Optimized Electrostatic Linear Ion Trap for Charge Detection Mass Spectrometry

In charge detection mass spectrometry (CDMS), ions are passed through a detection tube and the m/z ratio and charge are determined for each ion. The uncertainty in the charge and m/z determinations can be dramatically reduced by embedding the detection tube in an electrostatic linear ion trap (ELIT) so that ions oscillate back and forth through the detection tube. The resulting time domain signal can be analyzed by fast Fourier transforms (FFTs). The ion’s m/z is proportional to the square of the oscillation frequency, and its charge is derived from the FFT magnitude. The ion oscillation frequency is dependent on the physical dimensions of the trap as well as the ion energy. A new ELIT has been designed for CDMS using the central particle method. In the new design, the kinetic energy dependence of the ion oscillation frequency is reduced by an order of magnitude. An order of magnitude reduction in energy dependence should have led to an order of magnitude reduction in the uncertainty of the m/z determination. In practice, a factor of four improvements was achieved. This discrepancy is probably mainly due to the trajectory dependence of the ion oscillation frequency. The new ELIT design uses a duty cycle of 50%. We show that a 50% duty cycle produces the lowest uncertainty in the charge determination. This is due to the absence of even-numbered harmonics in the FFT, which in turn leads to an increase in the magnitude of the peak at the fundamental frequency.

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The FUNPET—a New Hybrid Ion Funnel-Ion Carpet Atmospheric Pressure Interface for the Simultaneous Transmission of a Broad Mass Range

An atmospheric pressure interface transports ions from ambient pressure to the low-pressure environment of a mass spectrometer. A capillary coupled to an ion funnel is widely used. However, conventional ion funnels do little to negate the large amount of energy picked up by high-mass ions from the gas flow through the capillary. There has been little work done on the effects of gas flow on ion transmission, and the previous studies have all been limited to low-mass, low-charge ions. In this work, we account for the effects of gas flow, diffusion, and electric fields (static and oscillating) on ion trajectories and use simulations to design a new hybrid ion funnel-ion carpet (FUNPET) interface that transmits a broad mass range with a single set of instrument conditions. The design incorporates a virtual jet disruptor where pressure buildup and counter flow dissipate the supersonic jet that results from gas flow into the interface. This, and the small exit aperture that can be used with the FUNPET, reduces the gas flow into the next stage of differential pumping. The virtual jet disruptor thermalizes ions with a broad range of masses (1 kDa to 1 GDa), and once thermalized, they are transmitted into next region of the mass spectrometer with low excess kinetic energy. The FUNPET interface is easy to fabricate from flexible printed circuit board and a support frame made by 3D printing. The performance of the interface was evaluated using charge detection mass spectrometry.

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Internal energy and angular momentum effects on collision induced dissociation fragmentation patterns

Data are reported on the effects of internal energy and angular momentum on the collision induced dissociation CID fragmentation pattern. For the ions studied changes in the relative intensities of the fragment ions as internal energy varied were found to be larger than suggested by McLafferty and coworkers. Possible effects of angular momentum on the CID fragmentation pattern are discussed. The charge stripping spectra of the ions studied were found to be strongly dependent on initial energy and/or angular momentum. Hence care must be taken if charge stripping spectra are used to distinguish ion structures.     

Electric susceptibility of unsolvated glycine-based peptides

The DC electric susceptibilities of unsolvated glycine-based peptides, WGn (W = tryptophan and G = glycine) with n = 1-5, have been measured by deflection of a molecular beam in an electric field. These are the first electric deflection measurements performed on peptides. At 300 K the susceptibilities are in the range of 200-400 A(3). By far the largest contribution to the susceptibilities is from the permanent dipole moment of the peptides. The results indicate that the peptides do not have rigid conformations with fixed dipoles. Instead the dipole is averaged as the peptides explore their energy landscape. For a given WGn peptide, all molecules have almost the same average dipole, which suggests that they all explore a similar energy landscape on the microsecond time scale of the measurement. The measured susceptibilities are in good overall agreement with values calculated from the average dipole moment deduced from Monte Carlo simulations.     

The initial steps in the hydration of unsolvated peptides: water molecule adsorption on alanine-based helices and globules

Equilibrium constants for the adsorption of the first water molecule onto a variety of unsolvated alanine-based peptides have been measured and Delta H degrees and DeltaS degrees have been determined. The studies were designed to examine the effects of conformation, charge, and composition on the propensity for peptides to bind water. In general, water adsorption occurs significantly more readily on the globular peptides than on helical ones: several of the singly charged helical peptides were not observed to adsorb a water molecule even at -50 degrees C. These results place a limit on the free energy change for interaction between a water molecule and the helical peptide group. Molecular dynamics simulations reproduce most of the main features of the results. The ability to establish a network of hydrogen bonds to several different hydrogen-bonding partners emerges as a critical factor for strong binding of the water molecule. Whether the charge site is involved in water adsorption depends on how well it is shielded. Peptides containing a protonated histidine bind water much more strongly that those containing a protonated lysine because the delocalized charge on histidine is difficult to shield. The entropy change for adsorption of the first water molecule is correlated with the enthalpy change.     

On the structure and photodissociation of cluster ions in the gas phase. (N2) (O2+) and (NO)2+

Results are presented for two experiments on N₂O₂⁺ cluster ions formed via the reactions O₂⁺ + N₂ + M → (N₂) (O₂⁺) + M (i), and NO⁺ + NO + M → (NO)₂⁺ + M (ii). In the first experiment the N₂O₂⁺ clusters are collisionally dissociated. The resulting collision-induced dissociation (CID) spectra show almost exclusively O₂⁺ and N₂⁺ products from N₂ O₂⁺ formed via the first reaction, and almost exclusively NO⁺ products from N₂O₂⁺ formed via the second reaction. In the second experiment, single-photon photodissociation of N₂O₂⁺ ions produced by both reactions (i) and (ii) was investigate using 514.5 and 634 nm radiation. The results indicate that the N₂O₂⁺ cluster from reaction (i) cannot be photodissociated while the N₂O₂⁺ cluster from reaction (ii) undergoes photodissociation at both wavelengths. These experiments indicate that two distinct N₂O₂⁺ cluster ions exist and that reactions (i) and (ii) selectively produce the two ions.     

The mobile proton in polyalanine peptides

Ion mobility measurements have been performed for protonated polyalanine peptides (A10 + H+, A15 + H+, A20 + H+, A25 + H+, and A15NH2 + H+) as a function of temperature using a new high-temperature drift tube. Peaks due to helices and globules were found at room temperature for all peptides, except for A10 + H+ (where only the globule is present). As the temperature is increased, the helix and globule peaks broaden and merge to give a single narrow peak. This indicates that the two conformations interconvert rapidly at elevated temperatures. The positions of the merged peaks show that A15 + H+ and A15NH2 + H+ spend most of their time as globules when heated, while A20 + H+ and A25 + H+ spend most of their time as helices. The helix/globule transitions are almost certainly accompanied by intramolecular proton transfer, and so, these results suggest that the proton becomes mobile (able to migrate freely along the backbone) at around 450 K. The peptides dissociate as the temperature is increased further to give predominantly the bn(+), b(n-1)(+), b(n-2)(+), ... series of fragment ions. There is a correlation between the ease of fragmentation and the time spent in the helical conformation for the An + H+ peptides. Helix formation promotes dissociation because it pools the proton at the C-terminus where it is required for dissociation to give the observed products. In addition to the helix and globule, an antiparallel helical dimer is observed for the larger peptides. The dimer can be collisionally dissociated by injection into the drift tube at elevated kinetic energies.     

Molecular dynamics simulations of the rehydration of folded and unfolded cytochrome C ions in the vapor phase.

Molecular dynamics (MD) simulations have been performed to study the rehydration of compact and unfolded cytochrome c ions in the vapor phase. Experimental studies have shown that the compact conformations adsorb many more water molecules than unfolded ones when exposed to water vapor. MD simulations performed with up to 150 water molecules reproduce the key experimental observations, including a partial refolding caused by hydration. According to the calculations it is more energetically favorable to hydrate the compact conformation in the initial stages of hydration, because it is easier for a water molecule to interact simultaneously with several polar groups (due to their proximity). The protonated side chains are not favored hydration sites in the simulations because they have "self-solvation" shells which must be disrupted for the water to penetrate. For both conformations, the adsorbed water molecules are mainly located in surface crevices.