High-Resolution Differential Ion Mobility Separations/Orbitrap Mass Spectrometry without Buffer Gas Limitations

Strong orthogonality between differential ion mobility spectrometry (FAIMS) and mass spectrometry (MS) makes their hybrid a powerful approach to separate isomers and isobars. Harnessing that power depends on high resolution in both dimensions. The ultimate mass resolution and accuracy are delivered by Fourier Transform MS increasingly realized in Orbitrap MS, whereas FAIMS resolution is generally maximized by buffers rich in He or H₂ that elevate ion mobility and lead to prominent non-Blanc effects. However, turbomolecular pumps have lower efficiency for light gas molecules and their flow from the FAIMS stage complicates maintaining the ultrahigh vacuum (UHV) needed for Orbitrap operation. Here we address this challenge via two hardware modifications: (i) a differential pumping step between FAIMS and MS stages and (ii) reconfiguration of vacuum lines to isolate pumping of the high vacuum (HV) region. Either greatly ameliorates the pressure increases upon He or H₂ aspiration. This development enables free optimization of FAIMS carrier gas without concerns about MS performance, maximizing the utility and flexibility of FAIMS/MS platforms.