Traveling wave ion mobility spectrometers (TW IMS) operate at significantly higher fields than drift tube ion mobility spectrometers. Here we measured the fragmentation of the fragile p-methoxybenzylpyridinium ion inside the TW ion mobility cell of the first-generation SYNAPT HDMS spectrometer. The ion's vibrational internal energy was quantified by a vibrational effective temperature T(eff,vib), which is the mean temperature of the ions inside the cell that would result in the same fragmentation yield as observed experimentally. Significant fragmentation of the probe ion inside the TW IMS cell was detected, indicating that field heating of the ions takes place in TW IMS. For typical small molecule IMS conditions, T(eff,vib) = 555 ± 2 K. The variations of the effective temperature were studied as a function of the IMS parameters, and we found that T(eff,vib) decreases when the wave height decreases, when the pressure increases, or when the wave speed increases. The energy transfer efficiency of argon is higher than for He, N(2), or CO(2). With T(eff,vib) being directly related to the ion speed inside the TW IMS, our results also provide new insight on the ion movement in TW IMS. We also discuss the influence of field heating of ions for calibration and structural studies in TW IMS.
Modern ion mobility instrumentation is typically operated above the low field limit, which may activate the ions and cause structural rearrangement or fragmentation during analysis. Here, we quantitatively assessed the internal heating experienced by ions during trapped ion mobility spectrometry (TIMS) experiments. To this end, the fragmentation yields of fragile benzylpyridinium “thermometer” ions were monitored during both the accumulation and analysis steps inside the TIMS tunnel. The corresponding fragmentation rate constants were translated into a vibrational effective temperature T eff,vib. Our results demonstrate significant fragmentation upstream and inside the TIMS tunnel that corresponds to T eff,vib ≈ 510 K during both the accumulation and analysis steps. Broadening our scope to cytochrome c and lysozyme, we showed that although compact “native” folds can be preserved, the collision cross section distributions are highly sensitive to the transmission voltages and the analysis time scale. Our results are discussed with regard to T eff,vib data previously acquired on traveling-wave (TWIMS) ion mobility in the context of native mass spectrometry and conformational landscape exploration.
During their travel inside a traveling wave ion mobility cell (TW IMS), ions are susceptible to heating because of the presence of high intensity electric fields. Here, we report effective temperatures Teff,vib obtained at the injection and inside the mobility cell of a SYNAPT G2 HDMS spectrometer for different probe ions: benzylpyridinium ions and leucine enkephalin. Using standard parameter sets, we obtained a temperature of ~800 K at injection and 728 ± 2 K into the IMS cell for p-methoxybenzylpyridinium. We found that Teff,vib inside the cell was dependent on the separation parameters and on the nature of the analyte. While the mean energy of the Boltzmann distributions increases with ion size, the corresponding temperature decreases because of increasing numbers of vibrational normal modes. We also investigated conformational rearrangements of 7+ ions of cytochrome c and reveal isomerization of the most compact structure, therefore highlighting the effects of weak heating on the gas-phase structure of biologically relevant ions.
Hyperbranched and star shaped polymers have raised tremendous interest because of their unusual structural and photochemical properties, which provide them potent applications in various domains, namely in the biomedical field. In this context, the development of adequate tools aiming to probe particular three-dimensional features of such polymers is of crucial importance. In this present work, ion mobility coupled with mass spectrometry was used to experimentally derive structural information related to cationized linear and star shaped poly-ε-caprolactones as a function of their charge state and chain length. Two major conformations were observed and identified using theoretical modeling: (1) near spherical conformations whose sizes are invariant with the polymer topology for long and lightly charged chains and (2) elongated conformations whose sizes vary with the polymer topology for short and highly charged chains. These conformations were further confirmed by collisional activation experiments based on the ejection thresholds of the coordinated cations that vary according to the elongation amplitude of the polymer chains. Finally, a comparison between solution and gas-phase conformations highlights a compaction of the structure with a loss of specific chain arrangements during the ionization and desolvation steps of the electrospray process, fueling the long-time debated question related to the preservation of the analyte structure during the transfer into the mass spectrometer.
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