.…”