Charge detection mass spectrometry is used to measure the mass, charge, MS and mobility of an individual ion produced by electrospray ionization of a 8 MDa polyethylene glycol sample. The charge detection mass spectrometer is an electrostatic ion trap that uses cone electrodes and a single tube detector and can detect ions for up to the full trapping time of 4.0 s. The time-domain signal induced on the detector tube by a single multiply charged ion can be complex owing to sequential fragmentation of the original precursor ion as well as increasing oscillation frequencies of the single ion owing to collisions with background gas that reduce the kinetic energy of the ion inside the trap. Simulations show that the ratio of the time for the ion to turn around inside the cone region of the trap to the time for the ion to travel through the detector tube is constant with m/z and increases with the ion energy per charge. By measuring this ratio, the kinetic energy of an ion can be obtained with good precision (∼1%) and this method to measure ion kinetic energies eliminates the necessity of ion energy selection prior to trapping for high precision mass measurement of large molecules in complex mixtures. This method also makes it possible to measure the masses of each sequential fragment ion formed from the original precursor ion. MS of a single multiply charged PEG molecule is demonstrated, and from these ion energy measurements and effects of collisions on the ion motion inside the trap, information about the ion mobility of the precursor ion and its fragments is obtained.
Applications of charge detection mass spectrometry (CDMS) for measuring the masses of large molecules, macromolecular complexes, and synthetic polymers that are too large or heterogeneous for conventional mass spectrometry measurements are made possible by weighing individual ions in order to avoid interferences between ions. Here, a new multiplexing method that makes it possible to measure the masses of many ions simultaneously in CDMS is demonstrated. Ions with a broad range of kinetic energies are trapped. The energy of each ion is obtained from the ratio of the intensity of the fundamental to the second harmonic frequencies of the periodic trapping motion making it possible to measure both the m/z and charge of each ion. Because ions with the exact same m/z but with different energies appear at different frequencies, the probability of ion−ion interference is significantly reduced. We show that the measured mass of a protein complex consisting of 16 protomers, RuBisCO (517 kDa), is not affected by the number of trapped ions with up to 21 ions trapped simultaneously in these experiments. Ion−ion interactions do not affect the ion trapping lifetime up to 1 s, and there is no influence of the number of ions on the measured charge-state distribution of bovine serum albumin (66.5 kDa), indicating that ion−ion interactions do not adversely affect any of these measurements. Over an order of magnitude gain in measurement speed over single ion analysis is demonstrated, and significant additional gains are expected with this multi-ion measurement method.
A general method for in situ measurements of the energy of individual ions trapped and weighed using charge detection mass spectrometry (CDMS) is described. Highly charged (> 300 e), individual polyethylene glycol (PEG) ions are trapped and oscillate within an electrostatic trap, producing a time domain signal. A segmented Fourier transform (FT) of this signal yields the temporal evolution of the fundamental and harmonic frequencies of ion motion throughout the 500-ms trap time. The ratio of the fundamental frequency and second harmonic (HAR) depends on the ion energy, which is an essential parameter for measuring ion mass in CDMS. This relationship is calibrated using simulated ion signals, and the calibration is compared to the HAR values measured for PEG ion signals where the ion energy was also determined using an independent method that requires that the ions be highly charged (> 300 e). The mean error of 0.6% between the two measurements indicates that the HAR method is an accurate means of ion energy determination that does not depend on ion size or charge. The HAR is determined dynamically over the entire trapping period, making it possible to observe the change in ion energy that takes place as solvent evaporates from the ion and collisions with background gas occur. This method makes it possible to measure mass changes, either from solvent evaporation or from molecular fragmentation (MS), as well as the cross sections of ions measured using CDMS. Graphical Abstract.
Mechanistic information about how gaseous ions are formed from charged droplets has been difficult to establish because direct observation of nanodrops in a size range relevant to gaseous macromolecular ion...
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