Charles S. Harden

Enhanced selectivity in ion mobility spectrometry analysis of complex mixtures by alternate reagent gas chemistry

Abstract Ion mobility spectrometry (IMS) analysis of a complex mixture of volatile organic compounds (VOCs) and organophosphorus compounds (OPCs) at vapor levels of 10–40 mg/m 3 produced mobility spectra with broad profiles illustrating limitations of ion mobility spectrometry (IMS) for screening such mixtures. Preseparation of this mixture with a gas Chromatograph inlet to an ion mobility spectrometer enhanced analytical selectivity although OPC detection was complicated by co-elution with other VOCs. Water reagent gas in the ion source of the ion mobility spectrometer yielded 46 gas Chromatographic peaks in a mixture of 45 VOCs and 19 OPCs. Co-elution of two materials was observed in eight of the Chromatographic peaks and co-elution of three materials occurred in four instances. Further selectivity was gained using reagent gases of elevated proton affinity in the ion source. Reagent gas chemistry for acetone and dimethylsulfoxide reduced the number of GC peaks to 26 and 20, respectively. Moreover, spectral integrity and quantitative response for OPCs were retained at 50 to 1000 pg levels with these reagent gases. For OPCs, analyte ions were shown to be of the type M 2 H + under these conditions of analysis and the mobilities of the product ions were independent of reagent gas. Reduced mobility values were assigned to OPC spectra using a well-characterized OPC ion as the reference. Spectral profiles and reduced mobilities suggested that the OPC product ions were not clustered with reagent gas molecules at 100 °C.

Fragmentation of butyl acetate isomers in the drift region of an ion mobility spectrometer

Abstract Peak distortion and baseline perturbations in ion mobility spectra of butyl acetate isomers were caused by fragmentation of product ions during transport through the drift region of an ion mobility spectrometer (IMS). Extensive fragmentation of MH + and M 2 H + at atmospheric pressure in air occurred with thermal energy changes ( 3 kT 2 ) of only 0.013 eV and the extent of fragmentation was influenced by the butyl isomer structure. At 150°C, major fragment ions were mass identified by IMS/mass spectrometry for t - and iso-butyl acetates as m/z 57 and for sec-butyl acetate as m/z 61. No fragmentation and only declustering from dimer to monomer ions were observed with n -butyl acetate. Fragmentation in the IMS drift region is proposed to occur by an intermolecular rearrangement mechanism through the monomer ion.

Collision induced dissociation studies of protonated alcohol and alcohol—water clusters by atmospheric pressure ionization tandem mass spectrometry.: Part 2. Ethanol, propanol and butanol

Abstract Protonated clusters of alcohols, (ROH) n H + , and alcohol—water heteroclusters, (ROH) n ((H 2 O) m H + , where R = C 2 H 5 , n -C 3 H 7 , iso-C 3 H 7 , n -C 4 H 9 , iso-C 4 H 9 , sec -C 4 H 9 and tert -C 4 H 9 , were formed in an atmospheric pressure ionization (API) corona discharge source, through proton transfer and displacement ion—molecule reactions with (H 2 O) n H + . The cluster ions were then subjected to collision induced dissociation (CID) in a tandem mass spectrometer (API-MS—MS). Stabilities of the clusters were examined through cluster size distribution analysis and CID reaction channels. The results gave insights about the structure and energetics of the clusters. The heteroclusters demonstrated a strong preference for water elimination over alcohol elimination, indicating that the alcohol moiety was the favored protonation site. The CID results indicated that in the heteroclusters water ligands were near the periphery of a chain, along which water and alcohol molecules were hydrogen bonded. This structural model could rationalize product ion formation through a single hydrogen bond cleavage for mild CID conditions and through breaking of two hydrogen bonds or a single bond after proton migration along the chain under enhanced fragmentation conditions. CID of protonated alcohols showed differences in the cleavage of CO vs. OH + bonds, as well as variance in product ion distributions in the alcohols.

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

E/N effects on K0 values revealed by high precision measurements under low field conditions

Ion mobility spectrometry (IMS) is used to detect chemical warfare agents, explosives, and narcotics. While IMS has a low rate of false positives, their occurrence causes the loss of time and money as the alarm is verified. Because numerous variables affect the reduced mobility (K0) of an ion, wide detection windows are required in order to ensure a low false negative response rate. Wide detection windows, however, reduce response selectivity, and interferents with similar K0 values may be mistaken for targeted compounds and trigger a false positive alarm. Detection windows could be narrowed if reference K0 values were accurately known for specific instrumental conditions. Unfortunately, there is a lack of confidence in the literature values due to discrepancies in the reported K0 values and their lack of reported error. This creates the need for the accurate control and measurement of each variable affecting ion mobility, as well as for a central accurate IMS database for reference and calibration. A new ion mobility spectrometer has been built that reduces the error of measurements affecting K0 by an order of magnitude less than ±0.2%. Precise measurements of ±0.002 cm(2) V(-1) s(-1) or better have been produced and, as a result, an unexpected relationship between K0 and the electric field to number density ratio (E/N) has been discovered in which the K0 values of ions decreased as a function of E/N along a second degree polynomial trend line towards an apparent asymptote at approximately 4 Td.

Detection of methyl isocyanate in air with the use of hand-held ion mobility spectrometers

Methyl isocyanate (MIC), CH3NCO, is a relatively simple molecule, but ion mobility spectra derived from studies of this molecule are complex. MIC is known to polymerize, which would lead one to expect that proton-bound monomer, proton-bound dimer, and even larger proton-bound ions could be observed. Indeed, this is the case, and a number of other species can also be observed. In this case headspace above a relatively fresh (i.e., recently purchased) MIC sample was analyzed, and numerous peaks were observed in a single spectrum. Peak identities and intensities were, of course, concentration dependent. Over a range of concentrations, as many as 16 peaks were observed. IMS systems used for these studies included chemical agent monitors (both water and acetone chemistry), a miniaturized hand-held IMS device (Mini-IMS) and an IMS-MS/MS instrument. Although ion mobility spectra are complex, it has been shown that hand-held IMS devices can be useful for detecting or monitoring airborne concentrations of this toxic and hazardous compound. IMS/MS/MS experimentation yielded some mass identifications, and possible ion compositions are proposed. Reduced ion mobility of H+(CH3NCO)(H2O)n was tentatively determined to be 1.91±0.02 cm2 / V s.

Analysis of ion mobility spectra for mixed vapors using Gaussian deconvolution

Abstract A central issue in the utilization of ion mobility spectrometry for chemical analysis is the proper interpretation of ion mobility spectra and the assignment of peak identities. Ion mobility spectra for contemporary drift tubes generally produce broad peaks and simple patterns without obvious details associated with structures. These features can hinder the analyses of spectra derived from mixed vapors. However, additional information from such spectra parameters and for boundaries of operation using ion mobility spectra from binary mixtures. An ion-molecule cluster ion, not obvious in traditional spectra analysis, was disclosed by deconvolution analysis and confirmed by independent ion mobility spectrometry-mass spectrometry.

Determination of E/N Influence on K0 Values within the Low Field Region of Ion Mobility Spectrometry

The established theory of ion motion within weak electric fields predicts that reduced ion mobility (K0) remains constant as a function of the ratio of electric field strength to drift gas number density (E/N). However, upon increasing the accuracy and precision of K0 value measurements during a previous study, a new relationship was seen in which the K0 values of ions decreased as a function of increasing E/N at field strengths below 4 Td. Here the effect of E/N on the K0 value of an ion has been investigated in order to validate the reality of the phenomenon and determine its cause. The pertinent measurements of voltage and drift time were verified in order to ensure the authenticity of the trend and that it was not a result of a systematic error in parametric measurements. The trend was also replicated on a separate ion mobility spectrometer drift tube in order to further validate its authenticity. As a result, the theory of ion motion within weak electric fields should be revised to reflect the behavior seen here.

High Accuracy Ion Mobility Spectrometry for Instrument Calibration

Ion mobility spectrometry (IMS) is widely used to characterize compounds of interest (COIs) based on their reduced mobility ( K0) values. In an attempt to increase the accuracy and agreement of studies, the most recommended method has been to use a reference compound with a known K0 value to calibrate the instrument and calculate COI K0 values from normalized spectra. Researchers are limited by the accuracy of previous K0 value reference measurements on which to base their calibrations. Any inaccuracy in these reference K0 values, typically ±2%, will propagate through to the calculated K0 value of the COI. For this reason, there is a need to standardize reference K0 values with improved accuracy. Through improvement of the accuracy of reference measurements, a lower degree of error will propagate through new K0 value calculations. The K0 values of the ammonium reactant ion, the potential reference standard dimethyl methylphosphonate (DMMP), and three explosive COIs were characterized at multiple drift gas temperatures, drift gas water contents, and electric field strengths on an accurate ion mobility spectrometry instrument. K0 values reported here are known to ±0.1% as a result of reducing the error of all instrumental parameters.

Construction and evaluation of a hermetically sealed accurate ion mobility instrument

Ion mobility spectrometry (IMS) is widely used to detect and identify chemical warfare agents, narcotics, and explosives in the field based on their reduced mobility (K0) values. Current detection windows for these analytes can only be as narrow as ±2% of the K0 values for the analyte being sought. These wide detection windows cause false positive alarms when an interferent with a similar reduced mobility falls within the detection window and triggers an alarm. This results in the loss of time and money as resources are diverted to verify the alarm. A high rate of false positive alarms is caused by a discrepancy in the reported K0 values across the literature that is, at best, ± 2% of the average available values. By accurately and precisely measuring the variables affecting an ion’s K0 value, an accurate K0 value can be produced and the detection windows widths that are established using these reference values can be reduced. Components for accurate analyses have been assembled in the past and here the construction of an accurate ion mobility spectrometry drift tube is described that is accurate to 0.1% of the calculated K0 value and can be hermetically sealed without inserting the drift tube into a large vacuum chamber. Having a pressure sealed accurate ion mobility spectrometer will allow for the control of the pressure variable within the K0 equation and the safe analysis of hazardous chemicals. Here the construction of an inexpensive and easily reparable sealed drift tube is described.