Combining Fourier\nTransform Ion Mobility with Charge Detection Mass\nSpectrometry for the Analysis of Multimeric Protein Complexes

Charge detection mass spectrometry\n(CDMS) allows direct mass measurement of heterogeneous samples by\nsimultaneously determining the charge state and the mass-to-charge\nratio (<i>m</i>/<i>z</i>) of individual ions,\nunlike conventional MS methods that use large ensembles of ions. CDMS\ntypically requires long acquisition times and the collection of thousands\nof spectra, each containing tens to hundreds of ions, to generate\nsufficient ion statistics, making it difficult to interface with the\ntime scales of online separation techniques such as ion mobility.\nHere, we demonstrate the application of Fourier transform multiplexing\nand drift tube ion mobility joined with Orbitrap-based CDMS for the\nanalysis of multimeric protein complexes. Stepped frequency modulation\nwas utilized to enable unambiguous frequency assignment during mobility\nsweeps and allow spectral averaging, which improves the accuracy and\nsignal-to-noise of ion mobility spectra and CDMS measurements. Fourier\ntransformation of the signal reveals the arrival times and collision\ncross sections of ions while simultaneously collecting charge information\nfor thousands of individual ions. Combining Fourier transform multiplexing\nion mobility and CDMS provides insight into each ion’s size\nand mass while showcasing a potential solution to the duty cycle mismatch\nof online separation techniques in the single ion regime.