Advancing Isotopic Analysis of Particles with the LS-APGD/Orbitrap-FTMS Booster: An Ultrahigh Mass Resolution Approach

Particle analysis plays a crucial role in many application areas, including nuclear forensics, environmental monitoring, and food safety, due to the isotopic fingerprinting information possessed by individual particles. Several mass spectrometric techniques, such as secondary ion mass spectrometry (SIMS), thermal ionization mass spectrometry (TIMS), single particle multi collector inductively coupled plasma mass spectrometry (SP-MC-ICP-MS), and single particle inductively coupled plasma time of flight mass spectrometry (SP-ICP-TOF-MS) have been explored for particle analysis, each offering varying degrees of accuracy and precision. However, most of these techniques suffer from low mass resolution, necessitating extensive prior chemical separation. For instance, if both uranium and plutonium particles are present in a complex sample, isotopic analysis cannot be performed without prior separation because of isobaric interferences, such as those between ²³⁸U and ²³⁸Pu. To address these challenges, we propose a novel ultrahigh mass resolution approach for isotopic analysis of particle suspension using the liquid sampling-atmospheric pressure glow discharge (LS-APGD) ionization source coupled with an Orbitrap mass spectrometer. The inherent high mass resolution of the Orbitrap is further augmented by incorporating the FTMS Booster, an external data acquisition and processing (DAQ/P) system. The ultrahigh resolution provided by the technique alleviates the need for chemical separations, enabling isotopic analysis of the analyte even in the presence of interfering isobars. This significantly reduces both turnaround time and the use of expensive consumables. In this study, a combination of rapid C-trap capture time and a balanced dilution of particle suspension was employed to capture fast particle events, followed by the detection of transients up to 3 s. The extended transient time significantly enhances mass resolution and improves isotopic results of particle events on a scan-to-scan basis by increasing the signal-to-noise ratios (S/N). Well-characterized particles relevant to nuclear forensics, with average diameters ranging from a few hundred nanometers to micrometers, were used to demonstrate the concept. The isotope ratios of the particle population and scan-to-scan particle events aimed at single particle detection will be discussed, along with a comparison to the conventional bulk digestion method. Additionally, the precision of the method, both run-to-run and scan-to-scan, will be presented.