Bath Gas Temperature Research Articles

Validation of Field-Dependent Ion-Solvent Cluster Modeling via Direct Measurement of Cluster Size Distributions.

Ion mobility spectrometry is widely used in analytical chemistry, either as a stand-alone technique or coupled to mass spectrometry. Ions in the gas phase tend to form loosely bound clusters with surrounding solvent vapors, artificially increasing the collisional cross section and the mass of the ion. This, in turn, affects ion mobility and influences separation. Further, ion-solvent clusters play an important role in most ionization mechanisms occurring in the gas phase. Consequently, a deeper understanding of ion-solvent cluster association and dissociation processes is desirable to guide experimental design and interpretation. A few computational models exist, which aim to describe the amount of clustering as a function of the reduced electric field strength, bath gas pressure and temperature, and the chemical species probed. It is especially challenging to model ion mobility under high reduced electrical field strengths due to the nonthermal conditions created by the field. In this work, we aim to validate a recently proposed first-principles model by comparing its predictions with direct measurements of cluster size distributions over a range of 20-120 Td as observed using a High Kinetic Energy Ion Mobility Spectrometer coupled to a mass spectrometer (HiKE-IMS-MS). By studying H+(H2O)n, [MeOH + H + n(H2O)]+, [ACE + H + n(H2O)]+, and [PhNH2 + H + n(H2O)]+ as test systems, we find very good agreement between model and experiment, supporting the validity of the computational workflow. Further, the detailed information gained from the modeling yields important insights into the cluster dynamics within the HiKE-IMS, allowing for better interpretation of the measured ion mobility spectra.

Three-dimensional particle size determination in a laminar diffusion flame by tomographic laser-induced incandescence

Non-intrusive measurement techniques are required to gain a comprehensive understanding about the processes of soot formation, growth and oxidation. Time-resolved laser-induced incandescence (TiRe-LII), commonly performed 0D or 2D within a flame, has proven to be a very suitable tool for the in situ sizing of soot primary particles. In this work, the technique is expanded to the third dimension by employing volumetric illumination and coupling it with a tomographic approach, which allows to computationally gain 3D information from 2D images taken at various angles. To minimize experimental cost, an approach using nine fiber bundles arranged in a semicircle around the flame and imaging the light onto a single camera is used. The technique is demonstrated on an ethene diffusion flame on a standard burner, providing spatially resolved 3D particle sizes. One focus of this work is to reveal the influence of input parameters such as the local bath gas temperature, which we measured by two-color pyrometry, and local laser fluence, which are both required for an accurate evaluation of the local particle size. It is shown that the assumption of an average temperature may result in a wrong picture even of qualitative soot size evaluation. In the end, a concept is proposed for a simultaneous determination of the 3D distribution of particle sizes through TiRe-LII and the required bath gas temperature via two-color pyrometry using a tomographic approach with only three cameras.

Simplified Representation of Multichannel Thermal Unimolecular Reactions. II. Refined Parametrization of Formaldehyde Dissociation

Abstract Simplified representations of branching fractions for thermal unimolecular two-channel reactions are discussed. The dissociation of formaldehyde serves as an illustrative example. Quantum-corrected classical trajectory calculations on an ab initio potential energy surface are combined with master equation calculations for collisional energy transfer. The treatment accounts for roaming atom dynamics. The dependence of the channel branching fractions on the bath gas pressure and temperature, on the collision efficiencies, and on the difference of channel threshold energies, are explored. It is discussed to what extent the derived simplified representations of channel branching fractions can be generalized.

How Hot Are Your Ions in Differential Mobility Spectrometry?

Ions can experience significant field-induced heating in a differential mobility cell. To investigate this phenomenon, the fragmentation of several para-substituted benzylpyridinium "thermometer" ions (R = OMe, Me, F, Cl, H, CN) was monitored in a commercial differential mobility spectrometer (DMS). The internal energy of each benzylpyridinium derivative was characterized by monitoring the degree of fragmentation to obtain an effective temperature, Teff, which corresponds to a temperature consistent with treating the observed fragmentation ratio using a unimolecular dissociation rate weighted by a Boltzmann distribution at a temperature T. It was found that ions are sufficiently thermalized after initial activation from the ESI process to the temperature of the bath gas, Tbath. Once a critical field strength was surpassed, significant fragmentation of the benzylpyridinium ions was detected. At the maximum bath gas temperature (450 K) and separation voltage (SV; 4400 V) for our instrument, Teff for the benzylpyridinium derivatives ranged from 664 ± 9 K (p-OMe) to 759 ± 17 K (p-H). The extent of activation at a given SV depends on the ion's mass, degrees of freedom, (NDoF), and collision frequency as represented by the ion's collision cross section. Plots of Teff vs the product of ion mass and NDoF and the inverse of collision cross section produce strong linear relationships. This provides an attractive avenue to estimate ion temperatures at a given SV using only intrinsic properties. Moreover, experimentally determined Teff correlate with theoretically predicted Teff using with a self-consistent method based on two-temperature theory. The various instrumental and external parameters that influence Teff are additionally discussed.

Energy transfer in intermolecular collisions of polycyclic aromatic hydrocarbons with bath gases He and Ar.

Classical trajectory simulations of intermolecular collisions were performed for a series of polycyclic aromatic hydrocarbons interacting with the bath gases helium and argon for bath gas temperature from 300 to 2500 K. The phase-space average energy transferred per deactivating collision, ⟨∆Edown⟩, was obtained. The Buckingham pairwise intermolecular potentials were validated against high-level quantum chemistry calculations and used in the simulations. The reactive force-field was used to describe intramolecular potentials. The dependence of ⟨∆Edown⟩ on initial vibrational energy is discussed. A canonical sampling method was compared with a microcanonical sampling method for selecting initial vibrational energy at high bath gas temperatures. Uncertainties introduced by the initial angular momentum distribution were identified. The dependence of the collisional energy transfer parameters on the type of bath gas and the molecular structure of polycyclic aromatic hydrocarbons was examined.

Sensitivity analysis for soot particle size imaging with laser-induced incandescence at high pressure

Soot particle sizes can be determined from time-resolved laser-induced incandescence (LII) in point measurements where full signal traces are detected. For instantaneous imaging, strategies are required that must cope with time-gated information and that rely on assumptions on the local boundary conditions. A model-based analysis is performed to identify the dependence of LII particle-size imaging on the assumed boundary conditions such as bath gas temperature, pressure, particle heat-up temperature, accommodation coefficients, and soot morphology. Various laser-fluence regimes and gas pressures are considered. For 60 bar, fluences that lead to particle heat-up temperatures of 3400– 3900 K provided the lowest sensitivity on particle-sizing. Effects of laser attenuation are evaluated. A combination of one detection gate starting at the signal peak and the other starting with 5 ns delay was found to provide the highest sensitivity at 60 bar. The optimum gate delays for different pressures are shown. The effects of timing jitter and poly-dispersity are investigated. Systematic errors in pyrometry imaging at 60 bar is evaluated.

Water ice nanoparticles: size and temperature effects on the mid-infrared spectrum

Mid-infrared spectra have been measured for cubic ice (I(c)) nanoparticles (3-150 nm diameter) formed by rapid collisional cooling over a wide range of temperatures (5-209 K). Spectral diagnostics, such as the ratio of surface related dangling OH to interior H-bonded OH stretch bands, reveal the manner in which particle size depends on bath gas temperature and density, and on water molecule concentration. For particles smaller than 5 nm strained intermolecular bonds on the surface and subsurface cause the predominant OH stretch peak position to be dramatically blue shifted by up to 40 cm(-1). In the size regime of 8-200 nm the position of the OH stretch absorption band maximum is relatively unaffected by particle size and it is possible to measure the temperature dependence of the peak location without influences from the surface or scattering. The band maximum shifts in a linear fashion from 3218 cm(-1) at 30 K to 3253 cm(-1) at 209 K, which may assist with temperature profiling of ice particles in atmospheric clouds and extraterrestrial systems. Over the same temperature range the librational mode band shifts very little, from 870 to 860 cm(-1). In the water stretching and bending regions discrete spectral features associated with the surface or sub-surface layers have been detected in particles as large as 80 nm.

The effect of ion trap temperature on the dissociation of peptide ions in a quadrupole ion trap

Thermally-assisted collision-induced dissociation can substantially increase the amount of dissociation in quadrupole ion trap mass spectrometer experiments. The experiments discussed here were performed to assess the effect on dissociation pathways as the bath gas temperature is increased in thermally-assisted collision-induced dissociation (TA-CID) experiments. Double resonance experiments in which a product ion was ejected during collision-induced dissociation of the parent ion provided data to asses competitive versus consecutive dissociation pathways. Consecutive dissociation pathways are indicated when lower mass product ions decrease in intensity when a higher mass product ion is ejected during CID. For the peptide ions studied, those that dissociate to give predominately N-terminal product ions show increased consecutive dissociation with increased temperature during TA-CID. For peptide ions that dissociate via formation of C-terminal product ions, competitive dissociation pathways were more prevalent and increasing the temperature had much less effect. N-terminal product ions consecutively dissociate to smaller N-terminal ions whereas C-terminal product ions dissociate to internal fragments.

On the Structure Elucidation Using Ion Mobility Spectrometry and Molecular Dynamics

A new approach is described for the elucidation of gas-phase peptide ion structures combining ion mobility spectrometry (IMS) data and molecular dynamics (MD)-cluster analysis (CA) prediction. The new approach is based on the determination of the gas-phase ion structure identity vectors (e.g., structure and population vectors) that generate the total conformational space of the gas-phase ion as a function of the IMS experimental conditions (e.g., field strength, pressure, bath gas temperature, and IM cell geometry). Two methods to efficiently sample the gas-phase conformational space of molecular ions as a function of the effective ion temperature characteristic of the IMS experiments are described: (i) a simulated annealing MD-CA-constant temperature MD-CA, and (ii) a generalized non-Boltzmann sampling MD-free energy analysis-CA. The new theoretical method has been successfully applied to two model peptide ions (Bradykinin fragments 1-5 and 1-8, RPPGF and RPPGFSPF, respectively) for which multiple conformations sensitive to the effective ion temperature have been suggested in previous studies.

Approach to Thermal Equilibrium in Atomic Collisions

The energy relaxation of fast atoms moving in a thermal bath gas is explored experimentally and theoretically. Two time scales characterize the equilibration, one a short time, in which the isotropic energy distribution profile relaxes to a Maxwellian shape at some intermediate effective temperature, and the second, a longer time in which the relaxation preserves a Maxwellian distribution and its effective temperature decreases continuously to the bath gas temperature. The formation and preservation of a Maxwellian distribution does not depend on the projectile to bath gas atom mass ratio. This two-stage behavior arises due to the dominance of small angle scattering and small energy transfer in the collisions of neutral particles. Measurements of the evolving Doppler profiles of emission from excited initially energetic nitrogen atoms traversing bath gases of helium and argon confirm the theoretical predictions.

Determination of Cooling Rates in a Quadrupole Ion Trap

Collisional cooling rates of infrared excited ions are measured in a quadrupole ion trap (QIT) mass spectrometer at different combinations of temperature and pressure. Measurements are carried out by monitoring fragmentation efficiency of leucine enkephalin as a function of irradiation time by an infrared laser after a short excitation and incrementally increasing cooling periods. Cooling rates are observed to be directly related to bath gas pressure and inversely related to bath gas temperature. The cooling rate at typical ion trap operating pressure (1 mTorr) and temperature (room T) is faster than can be measured. At elevated temperature and the lowest pressure used for the studies, the rate of collisional cooling becomes negligible compared to the rate of radiative cooling.

Electron-transfer ion/ion reactions of doubly protonated peptides: effect of elevated bath gas temperature.

In this study, the electron-transfer dissociation (ETD) behavior of cations derived from 27 different peptides (22 of which are tryptic peptides) has been studied in a 3D quadrupole ion trap mass spectrometer. Ion/ion reactions between peptide cations and nitrobenzene anions have been examined at both room temperature and in an elevated temperature bath gas environment to form ETD product ions. From the peptides studied, the ETD sequence coverage tends to be inversely related to peptide size. At room temperature, very high sequence coverage (approximately 100%) was observed for small peptides (< or =7 amino acids). For medium-sized peptides composed of 8-11 amino acids, the average sequence coverage was 46%. Larger peptides with 14 or more amino acids yielded an average sequence coverage of 23%. Elevated-temperature ETD provided increased sequence coverage over room-temperature experiments for the peptides of greater than 7 residues, giving an average of 67% for medium-sized peptides and 63% for larger peptides. Percent ETD, a measure of the extent of electron transfer, has also been calculated for the peptides and also shows an inverse relation with peptide size. Bath gas temperature does not have a consistent effect on percent ETD, however. For the tryptic peptides, fragmentation is localized at the ends of the peptides suggesting that the distribution of charge within the peptide may play an important role in determining fragmentation sites. A triply protonated peptide has also been studied and shows behavior similar to the doubly charged peptides. These preliminary results suggest that for a given charge state there is a maximum size for which high sequence coverage is obtained and that increasing the bath gas temperature can increase this maximum.

Application of the Monte Carlo method to modeling of kinetic processes with the participation of polyatomic molecules and clusters: 1. Kinetics of energy transfer during collisions of polyatomic molecules

The Monte Carlo method was used to model the collisional energy transfer for polyatomic molecules within the framework of the statistical theory of reactions. A model describing energy transfer through the formation of a statistical collisional complex was suggested. It was assumed that the total energy of the complex was randomized in the course of collisions and statistically distributed among the internal and translational degrees of freedom. The method was verified by comparing the equilibrium distribution functions for the vibrational, rotational, and total energies of the molecule. The mean energy portion and the root-mean-square energy portion transferred per collision, as functions of the total molecular energy, were determined. The relaxation parameters of the population distribution over energy after a sharp increase in the bath-gas temperature were calculated.

Thermal Dissociation of Gaseous Bradykinin Ions

Singly, doubly, and triply charged bradykinin ions have been dissociated thermally in a quadrupole ion trap operated with a bath gas comprised principally of helium at 1 mTorr. Dissociation rates as a function of bath gas temperature were measured along with product ion spectra. Studies were conducted to show that dissociation rates were independent of ion storage conditions and bath gas pressure. These studies indicate that good approximations to true Arrhenius parameters could be determined from Arrhenius plots using the bath gas temperature as a measure of the internal energy distribution of the parent ion population. Arrhenius parameters derived in this work agree with those reported recently using blackbody-induced radiative dissociation for the singly and doubly charged ions within experimental error. The spectrum of dissociation products were also very similar, reflecting the equivalency of the two methods for deriving Arrhenius parameters. Arrhenius parameters for the triply charged ion (Ea= 0.79 ...

Bath gas temperature and the appearance of ion trap tandem mass spectra of high-mass ions

Bath gas temperature effects upon the appearance of ion trap tandem mass spectra of protonated leucine enkephalin have been studied under a variety of ion activation conditions and over a bath gas temperature range of 298–486 K. Bath gas temperature was found to have two possible effects upon the identities and abundances of product ions observed in tandem mass spectra. At high parent ion dissociation rates (>10 s −1) and short activation times (<30 ms), bath gas temperature principally affects the appearance of tandem mass spectra via its role in determining the kinetics for first generation product ion cooling. In the case of protonated leucine enkephalin, this is primarily reflected in the extent to which the first generation b 4 + ion fragments further to yield the a 4 + ion, the energy supplied for this process arising from the parent ion internal energy. Bath gas temperature is most likely to be an important factor in the appearance of tandem mass spectra collected under the conditions just mentioned, when a first generation product ion tends to fragment at rates comparable to or greater than the parent ion dissociation rate. At low parent ion dissociation rates (>10 s −1), relatively long ion activation times (<100 ms), and elevated bath gas temperatures, dissociation resulting from thermal activation of the first and higher generation fragments can make significant contributions to the tandem mass spectra.

Effective ion internal temperatures achieved via boundary activation in the quadrupole ion trap: protonated leucine enkephalin

The dissociation rate of protonated leucine enkephalin in a quadrupole ion trap was measured as a function of the proximity to which the ions are brought to a stability boundary. A bath gas temperature of 480 K was used so that a dissociation rate could be readily measured even when the parent ions were remote from a stability boundary. Changes in the dissociation rate as a function of d.c. potential applied to the ring electrode of the ion trap, used to bring ions close to a stability boundary, could then be attributed to the ‘boundary activation’ process. A relationship between dissociation rate and parent ion effective internal temperature, derived from a study involving conventional ion trap resonance excitation, was used to estimate effective ion internal temperatures achieved under the boundary activation conditions used here. Effective ion internal temperatures 170 K above the bath gas temperature could readily be achieved using boundary activation. However, the efficiency of the overall boundary activation experiment was compromised by an initial rapid parent ion loss that occurred upon application of the d.c. potential. After correction for this initial ion loss, dissociation rates and relatively low ion ejection rates could be measured. Effective ion internal temperatures achieved with boundary activation are similar to those observed previously using conventional resonance excitation. However, the generally poorer efficiencies associated with boundary activation that are observed with resonance excitation at high dissociation rates suggests that higher ion internal temperatures can be achieved with conventional resonance excitation. Copyright © 1999 John Wiley & Sons, Ltd.

Thermal dissociation in the quadrupole ion trap: ions derived from leucine enkephalin

Rates of dissociation of protonated leucine enkephalin and the b 4 + fragment ion derived from protonated leucine enkephalin have been measured as a function of helium bath gas temperature in a quadrupole ion trap. Dissociation rates were observed to be insensitive to the amplitude of the trapping voltage over the range of values studied. This observation, along with theoretical arguments based on predicted levels of “rf heating,” indicates that any internal excitation of the ions due to ion trap storage is minimal. The bath gas temperature can therefore be used to characterize the internal temperatures of the ions. This approximation is expected to be most valid for high mass ions and low mass bath gases, such as helium. Activation parameters were obtained from Arrhenius plots of the rate data, and master equation modeling of the activation, deactivation, and dissociation processes was performed to provide an indication as to how closely these ions approached high-pressure limit behavior. Protonated leucine enkephalin more closely approached the high-pressure limit than the b 4 + ion due to its larger size and the fact that the activation parameters were derived from somewhat lower dissociation rates. These studies suggest that the quadrupole ion trap operated in the presence of a light, heated bath gas can be used to obtain Arrhenius activation parameters from the dissociation kinetics of relatively high mass ions.

Ion internal temperature and ion trap collisional activation: protonated leucine enkephalin

Protonated leucine enkephalin has been used as a prototypical high-mass ion to yield a quantitative estimate of the relationship between the amplitude of the resonance excitation voltage used in an ion trap collisional activation experiment, and the internal temperature to which an ion can be elevated over the bath gas temperature. The approach involves the measurement of the ion dissociation rate as a function of resonance excitation voltage, and the correlation of dissociation rate with ion internal temperature. The relatively high ion trap dissociation rates observed under typical resonance excitation conditions preclude the direct application of the Arrhenius equation to derive internal temperatures. An empirical determination of the relationship between ion internal temperature and dissociation rate over the rate range of interest here was made via the systematic variation of bath gas temperature. The data suggest a very nearly linear relationship between ion internal temperature and resonance excitation voltage, at least under conditions in which ion ejection is minimal. It is shown that protonated leucine enkephalin ions can be elevated by about 357 K over the bath gas temperature using a monopolar resonance excitation voltage of 540 mV p − p( q z = 0.163) without significant ion ejection. It is also demonstrated that ion internal temperature can be readily increased by increasing the bath gas temperature, by accelerating the ions in the presence of a room temperature bath gas (i.e. conventional ion trap collisional activation), or by a combination of the two approaches.

High-pressure methane oxidation behind reflected shock waves

Experiments on CH 4 oxidation behind reflected shock waves were conducted at elevated pressures, and the results compared to a detailed kinetics model. Mixtures of CH 4 and O 2 dilute in either argon or nitrogen were studied over a wide range of stoichiometry (=0.5–4.0), bath gas dilution (90.0–99.5%), pressure (9–480 atm), and temperature (1410–2040 K), corresponding to total concentrations from 5.6×10 −5 to 3.6×10 −3 mol/cm 3 . Reaction progress was monitored using narrow-line laser absorption of OH at 306 nm, infrared emission of CH 4 near 3.4 μm, and pressure measurements. The measured species time-histories and pressure traces were assembled into an extensive database of characteristic reaction times, peak OH mole fractions, and ignition delay times that can be used for comparisons with detailed kinetics mechanisms. The chemical kinetics model utilized in the present comparisons is the latest GRI mechanism, GRI-Mech 1.2. As a whole, agreement between the model predictions and the experimental measurements is good, particularly for ignition delay times. However, based on the results of certain CH 4 profiles, improvements in the model for high-pressure, fuel-rich conditions are needed. Sensitivity and species contribution analyses were used to identify the most important reactions at pressures up to 500 atm, some of which require more accurate rate coefficients. As pressure increases, the CH 3 removal pathways are altered, the CH 3 formation pathways remain the same, and reactions involving HO 2 become important.

Collisional energy transfer in highly excited molecules: Calculations of the dependence on temperature and internal, rotational, and translational energy

Classical trajectory calculations of the rate of collisional energy transfer between a bath gas and a highly excited polyatomic method, and the average energy transferred per collision, as functions of the bath gas translational energy and temperature, are reported. The method used is that of Lim and Gilbert [J. Phys. Chem. 94, 72 (1990)], which requires only about 500 trajectories for convergence, and generates extensive data on the collisional energy transfer between Xe and azulene, as a function of temperature, initial relative translational energy (E′T), and azulene initial internal energy (E′). The observed behavior can be explained qualitatively in terms of the Xe interacting in a chattering collision with a few substrate atoms, with the collision duration being much too brief to permit ergodicity but with a general tendency to transfer energy from hotter to colder modes (both internal and translational). At thermal energies, trajectory and experimental data show that the root-mean-squared energy transfer per collision, 〈ΔE2〉1/2, is relatively less dependent on E′ than the mean energy transfer 〈ΔE〉. The calculated temperature dependence is weak: 〈ΔE2〉1/2∝T0.3, corresponding to 〈ΔEdown〉∝T0.23. Values for the calculated average rotational energy transferred per collision (data currently only available from trajectories, and required for falloff calculations for radical–radical and ion-molecule reactions) are of the order of kBT, and similar to those for the internal energy; there is extensive collision-induced internal-rotational energy transfer. The biased random walk ‘‘model B,’’ as discussed in text, is found to be in accord with much of the trajectory data and with experiment. This suggests that energy transfer is through pseudorandom multiple interactions between the bath gas and a few reactant atoms; the ‘‘kick’’ given by the force at the turning point of each atom–atom encounter governs the amount of energy transferred. Moreover, a highly simplified version of this model explains why average energies transferred per collision for simple bath gases have the order-of-magnitude values seen experimentally, an explanation which has not been provided hitherto.