Resolving Severe Elemental Isobaric Interferences with a Combined Atomic and Molecular Ionization Source-Orbitrap...

Many fields of basic and applied sciences, including geochronology, astronomy, metabolism, etc., rely on the ability of mass spectrometry to obtain isotope ratio measurements having a high degree of certainty. The inability to resolve difficult isobaric interferences plagues certain measurements. A combined atomic and molecular (CAM) ionization source has been interfaced to a high-field Orbitrap mass spectrometer to alleviate severe atomic, isobaric interferences. This work examines the geochronologically significant ⁸⁷Sr and ⁸⁷Rb isotope pair. The mass difference between ⁸⁷Sr and ⁸⁷Rb is approximately 0.3 mDa, requiring a minimum resolving power (R = m/Δm) of ∼290,000, a value ∼30× higher than available with sector-field elemental mass spectrometers. Under ultrahigh-resolution conditions, Sr isotope ratio accuracy and precision were evaluated using NIST Sr SRM 987, yielding precision values of <0.1% relative standard deviation (RSD) for the major isotopes and a calculated LOD of 2 pg mL^-1 (120 fg of Sr for a 60 μL injection). In addition to manipulating the signal transient length, the total number of ions in the electrostatic trap and the ⁸⁷Sr/⁸⁷Rb concentration ratio were found to influence resolution. Ultimately, the isotopes were baseline-resolved with a calculated mass resolution of >1.7M. At equal ⁸⁷Sr and ⁸⁷Rb intensities, ⁸⁷Sr/⁸⁶Sr was measured as 0.71294 (a relative error of only 0.37%) with a precision of 0.097% RSD, clearly reflecting the alleviation of the isobaric interference.