Resolving Severe Elemental Isobaric Interferences with a Combined Atomic and Molecular Ionization Source-Orbitrap Mass Spectrometry Approach: The Sr and Rb Geochronology Pair.

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 Sr and Rb isotope pair. The mass difference between Sr and Rb is approximately 0.3 mDa, requiring a minimum resolving power ( = /Δ) 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 (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 Sr/Rb concentration ratio were found to influence resolution. Ultimately, the isotopes were baseline-resolved with a calculated mass resolution of >1.7M. At equal Sr and Rb intensities, Sr/Sr 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.