Simultaneous Measurements of Mass and Collisional Cross-Section of Single Ions with Charge Detection Mass Spectrometry

The masses and mobilities of single multiply charged ions of cytochrome c, ubiquitin, myoglobin, and bovine serum albumin formed by electrospray ionization are measured using charge detection mass spectrometry (CDMS). Single ions are trapped and repeatedly measured as they oscillate inside an electrostatic ion trap with cone electrodes for up to the maximum trapping time set at 500 ms. The histograms of the many single ion oscillation frequencies have resolved peaks that correspond to the different charge states of each protein. The m/z of each ion is determined from the initial oscillation frequency histogram, and the evolution of the ion energy with time is obtained from the changing frequency. A short-time Fourier transform of the time-domain data indicates that the increase in ion frequency occurs gradually with time with occasional sudden jumps in frequency. The frequency jumps are similar for each protein and may be caused by collision-induced changes in the ion trajectory. The rate of the gradual f...