Isotope ratio characteristics and sensitivity for uranium determinations using a liquid sampling-atmospheric pressure glow discharge ion source coupled to an Orbitrap mass analyzer

The continued development of the liquid sampling-atmospheric pressure glow discharge (LS-APGD) microplasma ion source, combined with Orbitrap™ mass spectrometry technology is described. Improved characterization of this new system for performing precise and accurate isotope ratio (IR) determinations is demonstrated. The instrumental parameters evaluated include the number of microscans and scans making up a data acquisition set, uranium concentration/signal level, sample make-up, and Fourier transform digitization window. A limiting precision of 0.41% relative standard deviation (RSD) can be achieved for a single determination with natural uranium, with a variability of 1.63% RSD over 10 separate analytical measurements, reflective of instrument drift. A study of matrix effects on IR measurements of U is also presented, highlighting the importance of pre-mass selection before injection into the Orbitrap MS. Dynamic range is demonstrated with response curves covering 4 orders of magnitude of concentration (∼1–1000 ng mL−1) and having an R2 value of >0.99. These results demonstrate the very promising pairing of the LS-APGD ionization source and the Orbitrap MS, pointing as well to definitive paths forward to better utilize both components in high quality isotope ratio mass spectrometry (IRMS).