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

Abstract

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 frequency shift increases with protein mass and charge state. This gradual frequency change is due to ion energy loss from collisions with the background gas. The total energy lost by an ion is determined from the latter frequency shifts normalized to a 500 ms lifetime, and these values increase nearly linearly with measured collisional cross-sections for these protein ions. These results show that the mass and collisional cross-section of single multiply charged ions can be obtained from these CDMS measurements by using proteins with known collisional cross-sections for calibration.

Recommendations for reporting ion mobility Mass Spectrometry measurements

“…For these reasons, we devoted special attention to define K 0 and IM‐derived CCS values, and to clarify what influences these quantities. There are other methods than ion mobility spectrometry to determine experimental collision cross sections in a more direct way, for example, by monitoring the loss of ion signal due to collisions (Covey & Douglas, ; Chen, Collings, & Douglas, ; Anupriya, Jones, & Dearden, ; Anupriya et al, ; Dziekonski et al, ; Elliott et al, ; Sanders et al, ). These methods to determine collision cross sections are however not based on ion mobility measurements, and will not be covered here.…”

Weighing synthetic polymers of ultra‐high molar mass and polymeric nanomaterials: What can we learn from charge detection mass spectrometry?

“…Recently, Williams and coworkers have shown that not only mass and charge but also cross-sectional information can be obtained by CDMS, which can be used to directly explore structures of heavy molecules. [84][85][86] Here, electrostatic mirrors at each side of the detection tube of the CDMS set-up have been implemented, and trapped ions collide with background gas, which leads to frequency shifts (see Figure 5). From the amount and the intervals of the frequency, the collisional cross section can be evaluated.…”

Recommendations for Reporting Ion Mobility Mass Spectrometry Measurements

“…For these reasons, we devoted special attention to define K 0 and IM-derived CCS values, and to clarify what influences these quantities. There are other methods than ion mobility spectrometry to determine experimental collision cross sections in a more direct way, for example, by monitoring the loss of ion signal due to collisions (Covey & Douglas, 1993;Chen, Collings, & Douglas, 1997;Anupriya, Jones, & Dearden, 2016;Anupriya et al, 2017;Dziekonski et al, 2017;Elliott et al, 2017;Sanders et al, 2018). These methods to determine collision cross sections are however not based on ion mobility measurements, and will not be covered here.…”