Realization\nof Higher Resolution Charge Detection\nMass Spectrometry

Charge detection mass spectrometry (CD-MS) allows mass\ndistributions\nto be measured for heterogeneous samples that cannot be analyzed by\nconventional MS. With CD-MS, the <i>m</i>/<i>z</i> and charge are measured for individual ions using a detection cylinder\nembedded in an electrostatic linear ion trap (ELIT). Imprecision in\nboth the <i>m</i>/<i>z</i> and charge measurements\ncontribute to the mass resolution. However, if the charge can be measured\nwith a precision of <0.2 e the charge state can be assigned with\na low error rate and the mass resolving power only depends on the <i>m</i>/<i>z</i> resolution. Prior to this work, the\nbest resolving power demonstrated experimentally for CD-MS was 700.\nHere we demonstrate a resolving power of >14,600, 20-times higher\nthan the previous best. Trajectory simulations were used to optimize\nthe geometry and electrostatic potentials of the ELIT. We found conditions\nwhere the energy dependence of the oscillation frequency becomes parabolic,\nand then operated with a nominal ion energy at the minimum of the\nparabola. The 14,600 resolving power was obtained with a beam collimator\nbefore the ELIT. With the collimator removed, the resolving power\nof the optimized ELIT is 7300, which is still an order of magnitude\nhigher than the previous best. The resolving power was demonstrated\nby resolving the isotope distributions for peptides and proteins.\nHigh resolution CD-MS measurements were then used to resolve the glycans\non a monoclonal antibody and applied to the analysis of hepatitis\nB virus capsids. The results indicate that procedures for adduct removal\nneed to be improved for the full benefit of the higher resolving power\nto be realized for higher mass species. However, these results represent\na key step toward using CD-MS to analyze very complex protein mixtures\nwhere charge states are not well resolved in the <i>m</i>/<i>z</i> spectrum because of congestion from numerous\noverlapping peaks.