Pressure Ion Mobility Research Articles

Study of atmospheric pressure chemical ionization of phthalates in air by ion mobility spectrometry/mass spectrometry.

Phthalates are widely used in consumer products in the chemical industries. Due to their abundance in the milieu, their potentially harmful effect on the environment, human and animal health there is a need for sensitive and fast methods for their detection. Positive polarity Corona Discharge Atmospheric Pressure Chemical Ionization (CD-APCI) in the air was applied for ionization of phthalates. The ionization method for the phthalates was studied by atmospheric pressure Ion Mobility Spectrometry (IMS) and hybrid IMS/orthogonal acceleration Time-of-Flight Mass spectrometry (IMS-oaTOF-MS). CD-APCI IMS and MS spectra of selected phthalates (dimethyl phthalate, diethyl phthalate, diethyl isophthalate, diethyl terephthalate, dipropyl phthalate, diisopropyl phthalate, dibutyl phthalate, diisobutyl phthalate, and dibutyl terephthalate) were recorded. In the case of the ortho- and "iso"-isomers exclusively the protonated molecular ions [M+H]+ were detected. In the case of the para- and meta-isomers and regioisomers, APCI resulted in the appearance of hydrated protonated molecular ions [M + H]+ ·(H2 O)0,1,2 . The ion mobilities, collision cross-sections of these ions in air, as well as the limits of detection (LODs) for the phthalate vapors, were determined. In the case of isomeric phthalates, we have demonstrated the potential of the IMS technique for their separation. The results show that CD-APCI in combination with IMS and IMS-oaTOF-MS is a suitable method for the fast and sensitive detection of phthalates with the potential to separate some isomers.

Possible strategy to use differential mobility spectrometry in real time applications

Differential Mobility Spectrometry (DMS), also called as FAIMS is a variation of atmospheric pressure ion mobility measurement techniques and is capable of providing information about the electric field - mobility dependence of ions. In this method, a combined electric field is used. This field consists of asymmetric oscillating electric field of high intensity and low static field component. Analytical information in DMS is 2-dimensional dependence of ionic current on oscillating field amplitude and the value of static field intensity. The measurement of DMS signal for whole ranges of both variables is time consuming and also generates lot of data. It is a disadvantage of DMS method, which limits the use of this otherwise powerful technology in real time applications that require a response time of few seconds. This paper presents a way to limit measurement time by heuristic knowledge of the properties of the data space and another method based on the concept of Shannon Entropy to find operating parameters satisfying both separation and signal to noise ratio requirements.

Stabilization of gas-phase uranyl complexes enables rapid speciation using electrospray ionization and ion mobility-mass spectrometry

Significant challenges exist when characterizing f-element complexes in solution using traditional approaches such as electrochemical and spectroscopic techniques as they do not always capture information for lower abundance species. However, provided a metal-complex with sufficient stability, soft ionization techniques such as electrospray offer a means to quantify and probe the characteristics of such systems using mass spectrometry. Unfortunately, the gas-phase species observed in ESI-MS systems do not always reflect the solution phase distributions due to the inherent electrochemical mechanism of the electrospray process, ion transfer from ambient to low pressures conditions, and other factors that are related to droplet evaporation. Even for simple systems (e.g. hydrated cations), it is not always clear whether the distribution observed reflects the solution phase populations or whether it is simply a result of the ionization process. This complexity is further compounded in mixed solvent systems and when multiply charged species are present. Despite these challenges, the benefits of mass spectrometry with respect to speed, sensitivity, and the ability to resolve isotopes continue to drive efforts to develop techniques for the speciation of metal complexes. Using an electrospray ionization atmospheric pressure ion mobility mass spectrometer (ESI-apIMS-MS), we demonstrate an approach to stabilize simple uranyl complexes during the ionization process and mobility separation to aid speciation and isotope profile analysis. Specifically, we outline and demonstrate the capacity of ESI-apIMS-MS methods to measure mobilities of different uranyl species, in simple mixtures, by promoting stable gas phase conformations with the addition of sulfoxides (i.e. dimethyl sulfoxide (DMSO), dibutyl sulfoxide (DBSO), and methyl phenyl sulfoxide (MPSO)). Addition of these sulfoxides, as observed in the mass spectrum and mobility domain, produce stable gas-phase conformations that enable the observation of the counter anion pair while minimizing the range of ligand exchange events as the ionized complex enters the gas-phase. Other enhancements include improved data acquisition times by applying multiplexing approaches to the IMS Bradbury-Nielsen (BN) gate to realize increased ion transmission and improve ion statistics measured at the m/z detector. Analyte identification using this approach is based on a multitude of combined measured gas-phase ion metrics, which include mass measurements, isotope profiling, and experimentally determined reduced mobilities measured at the low-field limit (<2 E/N). Though geared initially towards uranyl complexes, this approach may find application in fields where both chemical speciation and isotopic profiles provide diagnostic information for a given metal.

Dissociation Enthalpies of Chloride Adducts of Nitrate and Nitrite Explosives Determined by Ion Mobility Spectrometry.

The kinetics for thermal dissociations of the chloride adducts of the nitrate explosives 1,3-dinitroglycerin (1,3-NG), 1,2-dinitroglycerin (1,2-NG), the nitrite explosive 3,4-dinitrotoluene (3,4-DNT), and the explosive taggant 2,3-dimethyl-2,3-dinitrobutane (DMNB) have been studied by atmospheric pressure ion mobility spectrometry. Both 1,3-NG·Cl(-) and1,2-NG·Cl(-) decompose in a gas-phase SN2 reaction in which Cl(-) displaces NO3(-) while 3,4-DNT·Cl(-) and DMNB·Cl(-) decompose by loss of Cl(-). The determined activation energy (kJ mol(-1)) and pre-exponential factor (s(-1)) values for the dissociations respectively are 1,3-NG·Cl(-), 86 ± 2 and 2.2 × 10(12); 1,2-NG·Cl(-), 97 ± 2 and 3.5 × 10(12); 3,4-DNT·Cl(-), 81 ± 2 and 4.8 × 10(13); and DMNB·Cl(-), 68 ± 2 and 9.7 × 10(11). Calculations by density functional theory show the structures of the nitrate ester adducts involve three hydrogen bonds: one from the hydroxyl group and the other two from the two nitrated carbons. The relative Cl(-) dissociation energies of the nitrates together with the previously reported smaller value for glycerol trinitrate and the calculated highest value for glycerol 1-mononitrate are explicable in terms of the number of hydroxyl hydrogen bond participants. The theoretical enthalpy changes for the nitrate ester displacement reactions are in agreement with those derived from the experimental activation energies but considerably higher for the nitro compounds.

Influence of the coupling between an atmospheric pressure ion mobility spectrometer and the low pressure ion inlet of a mass spectrometer on the mobility measurement

Ion mobility spectrometers (IMS) are versatile gas analyzers. Due to their small size and robustness, combined with a very high sensitivity, they are often used in gas sensing applications such as environmental monitoring. In order to improve the selectivity, they are typically combined with a mass spectrometer (MS). Since IMS works at atmospheric pressure, and MS works at vacuum, a special interface reducing the pressure over normally two stages has to be used. In this paper the influence of this coupling of different pressure areas on the IMS signal will be analyzed with help of finite elements method simulations.

Decreased Gap Width in a Cylindrical High-Field Asymmetric Waveform Ion Mobility Spectrometry Device Improves Protein Discovery.

High-field asymmetric waveform ion mobility spectrometry (FAIMS) is an atmospheric pressure ion mobility technique that separates gas phase ions according to their characteristic dependence of ion mobility on electric field strength. FAIMS can be implemented as a means of automated gas-phase fractionation in liquid chromatography-tandem mass spectrometry (LC-MS/MS) experiments. We modified a commercially available cylindrical FAIMS device by enlarging the inner electrode, thereby narrowing the gap and increasing the effective field strength. This modification provided a nearly 4-fold increase in FAIMS peak capacity over the optimally configured unmodified device. We employed the modified FAIMS device for on-line fractionation in a proteomic analysis of a complex sample and observed major increases in protein discovery. NanoLC-FAIMS-MS/MS of an unfractionated yeast tryptic digest using the modified FAIMS device identified 53% more proteins than were identified using an unmodified FAIMS device and 98% more proteins than were identified with unaided nanoLC-MS/MS. We describe here the development of a nanoLC-FAIMS-MS/MS protocol that provides automated gas-phase fractionation for proteomic analysis of complex protein digests. We compare this protocol against prefractionation of peptides with isoelectric focusing and demonstrate that FAIMS fractionation yields comparable protein recovery while significantly reducing the amount of sample required and eliminating the need for additional sample handling.

Comprehensive analysis of fatty alcohol ethoxylates by ultra high pressure hydrophilic interaction chromatography coupled with ion mobility spectrometry mass spectrometry using a custom-designed sub-2 μm column

Comprehensive analysis of fatty alcohol ethoxylates has been conducted by coupling ultra high pressure hydrophilic interaction chromatography and ion mobility spectrometry mass spectrometry. A custom-designed sub-2 μm column was used for the chromatographic separation of fatty alcohol ethoxylates by hydrophilic interaction chromatography. Ion mobility spectrometry provided a post-ionization resolution during a very short period of 6.4 ms. Distinguishable families of singly, doubly, and triply charged fatty alcohol ethoxylates were clearly observed. By virtue of the combination of hydrophilic interaction chromatography and ion mobility spectrometry, comprehensive resolution based on both hydrophobicity difference and mobility disparity has been achieved for fatty alcohol ethoxylates. The orthogonality of the developed separation and analysis system was evaluated with the correlation coefficient and peak spreading angle of 0.0224 and 88.72°, respectively. The actual peak capacity obtained was individually 40 and 193 times than those when hydrophilic interaction chromatography and ion mobility spectrometry were used alone. The collision cross-sections of fatty alcohol ethoxylates were calculated by calibrating the traveling wave ion mobility device with polyalanine.

Evaluation of Hadamard Transform Atmospheric Pressure Ion Mobility Time-of-Flight Mass Spectrometry for Complex Mixture Analysis

Ion mobility mass spectrometry (IMMS) has gained popularity in the analysis of complex mixtures such as those encountered in metabolomics and proteomics. However, the challenge that exists in conventional pulsed IMMS is its inherent low duty cycle. The first application of Hadamard transform (HT)-type signal coupled with atmospheric pressure IMMS to complex mixtures is presented. Performance of the prototype was assessed by the analysis of metabolite standard mixture. With 200 times increased IMS duty cycle in HT mode compared with conventional pulsed mode, the limit of detection (LOD) was decreased by ∼10 times. Evaluation for application to complex mixtures was achieved using the NIST Standard Reference Material 1950 Metabolites in Human Plasma. Approximately 180 metabolite ions were detected within 1 min with an IMS resolving power (Rp) of ∼100. Rapid chromatographic separation prior to IMMS analysis was also demonstrated for improving the response of metabolite ions in rat brain tissue extract.

Desorption electrospray ionization (DESI) with atmospheric pressure ion mobility spectrometry for drug detection

Desorption electrospray ionization (DESI) was coupled to an ambient pressure drift tube ion mobility time-of-flight mass spectrometer (IM-TOFMS) for the direct analysis of active ingredients in pharmaceutical samples. The DESI source was also coupled with a standalone IMS demonstrating potential of portable and inexpensive drug-quality testing platforms. The DESI-IMS required no sample pretreatment as ions were generated directly from tablets and cream formulations. The analysis of a range of over-the-counter and prescription tablet formations was demonstrated for amphetamine (methylphenidate), antidepressant (venlafaxine), barbiturate (Barbituric acid), depressant (alprazolam), narcotic (3-methylmorphine) and sympatholytic (propranolol) drugs. Active ingredients from soft and liquid formulations, such as Icy Hot cream (methyl salicylate) and Nyquil cold medicine (acetaminophen, dextromethorphan, doxylamine) were also detected. Increased sensitivity for selective drug responses was demonstrated through the formation of sodiated adduct ions by introducing small quantities of NaCl into the DESI solvent. Of the drugs and pharmaceuticals tested in this study, 68% (22 total samples) provided a clear ion mobility response at characteristic mobilities either as (M + H)(+), (M - H)(-), or (M + Na)(+) ions.

Development of an Atmospheric Pressure Ion Mobility Spectrometer–Mass Spectrometer with an Orthogonal Acceleration Electrostatic Sector TOF Mass Analyzer

Recently developed ion mobility mass spectrometer is described. The instrument is based on a drift tube ion mobility spectrometer and an orthogonal acceleration electrostatic sector time-of-flight mass analyzer. Data collection is performed using a specially developed fast ADC-based recorder that allows real-time data integration in an interval between 3 and 100 s. Primary tests were done with positive ion electrospray. The tests have shown obtaining 100 ion mobility resolving power and 2000 mass resolving power. Obtained for 2,6-di-tert-butylpyridine in electrosprayed liquid samples during 100 s analysis and full IMS/MS data collection mode were 4 nM relative limits of detection and a 1 pg absolute limit of detection (S/N=3). Characteristic ion mobility/mass distributions were recorded for selected antibiotics, including amoxicillin, ampicillin, lomefloxacin, and ofloxacin. At studied conditions, lomefloxacin forms only a protonated molecule-producing reduced ion mobility peak at 1.082 cm(2)/(V s). Both amoxicillin and ampicillin produce [M + H](+), [M + CH3OH + H](+), and [M + CH3CN + H](+). Amoxicillin shows two peaks at 0.909 cm(2)/(V s) and 0.905 cm(2)/(V s). Ampicillin shows one peak at 0.945 cm(2)/(V s). Intensity of protonated methanol containing cluster for both ampicillin and amoxicillin has a clear tendency to rise with sample keeping time. Ofloxacin produces two peaks in the ion mobility distribution. A lower ion mobility peak at 1.051 cm(2)/(V s) is shown to be formed by [M + H](+) ions. A higher ion mobility peak appearing for samples kept more than 48 h is shown to be formed by both [M + H](+) ion and a component identified as the [M + 2H + M](+2) cluster. The cluster probably partly dissociates in the interface producing the [M + H](+) ion.

Analysis of Psychoactive Cathinones and Tryptamines by Electrospray Ionization Atmospheric Pressure Ion Mobility Time-of-Flight Mass Spectrometry

The ability to use positive ion monitoring mode with an atmospheric pressure ion mobility time-of-flight mass spectrometer (APIM(tof)MS) to detect psychoactive cathinones and tryptamines from aqueous phase samples was evaluated. The study used a traditional electrospray ionization (ESI) source for sample introduction and ionization. A total of four cathinones (mephedrone, butylone, 4-Me-PPP, and 4-MEC) and five tryptamines (5-EtO-DPT, 5-EtO-DALT, 5-EtO-MIPT, 5-EtO-ALCHT, and 5-EtO-2MALET) were investigated, and we report on parent ions, collision induced dissociation (CID) fragment ions, reduced mobility (Ko), mass flight times, and detection limits obtained from a single instrument run for the psychoactive substances. Detection limits reported ranged from 3 to 11 μM concentration for the compounds studied. This detection limit range corresponded to 1-5 ng of material needed for improved detection on the instrument. This article demonstrates that it was possible to use a single instrument platform for the separation, detection, and identification of cathinones and tryptamines in less than 1 min. The application holds great promise for detecting and identifying a new class of drugs often referred to as "bath salts" or "legal highs" distributed over the Internet.

Quantitative ion mobility spectroscopy based on molecular collision rate theory

Atmospheric pressure chemical ionization and ion mobility spectrometry (IMS) have traditionally been viewed as a qualitative analytical technique for identifying specific chemicals in the atmosphere. This work employs a nonlinear model based on molecular collision rate theory for quantitative modeling of chemical analyte concentrations. The collision rate between any two molecules depends on the relative populations of each chemical species in the volume of air analyzed where most collisions between ions, or neutral molecules and ions, result in no charge transfer. The rate constants for formation of product ions and consumption of source ions are estimated using empirical data over a wide concentration range for several analytes and reagent gases. The rate constants are unique to the analyte and the reagent gas as well as the sensitivity of the particular IMS instrument and provide a quantitative model to relate the mobility peak amplitudes to the analyte concentration. The rate constants can also be normalized by the reaction ion consumption rate constant to remove the IMS instrument sensitivity and provide a qualitative metric for analyte identification independent of a particular IMS instrument. A quantitative example is given for an acetic acid plume measured by a hand-held IMS detector outdoors has the plume passes. The quantitative rate constants provide a reasonable basis for estimating analyte concentration from the ion mobility spectra over a wide range of analyte concentrations.

High-field asymmetric waveform ion mobility spectrometry for mass spectrometry-based proteomics

High-field asymmetric waveform ion mobility spectrometry (FAIMS) is an atmospheric pressure ion mobility technique that separates gas-phase ions by their behavior in strong and weak electric fields. FAIMS is easily interfaced with electrospray ionization and has been implemented as an additional separation mode between liquid chromatography (LC) and mass spectrometry (MS) in proteomic studies. FAIMS separation is orthogonal to both LC and MS and is used as a means of on-line fractionation to improve the detection of peptides in complex samples. FAIMS improves dynamic range and concomitantly the detection limits of ions by filtering out chemical noise. FAIMS can also be used to remove interfering ion species and to select peptide charge states optimal for identification by tandem MS. Here, the authors review recent developments in LC-FAIMS-MS and its application to MS-based proteomics.

Improved Ion Mobility Resolving Power with Increased Buffer Gas Pressure

Security and military applications of analytical techniques demand a small, rugged, reliable instrument that has traditionally been served well by atmospheric pressure ion mobility spectrometry (IMS) systems. Modern threats stipulate these instruments must reliably operate in increasingly complex environments. Previous work has demonstrated that increasing the pressure of an IMS drift tube has the potential to increase the resolving power of IMS, but operation at low temperatures resulted in a leveling of the measured resolving power as a function of pressure. By creating a novel aperture grid/Faraday plate design, a high-pressure IMS (HPIMS) system has been created that maintains a resolving power efficiency of 80% regardless of the pressure applied to the cell. This allows previously unattainable resolving powers to be achieved utilizing a small (10.7 cm) IMS cell. Using high pressure, a stand-alone IMS cell of 10.7 cm length has demonstrated a resolving power of 102 when operated at 2.5 atm. An increase in peak-to-peak resolution was also noted as pressure increased. Finally, the slope of the resulting inverse mobility/pressure curve for a single analyte has been shown to be proportional to the collision-cross-section of the analyte of interest, providing a novel method for the calculation of collision-cross-section of target ions from the HPIMS data.

Resolving Structural Isomers of Monosaccharide Methyl Glycosides Using Drift Tube and Traveling Wave Ion Mobility Mass Spectrometry

Monosaccharide structural isomers including sixteen methyl-D-glycopyranosides and four methyl-N-acetylhexosamines were subjected to ion mobility measurements by electrospray ion mobility mass spectrometry. Two ion mobility-MS systems were employed: atmospheric pressure drift tube ion mobility time-of-flight mass spectrometry and a Synapt G2 HDMS system which incorporates a low pressure traveling wave ion mobility separator. All the compounds were investigated as [M + Na](+) ions in the positive mode. A majority of the monosaccharide structural isomers exhibited different mobility drift times in either system, depending on differences in their anomeric and stereochemical configurations. In general, drift time patterns (relative drift times of isomers) matched between the two instruments. Higher resolving power was observed using the atmospheric pressure drift tube. Collision cross section values of monosaccharide structural isomers were directly calculated from the atmospheric pressure ion mobility experiments, and a collision cross section calibration curve was made for the traveling wave ion mobility instrument. Overall, it was demonstrated that ion mobility-mass spectrometry using either drift tube or traveling wave ion mobility is a valuable technique for resolving subtle variations in stereochemistry among the sodium adducts of monosaccharide methyl glycosides.

Nanospray FAIMS Fractionation Provides Significant Increases in Proteome Coverage of Unfractionated Complex Protein Digests

High-field asymmetric waveform ion mobility spectrometry (FAIMS) is an atmospheric pressure ion mobility technique that can be used to reduce sample complexity and increase dynamic range in tandem mass spectrometry experiments. FAIMS fractionates ions in the gas-phase according to characteristic differences in mobilities in electric fields of different strengths. Undesired ion species such as solvated clusters and singly charged chemical background ions can be prevented from reaching the mass analyzer, thus decreasing chemical noise. To date, there has been limited success using the commercially available Thermo Fisher FAIMS device with both standard ESI and nanoLC-MS. We have modified a Thermo Fisher electrospray source to accommodate a fused silica pulled tip capillary column for nanospray ionization, which will enable standard laboratories access to FAIMS technology. Our modified source allows easily obtainable stable spray at flow rates of 300 nL/min when coupled with FAIMS. The modified electrospray source allows the use of sheath gas, which provides a fivefold increase in signal obtained when nanoLC is coupled to FAIMS. In this work, nanoLC-FAIMS-MS and nanoLC-MS were compared by analyzing a tryptic digest of a 1:1 mixture of SILAC-labeled haploid and diploid yeast to demonstrate the performance of nanoLC-FAIMS-MS, at different compensation voltages, for post-column fractionation of complex protein digests. The effective dynamic range more than doubled when FAIMS was used. In total, 10,377 unique stripped peptides and 1649 unique proteins with SILAC ratios were identified from the combined nanoLC-FAIMS-MS experiments, compared with 6908 unique stripped peptides and 1003 unique proteins with SILAC ratios identified from the combined nanoLC-MS experiments. This work demonstrates how a commercially available FAIMS device can be combined with nanoLC to improve proteome coverage in shotgun and targeted type proteomics experiments.

Atmospheric pressure gas-phase ammonium/alkyl ammonium exchange studies of some crown ethers complexes using ion mobility spectrometry: A thermodynamic investigation and collision cross section measurements

A systematic evaluation of the intermolecular binding forces responsible for the association of crown ethers (hosts) with alkyl amines (guests) is presented by exchanging the ammonium and alkyl ammonium in the ion source of atmospheric pressure ion mobility spectrometry. The hydrogen bonding interactions of crown ether/alkyl ammonium ion complexes are examined with respect to the macrocyclic size effects of five crown ethers (12-crown-4, 15-crown-5, benzo-15-crown-5, 18-crown-6 and aza-18-crown-6) and the gas-phase basicities of five homologous series of alkyl amines. Our approach is based on a formalism assuming an equilibrium: [host + NH 4] + + [guest + H] + ↔ [host + guestH] + + [NH 4] +. The exchange equilibrium constant for this process was calculated using the relative peak intensities of the corresponding species in the IMS spectra. Determination of the equilibrium constants at different temperature via Van’t Hoff plots led to the standard molar enthalpy change Δ H° that is equivalent to the guest-exchange affinity of different crown ethers. The collision cross-sections are measured for the formation of complexes ions and different alkyl ammonium ions, which are produced. Complexes of 15C5 with two different normal and iso-alkyl amine showed a gas phase selectivity binding to normal alkyl amines relative to iso-alkyl amines.

Decomposition of overlapping plasmagram peaks by spectral subtraction

Low field atmospheric pressure Ion Mobility Spectroscopy (IMS) involves the careful analysis of plasmagrams with multiple peaks which can mask one another when they are closely spaced in drift time or corresponding reduced mobility. A typical signal processing approach to decomposing overlapped peaks would be to use an orthogonal decomposition technique, but unfortunately Gaussian-like functions are not orthogonal, so no unique decomposition can be guaranteed. However, each ion species in the drift tube will arrive at the Faraday plate with a known statistical distribution determined by the IMS instrument’s drift tube design, electric field strength, reagent gas flow and other instrument-specific factors such as the ion gate function. This paper presents a straightforward algorithm for decomposing plasmagrams into distinct peaks using a subtractive technique that independently estimates the statistical parameters of each peak, rejecting spurious peaks and electrical noise. The results show that for relatively short gate times, the plasmagram peaks are nearly Gaussian-shaped, but slightly fatter and asymmetric. We show that including of the gate rise and fall times is also significant in matching the plasmagram peak shape. We also show that the diffusion effects on resolution can be attributed to combinations of non-uniform ion distributions in the reaction chamber as well as detritus effects in the drift tube. Given the known peaks statistical parameters, one can then separate overlapping peaks using a straightforward spectral subtractive technique.

Theoretical and experimental study of the achievable separation power in resistive‐glass atmospheric pressure ion mobility spectrometry

We present a detailed investigation of the performance of our previously reported nanoelectrospray high-resolution resistive-glass atmospheric pressure drift tube ion mobility spectrometer constructed with monolithic resistive-glass desolvation and drift regions. Using experimental spectral data and theoretical pulse width and diffusion variables, we compare theoretical and experimental resolving powers achievable under a variety of field strengths and ion gate pulse widths. The effects of instrumental and operational parameters on the resolution achievable in chromatographic terms are also discussed. Following characterization of the separation power of the instrument, experimental spectral peak width data is fitted by a least-squares procedure to a pre-existing semi-empirical model developed to study contributions to peak width other than initial pulse width and diffusional broadening. The model suggests possible contributions to the final measured peak width from electric field inhomogeneity and minor contributions from instrumental parameters such as anode size, anode-to-anode grid distance and drift gas flow rate. The model also reveals an unexpected ion gate width dependence on the final measured peak width that we attribute to non-ideal performance of the Bradbury-Nielsen ion gate and limitations in the design of our pulsing-electronics.

Atmospheric Pressure Corona Discharge Ionisation and Ion Mobility Spectrometry/Mass Spectrometry study of the negative corona discharge in high purity oxygen and oxygen/nitrogen mixtures

Negative ions formed in negative corona discharge in high purity O 2 and O 2/N 2 mixtures have been studied using the Atmospheric Pressure Corona Discharge Ionisation Mass spectrometry (APCDI/MS) and Ion Mobility Spectrometry/Mass Spectrometry techniques (IMS/MS). The primary negative ions have been detected and identified using APCDI/MS technique. The primary ions are fast transformed via ion-molecule reaction with N 2O into ions with m/ z = 60 (N 2O 2 −). The N 2O is formed in the negative corona discharge in the reaction of O with N 2 (∼5 ppm). In the case of N 2 admixtures to O 2 from 10 to 80%, additional negative ion m/ z = 62, NO 3 − appears in the IMS and MS spectra. The mobilities of the negative ions were determined from IMS and mass selected IMS spectra. The m/ z = 60 (N 2O 2 −) has reduced ion mobility of 2.52 ± 0.05 cm 2 V −1 s −1. In O 2/N 2 mixtures, additionally to m/ z = 60, negative ion with reduced ion mobility of 2.14 ± 0.05 cm 2 V −1 s −1 and m/ z = 62 have been detected and assigned to NO 3 −. Formation of this ion was associated with the presence of NO 2 generated in the negative corona discharge in O 2/N 2 mixtures.