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Managing argon interference during measurements of 18O/16O ratios in O2 by continuous-flow isotope ratio mass spectrometry (CAT#: STEM-ST-0003-LJX)

Introduction

Monitoring changes in stable oxygen isotope ratios in molecular oxygen allows for studying many fundamental processes in bio(geo)chemistry and environmental sciences. While the measurement of 18O/16O ratios of O2 in gaseous samples can be carried out conveniently and from extracting moderately small aqueous samples for analyses by continuous-flow isotope ratio mass spectrometry (CF-IRMS), oxygen isotope signatures, δ18O, could be overestimated by more than 6‱ because of interferences from argon in air. Here, we systematically evaluated the extent of such Ar interferences on 18O/16O ratios of O2 for measurements by gas chromatography/IRMS and GasBench/IRMS and propose simple instrumental modifications for improved Ar and O2 separation as well as post-measurement correction procedures for obtaining accurate δ18O. We subsequently evaluated the consequences of Ar interferences for the quantification of O isotope fractionation in terms of isotope enrichment factors by continuous-flow isotope ratio mass spectrometry.

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Principle Applications Procedure Materials Notes

Principle

Isotope ratio mass spectrometry (IRMS) leverages magnetic sector mass spectrometry to enable high-precision measurement of the stable isotope content of a sample. Typical measurements target hydrogen, carbon, nitrogen, and oxygen analyses—although elements with masses up to and including sulfur can be measured. Solid, liquid, or gas phase samples are converted to simple gases then introduced to the IRMS. During analysis, an electron impact source ionizes sample-derived gas which is then accelerated down a flight tube, separated by mass, and quantified using a series of Faraday cups. The high precision of IRMS enables enumeration of even very small isotopic fractionation associated with physical, chemical, and biological transformations or natural abundance measurements.

Applications

For explaining the detailed molecular mechanisms behind biological processes
For understanding and quantifying nutrient and material exchanges between ecosystems
For providing ultra-precise stable isotope analyses
For understanding the geological history of the Earth
For food authenticity, forensic science, medical research and anti-doping testing

Procedure

1. Fill the reaction tube and install it, connect the gas path
2. Check for helium leaks
3. Heat up the reactor, wait for the reaction tube to burn stable, adjust the state of the equipment
4. Wrap the sample in a tin cup and test the sample
5. Store and process data

Materials

• Sample Type:
Oxygen

Notes

1.The approach is also valuable for quantifying the reactivity and progression of an applied stable isotope tracer to help determine reaction rates and final disposition of applied substrates.
2.IRMS offers a way of measuring isotopic variations with extremely high levels of accuracy. It can be used to detect isotope values of lighter elements with no issues, making it instrumental in the analysis of organic and natural samples.
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