Determination of uranium isotope ratios using a liquid sampling atmospheric pressure glow discharge/Orbitrap mass spectrometer system.

RATIONALE: The field of highly accurate and precise isotope ratio analysis, for use in nonproliferation, has been dominated by thermal ionization and inductively coupled plasma mass spectrometry. While these techniques are considered the gold standard for isotope ratio analysis, a downsized instrument capable of accurately and precisely measuring uranium (U) isotope ratios is desirable for field studies or in laboratories with limited infrastructure. METHODS: The developed system interfaces the liquid sampling, an atmospheric pressure glow discharge (LS-APGD) ion source, with a high-resolution Exactive Orbitrap mass spectrometer. With this experimental setup certified U isotope standards and unknown samples were analyzed. The accuracy and precision of the system were then determined. RESULTS: The LS-APGD/Exactive instrument measured a certified reference material of natural U (235 U/238 U = 0.007261) with a 235 U/238 U ratio of 0.007065 and a % relative standard uncertainty of 0.082, meeting the International Target Values for the destructive analysis of U. In addition, when three unknowns were measured and these measurements were compared with the results from an ICP multi-collector instrument, there were no statistical differences between the two instruments. CONCLUSIONS: The LS-APGD/Orbitrap system, while still in the preliminary stages of development, offers highly accurate and precise isotope ratio results that suggest a potential paradigm shift in the world of isotope ratio analysis. Furthermore, the portability of the LS-APGD as an elemental ion source, combined with the small size and smaller operating demands of the Orbitrap, suggests that the instrumentation is capable of being field-deployable.

Preliminary Figures of Merit for Isotope Ratio Measurements: The Liquid Sampling-Atmospheric Pressure Glow Discharge Microplasma Ionization Source Coupled to an Orbitrap Mass Analyzer.

In order to meet a growing need for fieldable mass spectrometer systems for precise elemental and isotopic analyses, the liquid sampling-atmospheric pressure glow discharge (LS-APGD) has a number of very promising characteristics. One key set of attributes that await validation deals with the performance characteristics relative to isotope ratio precision and accuracy. Owing to its availability and prior experience with this research team, the initial evaluation of isotope ratio (IR) performance was performed on a Thermo Scientific Exactive Orbitrap instrument. While the mass accuracy and resolution performance for Orbitrap analyzers are well-documented, no detailed evaluations of the IR performance have been published. Efforts described here involve two variables: the inherent IR precision and accuracy delivered by the LS-APGD microplasma and the inherent IR measurement qualities of Orbitrap analyzers. Important to the IR performance, the various operating parameters of the Orbitrap sampling interface, high-energy collisional dissociation (HCD) stage, and ion injection/data acquisition have been evaluated. The IR performance for a range of other elements, including natural, depleted, and enriched uranium isotopes was determined. In all cases, the precision and accuracy are degraded when measuring low abundance (<0.1% isotope fractions). In the best case, IR precision on the order of 0.1% RSD can be achieved, with values of 1%-3% RSD observed for low-abundance species. The results suggest that the LS-APGD is a promising candidate for field deployable MS analysis and that the high resolving powers of the Orbitrap may be complemented with a here-to-fore unknown capacity to deliver high-precision IRs. Graphical Abstract ᅟ.