Field induced fragmentation spectra from reactive stage-tandem differential mobility spectrometry.

A planar tandem differential mobility spectrometer was integrated with a middle reactive stage to fragment ions which were mobility selected in a first analyzer stage using characteristic compensation and separation fields. Fragmentation occurred in air at ambient pressure of 660 Torr (8.8 kPa) with electric fields of 10 to 35 kV cm-1 (E/N of 52 to 180 Td) between two 1 mm wide metal strips, located on each analyzer plate between the first and second mobility stages. Field induced fragmentation (FIF) spectra were produced by characterizing, in a last stage, the mobilities of fragment ions from protonated monomers of 43 oxygen-containing volatile organic compounds from five chemical classes. The extent of fragmentation was proportional to E/N with alcohols, aldehydes, and ethers undergoing multiples steps of fragmentation; acetates fragmented only to a single ion, protonated acetic acid. In contrast, fragmentation of ketones occurred only for methyl i-butyl ketone and 2-hexanone. Fragment ion identities were supported by mass-analysis and known fragmentation routes and suggested that field induced fragmentation at ambient pressure can introduce structural information into FIF spectra, establishing a foundation for chemical identification using mobility methods.

Fragmentation, auto-modification and post ionisation proton bound dimer ion formation: the differential mobility spectrometry of low molecular weight alcohols.

Differential mobility spectrometry (DMS) is currently being used for environmental monitoring of space craft atmospheres and has been proposed for the rapid assessment of patients at accident and emergency receptions. Three studies investigated hitherto undescribed complexity in the DMS spectra of methanol, ethanol, propan-1-ol and butan-1-ol product ions formed from a (63)Ni ionisation source. 54 000 DMS spectra obtained over a concentration range of 0.01 mg m(-3)(g) to 1.80 g m(-3)(g) revealed the phenomenon of auto-modification of the product ions. This occurred when the neutral vapour concentration exceeded the level required to induce a neutral-ion collision during the low field portion of the dispersion field waveform. Further, post-ionisation cluster-ion formation or protonated monomer/proton bound dimer inter-conversion within the ion-filter was indicated by apparent shifts in the values of the protonated monomer compensation field maximum; indicative of post-ionisation conversion of the protonated monomer to a proton-bound dimer. APCI-DMS-quadrupole mass spectrometry studies enabled the ion dissociation products from dispersion-field heating to be monitored and product ion fragmentation relationships to be proposed. Methanol was not observed to dissociate, while propan-1-ol and butan-1-ol underwent dissociation reactions consistent with dehydration processes that led ultimately to the generation of what is tentatively assigned as a cyclo-C3H3(+) ion (m/z 39) and hydrated protons. Studies of the interaction of ion filter temperature with dispersion-field heating of product ions isolated dissociation/fragmentation product ions that have not been previously described in DMS. The implications of these combined findings with regard to data sharing and data interpretation were highlighted.

Characterization of ion processes in a GC/DMS air quality monitor by integration of the instrument to a mass spectrometer.

The air quality monitor (AQM), which included a portable gas chromatograph (GC) and a detector was interfaced to a mass spectrometer (MS) by introducing flow from the GC detector to the atmospheric pressure ion source of the MS. This small GC system, with a gas recirculation loop for carrier and detector make-up gases, comprised an inlet to preconcentrate volatile organic compounds (VOCs) in air, a thermal desorber before the GC column, a differential mobility spectrometer (DMS), and another DMS as an atmospheric pressure ionization source for the MS. Return flow to the internally recirculated air system of the AQM's DMS was replenished using purified air. Although ions and unreacted neutral vapors flowed from the detector through Viton® tubing into the source of the MS, ions were not detected in the MS without the auxillary ion source, (63)Ni as in the mobility detector. The GC-DMS-MS instrument provided a 3-D measurement platform (GC, DMS, and MS analysis) to explore the gas composition inside the GC-DMS recirculation loop and provide DMS-MS measurement of the components of a complex VOC mixture with performance significantly enhanced by mass-analysis, either with mass spectral scans or with an extracted ion chromatogram. This combination of a mobility spectrometer and a mass spectrometer was possible as vapors and ions are carried together through the DMS analyzer, thereby preserving the chromatographic separation efficiency. The critical benefit of this instrument concept is that all flows in and through the thoroughly integrated GC-DMS analyzer are kept intact allowing a full measure of the ion and vapor composition in the complete system. Performance has been evaluated using a synthetic air sample and a sample of airborne vapors in a laboratory. Capabilities and performance values are described using results from AQM-MS analysis of purified air, ambient air from a research laboratory in a chemistry building, and a sample of synthetic air of known composition. Quantitative measures of a stand-alone AQM are disclosed for VOCs in the ppb to ppm levels with an average precision of 5.8% RSD and accuracy from 4% to 28% error against a standard method.

Tandem differential mobility spectrometry with ion dissociation in air at ambient pressure and temperature.

Proton-bound dimers were dissociated to protonated monomers in air at ambient pressure and temperature using electric fields of ultrahigh Field Asymmetric Ion Mobility Spectrometry (ultraFAIMS) with the onset of dissociation for ethyl acetate as 96 Td and for dimethyl methyl phosphonate as 170 Td. Ions then were measured by differential mobility spectrometry (DMS). Fragment ions were formed with propyl acetate at electric fields of 90 Td or greater. The dissociation in ultraFAIMS of ions, with compensation fields near zero, to form smaller ions with new compensation fields, provided a method to improve peak capacity in DMS without gas modifiers. These findings also lay the foundation for a triple stage DMS with a centre stage for ion dissociation or fragmentation.

Neural network classification of mobility spectra for volatile organic compounds using tandem differential mobility spectrometry with field induced fragmentation.

A spectral library of field induced fragmentation (FIF) spectra for 45 oxygen-containing volatile organic compounds from 5 chemical classes was obtained using tandem differential mobility spectrometry (DMS). Protonated monomers were mobility isolated in a first DMS stage, fragmented with electric fields >10,000 V/cm in a middle (or reactive) stage, and mobility characterized in a second DMS stage. Other spectral libraries were obtained for protonated monomers and for complete mobility spectra from a single DMS stage. Neural networks from Python/Tensorflow software, prepared in-house, and from commercial NeuralWorks Professional II/PLUS were trained to assign spectra into a chemical class. The success at classification was determined for familiar and unfamiliar spectra from these three libraries. Classification test scores were best with FIF spectra with >0.99 for familiar compounds and 0.52 for unfamiliar compounds and were consistent with neural network learning of structural information from fragment ions when compared to other spectral libraries. Radar charts are introduced as measures of classification and as a tool to explore mis-classification. This work shows that ion fragmentation with multi-stage tandem DMS portends molecular identification with the portability and robustness of ambient pressure ion mobility analyzers.

Volatile organic compounds in headspace over electrical components at 75 to 200°C - part 1. identification of constituents and emission rates.

The identity and emission rates of volatile organic compounds (VOCs) in headspace vapors over electronic components were determined at temperatures from 75 to 200°C using gas chromatography/mass spectrometry. The emission of VOCs may provide a basis to detect the onset of the overheating of electronic components in confined atmospheres near electronic bays on airplanes and submarines before smoldering or ignition. VOCs found in headspace vapors over components, including resistors, capacitors, diodes, transistors, and insulation from wires of a transformer, were composed of simple mixtures of substances with 6 to 10 carbon number from chemical families including ketones, aldehydes, substituted benzenes, alcohols, and phenols. Composition of the vapors was characteristic but not exclusive of a particular electrical component, except for phenols and methylstyrene, which were found only in a single component. Emission rates were expressed as nanogram of chemical per gram of component per minute, and increased from a low of 0.001 ng/g-min for nonanal from transformer wire at 100°C to a maximum of 2.5 ng/g-min at 150°C for isophorone from a resistor. Patterns of persistence with repeated sampling of headspace for components at 200°C over 5 hr suggested that VOCs arose from impurities in plastics rather than from thermal decomposition of the polymer.

Improved selectivity for the determination of trinitrotoluene through reactive stage tandem ion mobility spectrometry and a quantitative measure of source-based suppression of ionization.

A tandem ion mobility spectrometer was used to mobility isolate ions at the drift time for trinitrotoluene (TNT) in a first mobility stage, remove an interfering compound by ion decomposition in a middle reactive stage, and mobility characterize the remaining TNT ions in a second mobility stage. This sequential processing of ions provided decisive detection of TNT in the presence of an interfering peak differing from TNT in reduced mobility coefficient (Ko) by only 0.02 cm2/V. Even though ions of TNT (as M - 1)- and the interfering compound were more than 90% convolved, TNT could be selectively detected with more than 95% decomposition of the interferent at 123 Td to an ion now separated by ΔKo of 0.2 cm2/V from TNT. Ions for TNT were not decomposed in these electric fields though transmission efficiency was decreased by 20% through a wire grid assembly (the reactive stage). Although tandem ion mobility spectrometry with a reactive stage improves selectivity of measurement in the drift time dimension, the chemistry of ion formation in the ion source is affected still by ion suppression. Response to 1 ng TNT was decreased as much as 30% from 200 ng of interferent deposited on sample trap.

Limits of separation of a multi-capillary column with mixtures of volatile organic compounds for a flame ionization detector and a differential mobility detector.

The resolving power of a multi-capillary column (MCC) was evaluated using 14 mixtures of volatile organic compounds with known composition and complexity which was incremented stepwise up to 129 constituents. The number of constituents in these mixtures versus the number of components separated and detected with a flame ionization detector showed a proportional rise, with a decreasing slope, to 76 peaks after which a plateau was reached. This was improved 23.7% to 94 constituents, or 73% of all compounds in the mixture, after simplex optimization of carrier gas linear velocity, initial temperature and program rate. When the detection method was differential mobility spectrometry (DMS), additional selectivity was introduced through ion formation and separation. Fifty nine compounds were detected by DMS and 46 were separated by retention time; 13 were co-eluted and 7 of these were resolved by differential ion mobility (90% of all components ionized). A correlation of -0.412 between retention time for gas chromatography (GC) and differential mobility for DMS suggested a significant level of orthogonal character and the method of GC-DMS should not be seen as sequential only.

Differential Mobility Spectrometry of Ketones in Air at Extreme Levels of Moisture.

The performance of a differential mobility spectrometer was characterized at ambient pressure and ten values of water vapor concentration, from 1.0 × 102 to 1.7 × 104 ppm using a homologous series of seven ketones from acetone to 2-dodecanone. Dispersion plots at 30 °C with separation fields from 35 to 123 Td exhibited increased alpha functions for the hydrated proton, protonated monomers, and proton bound dimers with increased moisture levels. Increases in the level of moisture were accompanied by decreased quantitative response with progressive suppression in the formation of the proton bound dimer first and then protonated monomer. Product ions for 2-octanone at 7 ppb were not observed above a moisture level of 4.0 × 103 ppm, establishing a limit for observation of analyte ion formation. The observation limit increased from 1.1 × 103 ppm for acetone to 5.7 × 103 ppm for 2-dodecanone. These findings demonstrate that ketones can be determined with a differential mobility spectrometry (DMS) analyzer near room temperature in the presence of elevated levels of moisture expected with the use of membrane inlets or headspace sampling of surface or ground waters. Moisture levels entering this DMS analyzer employed as an environmental monitor should be kept at 1.0 × 103 ppm or below and quantitative studies for individual ketones should be made at a fixed moisture level.

Differential mobility separation of ions using a rectangular asymmetric waveform.

The performance of a planar differential mobility spectrometer (DMS) is investigated when operated in air at ambient pressure and driven by a rectangular asymmetric waveform, limited to frequencies of <1.2 MHz and voltage pulse amplitudes of <1 kV with steep rise times of the order of approximately 15 ns. Independent control of frequency, voltage pulse amplitude, and duty cycle allow for characterizing the DMS in terms of transmission, resolution and separation. The tradeoff between sensitivity and resolution and the effect of duty cycle on instrument performance are demonstrated experimentally. The dependence of ion mobility on the magnitude of the electric field determines the displacement of ions measured by the DC compensation voltage as a function of the duty cycle. Optimum values for the duty cycle exist for the separation of A- and C-type ions, while, B-type ions exhibit a more complex behavior. An analytical expression for describing the effect of duty cycle on the separation of the ions, determined by variations in the compensation voltage, is developed and compared to experimental results obtained in air below 75 Td using estimated alpha parameters for a set of ketones. In this context, errors associated with the calculation of alpha parameters using polynomials of even powers are highlighted.

Mobility resolution and mass analysis of ions from ammonia and hydrazine complexes with ketones formed in air at ambient pressure.

Protonated ammonia and hydrazines (MH(+)) form complexes with ketones and the differences in masses and mobilities of the resulting ions, MH(+)(ketone)(n), are sufficient for separation in an ion mobility spectrometer at ambient pressure. The highest mass ion for any of the protonated molecules is obtained when the ketone is present at elevated concentrations in the supporting atmosphere of both the source and drift regions of the spectrometer so that an ion maintains a discrete composition and mobility. The sizes of the ion-molecule complexes were found to depend on the number of H atoms on the protonated nitrogen atom--four for ammonia, three for hydrazine, two for monomethylhydrazine, and one for 1,1-dimethylhydrazine, and the drift times of these ions were proportional to the size of the ion-molecule complex. Unexpected side products, including protonated hydrazones and azines, and associated ketone clusters, were isolated to a single drift tube containing ceramic parts and could not, from CID studies, be attributed to gas-phase ion chemistry. These findings illustrate that mobility resolution of ions in IMS and IMS/MS experiments can be enhanced through chemical modification of the supporting gas atmosphere without changes in the core ion.

Ion mobility spectrometry of gas-phase ions from laser ablation of solids in air at ambient pressure.

A mobility spectrometer was used to characterize gas-phase ions produced from laser ablation of solids in air at 100 degrees C and at ambient pressure with a beam focused to a diameter of

A rapid and non-invasive method to determine toxic levels of alcohols and γ-hydroxybutyric acid in saliva samples by gas chromatography-differential mobility spectrometry.

A polydimethylsiloxane oral sampler was used to extract methanol, ethanol, ethylene glycol, 1,3-propandiol and γ-hydroxybutyric acid from samples of human saliva obtained using a passive drool approach. The extracted compounds were recovered by thermal desorption, isolated by gas chromatography and detected with differential mobility spectrometry, operating with a programmed dispersion field. Complex signal behaviours were also observed that were consistent with hitherto unobserved fragmentation behaviours in differential mobility spectrometry. These yielded high-mobility fragments obscured within the envelope of the water-based reactant ion peak. Further, compensation field maxima shifts were also observed which were attributable to transport gas modification phenomena. Nevertheless, the responses obtained indicated that in vivo saliva sampling with thermal desorption gas chromatography may be used to provide a semi-quantitative diagnostic screen over the toxicity threshold concentration ranges of 100 mg dm(-3) to 3 g dm(-3). A candidate method suitable for use in low resource settings for the non-invasive screening of patients intoxicated by alcohols and volatile sedatives has been demonstrated.

Gas chromatography with tandem differential mobility spectrometry of fatty acid alkyl esters and the selective detection of methyl linolenate in biodiesels by dual-stage ion filtering.

Alkyl esters of fatty acids (FAAEs) with carbon numbers from 8 to 20 formed protonated monomers and proton bound dimers through atmospheric pressure chemical ionization reactions and these gas ions were characterized for their field dependent mobility coefficients using differential mobility spectrometry (DMS). Separation of ion peaks with a vapor modifier was achieved for ions with masses of 317-1033 Da though the differences in these coefficients and the resolution of ion peaks decreased proportionally with increased ion mass. Differences in dispersion curves were sufficient to isolate ions from specific FAAEs in the effluent of a gas chromatograph by dual stage ion filtering using a tandem DMS detector. Methyl linolenate was isolated from nearby eluting methyl oleate, methyl stearate and methyl linoleate within analysis times of 10s without measureable complications from charge suppression in the ion source or leakage in filtering of ions with close proximity of dispersion behavior.

Atmospheric pressure chemical ionization studies of non-polar isomeric hydrocarbons using ion mobility spectrometry and mass spectrometry with different ionization techniques.

The ionization pathways were determined for sets of isomeric non-polar hydrocarbons (structural isomers, cis/trans isomers) using ion mobility spectrometry and mass spectrometry with different techniques of atmospheric pressure chemical ionization to assess the influence of structural features on ion formation. Depending on the structural features, different ions were observed using mass spectrometry. Unsaturated hydrocarbons formed mostly [M - 1]+ and [(M - 1)2H]+ ions while mainly [M - 3]+ and [(M - 3)H2O]+ ions were found for saturated cis/trans isomers using photoionization and 63Ni ionization. These ionization methods and corona discharge ionization were used for ion mobility measurements of these compounds. Different ions were detected for compounds with different structural features. 63Ni ionization and photoionization provide comparable ions for every set of isomers. The product ions formed can be clearly attributed to the structures identified. However, differences in relative abundance of product ions were found. Although corona discharge ionization permits the most sensitive detection of non-polar hydrocarbons, the spectra detected are complex and differ from those obtained with 63Ni ionization and photoionization.

Atmospheric pressure chemical ionization of alkanes, alkenes, and cycloalkanes.

Normal and cyclic alkanes and alkenes form stable gas-phase ions in air at atmospheric pressure from 40 to 200°C when moisture is below 1 ppm. Ionization of alkanes in a (63)Ni source favored charge transfer over proton transfer through pathways involving [M-1](+) and [M-3](+) ions. Ion mobility spectra for alkanes showed sharp and symmetrical profiles while spectra for alkenes suggested fragmentation. Ion identifications were made by using mass spectrometry, and ionization pathways were supported by using deuterated analogs of alkanes and alkenes. Alkanes were ionized seemingly through a hydrogen abstraction pathway and did not proceed through an alkene intermediate. New methods for interpretation of mobility spectra utilizing ion mobility spectrometry, atmospheric pressure chemical ionization mass spectrometry, chemical ionization mass spectrometry, and ion mobility spectrometry-mass spectrometry data were demonstrated.

On the structure of water-alcohol and ammonia-alcohol protonated clusters.

Collision-induced dissociation (CID) of protonated ammonia-alcohol and water-alcohol heteroclusters was studied using a triple quadrupole mass spectrometer with a corona discharge atmospheric pressure ionization source. CID results suggested that the ammonia-alcohol clusters had NH: at the core of the cluster and that hydrogen-bonded alcohol molecules solvated this central ion. In contrast, CID results in water-alcohol clusters showed that water loss was strongly favored over alcohol loss and that there was a preference for the charge to reside on an alcohol molecule. The results also indicated that a loose chain of hydrogen-bonded molecules was formed in the water-alcohol clusters and that there appeared to be no rigid protonation site or a fixed central ion. (J Am Soc Mass.

Atmospheric pressure chemical ionization of fluorinated phenols in atmospheric pressure chemical ionization mass spectrometry, tandem mass spectrometry, and ion mobility spectrometry.

Atmospheric pressure chemical ionization (APCI)-mass spectrometry (MS) for fluorinated phenols (C6H5-xFxOH Where x = 0-5) in nitrogen with Cl- as the reagent ion yielded product ions of M Cl- through ion associations or (M-H)- through proton abstractions. Proton abstraction was controllable by potentials on the orifice and first lens, suggesting that some proton abstraction occurs through collision induced dissociation (CID) in the interface region. This was proven using CID of adduct ions (M Cl-) with Q2 studies where adduct ions were dissociated to Cl- or proton abstracted to (M-H)-. The extent of proton abstraction depended upon ion energy and structure in order of calculated acidities: pentafluorophenol > tetrafluorophenol > trifluorophenol > difluorophenol. Little or no proton abstraction occurred for fluorophenol, phenol, or benzyl alcohol analogs. Ion mobility spectrometry was used to determine if proton abstraction reactions passed through an adduct intermediate with thermalized ions and mobility spectra for all chemicals were obtained from 25 to 200 degrees C. Proton abstraction from M Cl- was not observed at any temperature for phenol, monofluorophenol, or difluorophenol. Mobility spectra for trifluorophenol revealed the kinetic transformations to (M-H)- either from M Cl- or from M2 Cl- directly. Proton abstraction was the predominant reaction for tetra- and penta-fluorophenols. Consequently, the evidence suggests that proton abstraction occurs from an adduct ion where the reaction barrier is reduced with increasing acidity of the O-H bond in C6H5-xFxOH.

Miniature radio-frequency mobility analyzer as a gas chromatographic detector for oxygen-containing volatile organic compounds, pheromones and other insect attractants.

A high electric field, radio-frequency ion mobility spectrometry (RF-IMS) analyzer was used as a small detector in gas chromatographic separations of mixtures of volatile organic compounds including alcohols, aldehydes, esters, ethers, pheromones, and other chemical attractants for insects. The detector was equipped with a 2 mCi 63Ni ion source and the drift region for ion characterization was 5 mm wide, 15 mm long and 0.5 mm high. The rate of scanning for the compensation voltages was 60 V s(-1) and permitted four to six scans to be obtained across a capillary chromatographic elution profile for each component. The RF-IMS scans were characteristic of a compound and provided a second dimension of chemical identity to chromatographic retention adding specificity in instances of co-elution. Limits of detection were 1.6-55 x 10(-11) g with an average detection limit for all chemicals of 9.4 x 10(-11) g. Response to mass was linear from 2-50 x 10(-10) g with an average sensitivity of 4 pA ng(-1). Separations of pheromones and chemical attractants for insects illustrated the distinct patterns obtained from gas chromatography with RF-IMS scans in real time and suggest an analytical utility of the RF-IMS as a small, advanced detector for on-site gas chromatographs.

Classification of ion mobility spectra by functional groups using neural networks.

Neural networks were trained using whole ion mobility spectra from a standardized database of 3137 spectra for 204 chemicals at various concentrations. Performance of the network was measured by the success of classification into ten chemical classes. Eleven stages for evaluation of spectra and of spectral pre-processing were employed and minimums established for response thresholds and spectral purity. After optimization of the database, network, and pre-processing routines, the fraction of successful classifications by functional group was 0.91 throughout a range of concentrations. Network classification relied on a combination of features, including drift times, number of peaks, relative intensities, and other factors apparently including peak shape. The network was opportunistic, exploiting different features within different chemical classes. Application of neural networks in a two-tier design where chemicals were first identified by class and then individually eliminated all but one false positive out of 161 test spectra. These findings establish that ion mobility spectra, even with low resolution instrumentation, contain sufficient detail to permit the development of automated identification systems.