Differential mobility spectrometry improves uranium isotope ratio measurements on an ion trap mass spectrometer

Abstract

The characterization of uranium and other radionuclides is important to programs in nuclear forensics. Commonly, isotope-ratio mass spectrometry (IRMS) is employed as a laboratory-based system for quantification of isotope ratios (IR) to shed light on the degree of processing. Transporting nuclear material from the field to the laboratory is problematic while laborious sample processing and long analysis times make IRMS less amenable to real-time measurement. Because size, weight, and power (SWaP) requirements make conventional IRMS difficult to implement for on-site applications, it is attractive to develop IR analysis on an ion trap mass spectrometer which is amenable to field portability. A hindrance to in situ analysis includesmatrix interferences which are best handled by some separation method. Notably, ion mobility instruments have been commonly demonstrated in field use, and, in particular, differential mobility spectrometry (DMS) is a promising companion separation technique to ion traps. DMS enables tunable filtration prior to ion trap MS analysis, removing chemical interferences to address the heterogeneous nature of radionuclides and fission products. DMS may be particularly useful for removing molecular isobaric interferences and, as such, pairing DMS with ion trap MS has potential for fieldable IR analysis. Herein, DMS was coupled to a linear ion trap (LIT) for pre-filtration of uranyl ion (UO2+) from chemical background produced from nano electrospray ionization. Subsequently, isotopic analysis was conducted on the uranyl ion. The 235U/238U ratio measured with the DMS-MS system showed improved signal-to-background, compared to MS alone. Pairing DMS to LITMS improves the accuracy of the MS, while precision of the MS appears to be limited by the precision of the LIT. After DMS filtration, the isotope ratio measurement of 235U/238U decreased from 0.00913 to 0.00713, with the standardized uncertainty improving from 26.58% to 7.21%. Such results indicate that DMS may generally impart greater confidence of nuclear IRMS measurements.