Ions In Helium Gas Research Articles

Ion Mobility of Proteins in Nitrogen Gas: Effects of Charge State, Charge Distribution, and Structure.

Ion mobility is emerging as a rapid and sensitive tool for structural characterization. Collision cross-section (Ω) values determined using ion mobility are often compared to values calculated for candidate structures generated through molecular modeling. Several methods exist for calculating Ω values, but the trajectory method explicitly includes contributions from long-range, ion-neutral interactions. Recent implementations of the trajectory method have significantly reduced its expense and have made applications to proteins far more tractable. Here, we use ion mobility experiments and trajectory method calculations to characterize the effects of charge state, charge distribution, and structure on the ion mobility of proteins in nitrogen gas. These results show that ion-induced dipole interactions contribute significantly to the Ω values of these ions with nitrogen gas, even for the modestly charged ions commonly observed in native mass spectrometry experiments. Therefore, these interactions contribute significantly to the values measured in most structural biology and biophysics applications of ion mobility using nitrogen gas. Comparisons between the reciprocal mobilities of protein ions in helium gas and in nitrogen gas show that there are significant, noncorrelated differences between these values. As a consequence, it is challenging to estimate the errors associated with interconverting between helium- and nitrogen-based mobilities without extensive characterization in both gases, even for ions of proteins with similar sequences. Therefore, we recommend reporting Ω and mobility values that are based on the predominant gas present in the separation and applying additional caution when comparing results from mobility experiments performed using different gases.

Effects of Charge State, Charge Distribution, and Structure on the Ion Mobility of Protein Ions in Helium Gas: Results from Trajectory Method Calculations.

Collision cross section (Ω) values of gas-phase ions of proteins and protein complexes are used to probe the structures of the corresponding species in solution. Ions of many proteins exhibit increasing Ω-values with increasing charge state but most Ω-values calculated for protein ions have used simple collision models that do not explicitly account for charge. Here we use a combination of ion mobility mass spectrometry experiments with helium gas and trajectory method calculations to characterize the extents to which increases in experimental Ω-values with increasing charge state may be attributed to increased momentum transfer concomitant with enhanced long-range interactions between the protein ion and helium atoms. Ubiquitin and C-to-N terminally linked diubiquitin ions generated from different solution conditions exhibit more than a 2-fold increase in Ω with increasing charge state. For native and energy-relaxed models of the proteins and most methods for distributing charge, Ω-values calculated using the trajectory method increase by less than 1% over the range of charge states observed from typical solution conditions used for native mass spectrometry. However, the calculated Ω-values increase by 10% to 15% over the full range of charge states observed from all solution conditions. Therefore, contributions from enhanced ion-induced dipole interactions with increasing charge state are significant but without additional structural changes can account for only a fraction of the increase in Ω observed experimentally. On the basis of these results, we suggest guidelines for calculating Ω-values in the context of applications in biophysics and structural biology.

Mobilities of CH3O+, C2H5O+ and C3H7O+ ions in He gas

Mobilities of polyatomic molecule ions in helium gas were measured at 85K and room temperature using a low-temperature drift tube. They differed from the typical behavior shown by atomic or small molecular ions in helium owing to the influence of attractive interactions among the molecular ions and also their size and shape. To investigate the relationship between ion size and mobility, we measured the mobilities of CH3O+, C2H5O+, and C3H7O+.

State-selective ion mobility measurement for NHn+ (n = 1 - 3) ions in helium gas at low temperature

SynopsisWe have measured ion mobility in cooled helium gas for NH+, NH+2, and NH+3 at 77 and 4.3 K. In the measurement of NH+ and NH+3, we have observed double peaks in the arrival time spectra, which correspond to different electronic states of molecular ions. These results can be regarded as the first observation of the metastable states in mobility measurement for these ions.

Mobilities of Li+ - (2-butanol)n (n = 1-2) ions in He gas

SynopsisMobilities of Li+- (2-butanol)n (n = 1-2) ions in helium gas have been measured at 83 K and room temperature using a low temperature drift tube. The experimental mobilities are significantly smaller than the polarization limit values within the measured effective temperature range. Using the 12-6-4 potential, we have performed calculation of the mobilities in frame of the two-temperature momentum transfer theory. The agreement between the experimental and calculated mobilities is fairly good.

Determination of HeO + and HeO − interaction potentials from gaseous ion-mobility data

The ion–neutral interaction potentials of HeO + and HeO − are calculated from experimental gaseous ion-mobility data by using the direct inversion method. The obtained potentials are in good agreement with recent ab initio potentials, and they were used to obtain best parameters for the Morse–Spline–Van der Waals (MSV) potential model. The MSV potentials were then employed to compute the mobilities of O + and O − ions in helium gas. Results show that the MSV model potentials reproduce the experimental ion mobilities within their experimental accuracy.

Monte Carlo calculations of drift velocities and diffusion coefficients of Ar + ions in helium

Results are presented for the calculated drift velocities and diffusion coefficients for Ar + ions in helium at atmospheric pressure, temperature T=300 K and for reduced electric fields E/ N from about 1 Td up to about 150 Td, using Monte Carlo techniques. The drift velocities range from 5.94×10 3 to 559.0×10 3 cm s −1 for the Ar + ions in the ground state 2P 3/2 and from 5.85×10 3 to 545.0×10 3 cm s −1 for the Ar + ions in the metastable excited state 2P 1/2. These values are in good agreement (within about 5%) with the few experimental values available. The mobilities and diffusion coefficients for atomic Ar + ions in helium gas show no significant dependence on the spin state of the ion.

The mobility of He+ ions in helium gas

Recent values for the X 2u+ and A 2g+ He2+ potentials have been used to determine quantal phase shifts. These phase shifts have been used to compute transport cross sections that are highly accurate over a wide range of energy. A Gram-Charlier treatment of the Boltzmann kinetic equation has been used to calculate from the cross sections the mobility of He+ ions in He gas at temperatures of 2, 4.35, 5, 6, 77, 85 and 300 K for E/N values up to 40 Td, where E is the electric field strength and N is the gas number density. Excellent agreement is obtained with experimental values at 77 and 300 K. Differences of up to 4% are obtained with experiment at 4.35 K, supporting a suggestion that there may be additional uncertainties in the pressure measurements employed in this experiment. The 85 K observations are between 4% and 6% below our calculations, a somewhat larger difference than the quoted experimental uncertainty. Excellent agreement is obtained with the transverse diffusion coefficient measurements at 294 K.

Absence of quantum-mechanical effects on the mobility of argon ions in helium gas at 4.35 K

Quantum-mechanical transport cross sections are accurately computed from recent spectroscopic potentials for the three lowest energy levels of the molecular ion HeAr+. Statistical and ground state combinations of the cross sections are used to compute the transport coefficients describing the motion of Ar+ ions through He gas at 4.35 K. The calculated mobilities do not show the rapid decrease of the experimental values as the ratio of the electric field strength to the gas number density becomes very small. The experimental observations therefore are not due to the effects of orbiting resonance.

Transport coefficients for NO+ ions in helium gas: a test of the NO+He interaction potential

Abstract Transport cross-sections for the collision of positive nitrogen oxide ions with helium atoms have been computed from a theoretical NO + He interaction potential. These cross-sections have been used in a kinetic theory of diatomic ion motion in atomic gases to determine the mobility and diffusion coefficients parallel and perpendicular to an external electric field. Comparison of the calculated mobilities with experimental data shows that theory and experiment agree within their mutual uncertainties.

Symmetry-adapted perturbation theory potential for the HeK+ molecular ion and transport coefficients of potassium ions in helium

The interaction potential for the HeK+ system has been computed as the sum of attractive and repulsive contributions due to the electrostatic, exchange, induction, and dispersion interactions using the symmetry-adapted perturbation theory and a high-level treatment of electron correlation. The zero of the theoretical potential occurs at 4.704 bohr and the minimum occurs at 5.418 bohr where the potential value is −0.779 mhartree. The potential supports 36 bound rovibrational levels, and the ground state of the HeK+ molecular ion is bound by 125.1 cm−1. For all interatomic distances the ab initio potential agrees very well with the empirical potential obtained by direct inversion of the K+ mobilities in gaseous helium and disagrees with the potential obtained from the ion–beam scattering cross section data. The ab initio potential has been used to compute the transport coefficients of potassium ions in helium gas over a wide range of temperature and reduced field strength. A very good agreement of the calculated transport coefficients with the preponderance of the existing experimental data is observed. This agreement leads us to believe that the present interaction potential is accurate to within a few percent at all separations between 4.0 and 10.0 bohr and, consequently, represents the most accurate potential available for the HeK+ system.

The distribution of velocities for Ba+ ions in Ar gas

The ground-state potential for singly-charged barium ions interacting with argon atoms is inferred from laser-induced fluorescence measurements of the gaseous ion transport coefficients. The potential is used to compute velocity component distribution functions and fluorescence spectra. Comparison with the measured spectra provides insight into the accuracy with which the potential can be inferred and with which the distribution function and spectra can be calculated. Failure of a similar study of barium ions in helium gas is possibly due to inelastic collisions that invalidate a single-potential description of this system.

Search for Fractionally Charged Ions in Helium Gas

A Van de Graaff accelerator facility has been used as a supersensitive mass spectrograph in a search for free $+\frac{1}{3}|e|$ and $+\frac{2}{3}|e|$ charged ions in helium gas. No such ions were found at a level of sensitivity better than 1 part in 3.9\ifmmode\times\else\texttimes\fi{} ${10}^{14}$ He atoms. The mass ranges spanned were 0.13 to 7.5 amu for $+\frac{1}{3}|e|$ ions and 0.27 to 15.0 amu for $+\frac{2}{3}|e|$ ions.

Mobilities of H +3 and HeH + ions in helium gas and rate coefficient for HeH + + H 2 → H +3 + He

The mobilities of mass-identified H + 3 and HeH + ions in helium and the reaction rate coefficient for HeH + + H 2 → H + 3 + He have been measured by a drift-tube quadrupole mass spectrometer at 300 K. The zero-field reduced mobilities of H + 3 and HeH + ions, corrected to 273 K, are 31.0 ± 0.8 and 23.4 ± 0.6 cm 2 V −1 s −1 respectively. The reaction rate coefficient was found to be (1.26 + 0.16) × 10 −9 cm 3s −1 and was observed to be independent of the mean ion kinetic energy in the range from 0.04 to 0.3 eV.

Energy loss measurements of keV-ions in gases by time-of-flight spectroscopy

A time-of-flight technique which allows a fast and precise measurement of the stopping-power of gases for keV-ions has been developed. First results are reported for the stopping cross-section of lead ions in helium gas in the energy range 25 keV ≤ E 0 ≤ 75keV. The data are compared with the Linhard theory.

The mobility of mass-identified D+ions in helium gas

The mobility of mass identified D+ ions in helium gas has been measured by the 'four-gauze' electrical shutter method. The low field mobility corrected to 273K is 24.9+or-0.3cm2V-1s-1. The helium gas pressure in the mobility tube was in the range 1.85-4.12 Torr, and the deuterium gas concentration was about 0.01%.

Charge Equilibrium of Helium Ions in Helium Gas from 60 to 840 keV

Charge equilibrium fractions corresponding to the components ${(\mathrm{He})}^{0}$, ${(\mathrm{He})}^{+}$, and ${(\mathrm{He})}^{++}$ of a helium beam in helium gas have been measured. Singly ionized beams of ${\mathrm{He}}^{4}$ and doubly ionized beams of ${\mathrm{He}}^{3}$ were used. Ion velocities covered the range corresponding to energies of ${({\mathrm{He}}^{4})}^{+}$ ions from 60 to 840 keV. The charge-equilibrated beam was magnetically separated and charge spectra were recorded by means of a movable detector of equal sensitivity for all three charge components. In the energy interval where they overlap, namely between 60 and 200 keV, the measured equilibrium fractions are in excellent agreement with those of Barnett and Stier. No systematic discrepancy between the equilibrium fractions obtained at equal velocities with beams of the isotopes ${\mathrm{He}}^{3}$ and ${\mathrm{He}}^{4}$ was observed. The results were discussed in terms of the probabilities ${M}_{\mathrm{I}}$ for presence of any one of the two electrons in ${(\mathrm{He})}^{0}$ and ${M}_{\mathrm{II}}$ for presence of the one electron in ${(\mathrm{He})}^{+}$ as a function of the beam velocity $v$. Similarity of the curves for ${M}_{\mathrm{I}}$ and ${M}_{\mathrm{II}}$ allows for their superposition by a translation transformation on the velocity scale by a constant approximately equal to the "orbital" velocity ${u}_{\mathrm{I}}$ of either electron in ${(\mathrm{He})}^{0}$ as calculated from its first ionization energy. This is in disagreement with Dmitriev's assumption that the presence probabilities ${M}_{i}$ can be described by a universal function $M(\frac{v}{{u}_{i}})$.

Mobilities and Reaction Rates of Ions in Helium

The mobility of ${\mathrm{He}}^{+}$ ions in helium gas was measured, the result being 10.40\ifmmode\pm\else\textpm\fi{}0.10 (${\mathrm{cm}}^{2}$/V-sec). The ${\mathrm{He}}^{+}$ ions were observed to react with the helium gas, the attachment frequency being [1.08\ifmmode\pm\else\textpm\fi{}0.05]\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}31}$ ${n}^{2}(invsec)$, where $n$ is the gas density in atoms/${\mathrm{cm}}^{3}$. This value of the attachment frequency is roughly compatible with the theoretical estimate of Bates and with the mass-spectrometric measurement of Phelps and Brown for the rate of the three-body attachment reaction producing ${\mathrm{He}}_{2}^{+}$. The mobility of the production, ${\mathrm{He}}_{2}^{+}$, was found to be 16.70\ifmmode\pm\else\textpm\fi{}0.17. This mobility is compatible with recent ambipolar diffusion coefficient measurements on cold afterglows, where the identification of the ion is confirmed by the spectroscopic observations of Kerr. This mobility is inconsistent with the widely quoted value of Biondi and Chanin and with the theoretical analyses of Geltman and Dalgarno. Mobility determinations were also made on another ion, which is possibly what Biondi and Chanin called ${\mathrm{He}}_{2}^{+}$. The actual identity is unknown, the ion possibly being ${\mathrm{He}}^{2+}$, possibly a contaminant.

Double Charge Transfer and the Mobility of He2+Ions in Helium

The interaction energies between a He2+ ion and a He atom are calculated for a wide range of internuclear distances. These energies are used to determine the cross section for double charge transfer between He2+ ions and He atoms as well as the diffusion cross section and mobility of these ions in helium gas.

The mobility of positive ions in helium. Part I.―Helium ions

In a previous paper we gave an account of some experiments on the mobility of positive ions in helium gas. It was shown that minute traces of impurity profoundly affect the speed at which the charge is carried through the gas, so that very little significance can be attached to the values of the mobility of ions recorded in the literature. In particular we studied the behaviour of carefully purified helium gas when admitted at a pressure of 360 mm. of mercury into a baked-out apparatus. In our method of determining the mobility, a single group of ions gives rise to a peak in a current frequency curve, and it is easy to follow the changes produced by an impurity. Initially the ions in the helium had a high mobility and in one case we obtained a group with a mobility of the order of 17 cm./sec./volt/cm. at 760 mm. As the gas became contaminated by impurities gradually evolved from the glass walls and metal parts of the apparatus, peaks corresponding to groups of smaller mobility appeared, and eventually the charge was all carried by a group with mobility rather less than 8. The ions in these experiments were produced by means of α-particles from polonium. Traces of impurity in helium may affect the results in several ways. Firstly, they may lead to the formation of ionic clusters, particularly if the molecules of the impurity have a permanent dipole. In our experiments it seemed unlikely that molecules capable of forming clusters were present. Secondly, although with small concentrations of an impurity a negligible number of its ions will be produced by the direct action of the α-rays, they may be formed indirectly in either of the two following ways:—