Coupling of the Liquid Sampling–Atmospheric Pressure Glow Discharge to Orbitrap Mass Analyzers for Uranium Isotope Ratio Analysis: Evolution of the Methodology and Implications to the Field

Abstract

Just over a decade ago, a truly outside-of-the-box approach to isotope ratio mass spectrometry (IRMS) was undertaken between research groups at Clemson University and the Pacific Northwest National Laboratory. The original motivation dealt with projections as to whether or not microplasmas could be developed into practical elemental ionization sources, perhaps for transportable analysis applications. In particular, the use of the liquid sampling–atmospheric pressure glow discharge (LS-APGD) was pursued. Its interfacing to an ultra-high resolution Orbitrap platform, proved not only facile, but opened up a wealth of potential applications. Here, we lay out a historical, tutorial description of the interfacing and the evolution of the methodology regarding IRMS of uranium. Practical challenges and opportunities are described, which hopefully provide guidance to further applications in high resolution IRMS. It is hoped that, while detailed and lengthy, the didactic nature of the presentation provides experimental insights and tips, and also serves as an homage to our very good friend, Professor Gary M. Hieftje, whose scientific inspiration and comradery have been immeasurably important in our own careers.

Initial Characterization and Optimization of the Liquid Sampling-Atmospheric Pressure Glow Discharge Ionization Source Coupled to an Orbitrap Mass Spectrometer for the Determination of Plutonium

“…An HCD setting of 120 eV was chosen to be carried forward to be consistent with the HCD setting typically used for uranium isotope ratio determinations. 22 As depicted in Figure S3, the application of 120 eV HCD energy results in a substantial increase in the 242 Pu 16 O 2 + fraction of all plutoniumrelated molecular species. Very dramatically, the fraction of the target ion increases from ∼12% of 242 Pu 16 O 2 + with 20 eV of CID and no HCD to accounting for ∼85% of all Pu-related species at an energy of 120 eV HCD in conjunction with the CID process.…”

“…Indeed, the overlap of the standard deviations associated with the analyte responses indicates that there is no significant difference between HCD settings of 80–120 eV. An HCD setting of 120 eV was chosen to be carried forward to be consistent with the HCD setting typically used for uranium isotope ratio determinations . As depicted in Figure S3, the application of 120 eV HCD energy results in a substantial increase in the 242 Pu 16 O 2 + fraction of all plutonium-related molecular species.…”

“…As an optimized discharge condition was found for each experiment, that condition was carried forward to the next set of parametric optimizations. Since solution flow rate was not found to be a critical parameter, a flow rate of 30 μL min –1 was chosen for all further studies to be consistent with uranium isotope ratio settings and the typical starting conditions for LS-APGD-MS method development . As it showed the most significant influence during the DOE evaluation, the role of the interelectrode gap was first evaluated.…”

“…Previous work with the LS-APGD coupled with a benchtop Thermo Scientific Q Exactive Focus Orbitrap focused primarily on the challenging uranium isotope ratio determinations. ,,,,, A comparable level of performance was found across a wide range of uranium isotope depletion/enrichment levels during a first-level assessment of the LS-APGD-Orbitrap platform versus TIMS and scanning sector field and quadrupole ICP-MS isotope ratio determinations . Additional studies have looked at plasma discharge conditions, different collisional dissociation modalities, the effects of concomitant ions, advanced data acquisition systems, and data processing methods associated with uranium isotope ratio determinations. ,,,,− The major incentive for exploring isotope ratio determination of uranium includes leveraging the high mass resolution offered by the Orbitrap mass spectrometer to overcome isobaric and molecular species interferences that thwart straightforward analysis with traditional MS techniques.…”

“…20 A unique feature of the LS-APGD is the ability to couple the microplasma to mass spectrometers typically reserved for molecular mass spectrometry, including the high-resolution Orbitrap mass spectrometer. 22 This ability, when combined with the LS-APGD's practical advantages is a great stride forward toward simple and lower-cost high-resolution elemental analysis and isotope ratio determinations. While initially envisioned as an ionization source for elemental analysis, expanded studies have demonstrated the versatility of the LS-APGD.…”