Field dependence of mobilities for gas-phase-protonated monomers and proton-bound dimers of ketones by planar field asymmetric waveform ion mobility spectrometer (PFAIMS)

SPME arrow-based extraction for enhanced targeted and untargeted urinary volatilomics

BACKGROUND

Volatile organic compounds (VOCs) present in human urine are promising biomarkers for various health conditions and environmental exposures. However, their reliable detection is challenging due to the complexity of urinary matrices and the low concentrations of VOCs. Moreover, untargeted approaches present considerable challenges in terms of data interpretation, increasing the complexity of method development. Here we address these challenges by developing a new method that combines solid-phase microextraction (SPME) Arrow with gas chromatography-high resolution mass spectrometry (GC-HRMS), using a design of experiments (DOE) approach for targeted and untargeted compounds. This methodology, specifically tailored for SPME Arrow, represents a significant advancement in untargeted urinary analysis.

RESULTS

The method was developed based on targeted and untargeted outcomes, were ranking results focus on the highest response area of 11 spiked target VOCs representative of urinary volatilomics, and on identifying the maximum untargeted number of VOCs. The method was developed focusing on the highest response area of 11 spiked target VOCs representative of urinary volatilomics and identifying the maximum number of VOCs. A univariate method determined the optimal coating type, urine volume, and salt addition. Subsequently, a central composite design (CCD) DOE was used to determine ideal temperature, extraction, and incubation times. The best method obtained has an extraction time of 60 min at a temperature of 53 °C, with an SPME Arrow CAR/PDMS using 2 mL of urine, with 0.25 % w/v of NaCl and a pH of 2. Compared to conventional SPME fibers, the SPME Arrow showed improved extraction efficiency, detecting more VOCs. Finally, the enhanced method was successfully applied to urine samples from children exposed and non-exposed to tobacco smoke, identifying specific VOCs, like p-cymene and p-isopropenyl toluene related to tobacco exposure.

SIGNIFICANCE

By integrating both targeted and untargeted approaches, the developed method comprehensively captures the complexity of urinary metabolomics. This dual strategy ensures the precise identification of known compounds and the discovery of novel biomarkers, thereby providing a more complete metabolic profile. Such an approach is crucial for advancing in non-invasive diagnostics and environmental health studies, as it offers deeper insights into the intricate relationships between metabolic processes and various health conditions.

Reliable Detection of Chemical Warfare Agents Using High Kinetic Energy Ion Mobility Spectrometry

High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) ionize and separate ions at reduced pressures of 10-40 mbar and over a wide range of reduced electric field strengths E/N of up to 120 Td. Their reduced operating pressure is distinct from that of conventional drift tube ion mobility spectrometers that operate at ambient pressure for trace compound detection. High E/N can lead to a field-induced fragmentation pattern that provides more specific structural information about the analytes. In addition, operation at high E/N values adds the field dependence of ion mobility as an additional separation dimension to low-field ion mobility, making interfering compounds less likely to cause a false positive alarm. In this work, we study the chemical warfare agents tabun (GA), sarin (GB), soman (GD), cyclosarin (GF) and sulfur mustard (HD) in a HiKE-IMS at variable E/N in both the reaction and the drift region. The results show that varying E/N can lead to specific fragmentation patterns at high E/N values combined with molecular signals at low E/N. Compared to the operation at a single E/N value in the drift region, the variation of E/N in the drift region also provides the analyte-specific field dependence of ion mobility as additional information. The accumulated data establish a unique fingerprint for each analyte that allows for reliable detection of chemical warfare agents even in the presence of interfering compounds with similar low-field ion mobilities, thus reducing false positives.

Structural analysis of C8H6˙+ fragment ion from quinoline using ion-mobility spectrometry/mass spectrometry

This study investigated the structures of fragment ions derived from the quinoline (C9H7N) radical cation using ion-mobility spectrometry and mass spectrometry. Ion mobility and mass analysis revealed that C8H6˙+ is the primary dissociation product resulting from the loss of HCN during collision-induced dissociation of the quinoline radical cation. The reduced mobility (K0) of the C8H6˙+ fragment product in helium gas was measured over a range of reduced electric fields (E/N = 20.8-27.4 Td) at room temperature. The experimental K0 values indicated that C8H6˙+ is a mixture of phenylacetylene and pentalene radical cations. Furthermore, quantum chemical calculations revealed two potential energy surfaces delineating the loss of HCN from the quinoline radical cation to form phenylacetylene radical cations.

MobCal-MPI 2.0: an accurate and parallelized package for calculating field-dependent collision cross sections and ion mobilities

Ion mobility spectrometry (IMS), which can be employed as either a stand-alone instrument or coupled to mass spectrometry, has become an important tool for analytical chemistry. Because of the direct relation between an ion's mobility and its structure, which is intrinsically related to its collision cross section (CCS), IMS techniques can be used in tandem with computational tools to elucidate ion geometric structure. Here, we present MobCal-MPI 2.0, a software package that demonstrates excellent accuracy (RMSE 2.16%) and efficiency in calculating low-field CCSs via the trajectory method (≤30 minutes on 8 cores for ions with ≤70 atoms). MobCal-MPI 2.0 expands on its predecessor by enabling the calculation of high-field mobilities through the implementation of the 2^nd order approximation to two-temperature theory (2TT). By further introducing an empirical correction to account for deviations between 2TT and experiment, MobCal-MPI 2.0 can compute accurate high-field mobilities that exhibit a mean deviation of <4% from experimentally measured values. Moreover, the velocities used to sample ion-neutral collisions were updated from a weighted to a linear grid, enabling the near-instantaneous evaluation of mobility/CCS at any effective temperature from a single set of N₂ scattering trajectories. Several enhancements made to the code are also discussed, including updates to the statistical analysis of collision event sampling and benchmarking of overall performance.

First-Principles Modeling of Preferential Solvation in Mixed-Modifier Differential Mobility Spectrometry

Differential mobility spectrometry (DMS) separates ions based on mobility differences between high and low electric field conditions. To enhance resolution, solvents such as water and acetonitrile are often used to modify the collision environment and take advantage of differing dynamic clustering behavior between analytes that coelute in hard-sphere environments (e.g., N₂). When binary solvent mixtures are used to modify the DMS environment, one solvent can have a dominant influence over the other with respect to ion trajectories. For example, for quinoline derivatives, a 9:1 water:acetonitrile solvent mixture exhibited identical behavior to an environment containing only acetonitrile as a modifier. It was hypothesized that this effect arises due to the significantly different binding strengths of the two solvents. Here, we utilize a first-principles model of DMS to study analytes in single and binary solvent mixtures and explore the effects governing the dominance of one solvent over the other. Computed DMS dispersion curves of quinoline derivatives are in excellent agreement with those measured experimentally. For mixed-modifier environments, the predicted cluster populations show a clear preferential solvation of ions with the stronger binding solvent. The influence of ion-solvent binding energies, solvent concentration, and solvent molecule size is discussed in the context of the observed DMS behavior. This work can guide the usage of binary solvent mixtures for improving ion separations in cases where compounds coelute in pure N₂ and in single-solvent modifier environments. Moreover, our results indicate that binary solvent mixtures can be used to create a relative scale for solvent binding energies.

Investigation of Zero-/High-Field Ion Mobility Orthogonal Separation Using a Hyphenated DMA-FAIMS System and Validation of the Two-Temperature Theory at Arbitrary Field for Tetraalkylammonium Salts in Nitrogen

Toward greater separation techniques for ions, a differential mobility analyzer (DMA) has been coupled with field asymmetric waveform ion mobility spectrometry (FAIMS) to take advantage of two mobility-related but different methods of separation. The filtering effect of the DMA allows ions to be selected individually based on low-field mobility and studied in FAIMS at variable electric field, yielding mobility separations in two dimensions. Because spectra fully describe ion mobility at variable field strength, results are then compared with a two-temperature theory-predicted mobility up to the fourth-order approximation. The comparison yields excellent results up to at least 100 Td, beyond which the theory deviates from experiments. This is attributed to two effects, the enlargement of the structure due to ion heating and the inelasticity of the collisions with the nitrogen bath gas. The corrected mobility can then be used to predict the dispersion plot through a newly developed implicit equation that circumvents the possible issues related to the more elaborate Buryakov equation. Our results simultaneously show that the DMA-FAIMS coupling yields complete information on ion mobility versus the field-strength to gas-density ratio and works toward predicting such spectra from ion structures and gas properties.

Gas-Phase Characterization of Hypervalent Carbon Compounds Bearing 7-6-7-Ring Skeleton: Penta- versus Tetra-Coordinate Isomers

In this study, we afford explicit characterizations of the electronic and geometrical structures of recently reported hypervalent penta-coordinate carbon compounds by using gas-phase characterization techniques: photodissociation spectroscopy (PDS) and ion mobility-mass spectrometry (IM-MS). In particular for a compound with moderately electron-donating ligands, bearing p-methylthiophenyl substituents, the coexistence of tetra- and penta-coordinate isomers is confirmed, consistent with solution characterizations. It is in sharp contrast to the exclusive tetra-coordinate form (with normal valence of the central carbon atom) in the single crystal. This suggests that a non-polar environment makes the penta-coordinate structure thermodynamically most stable. This delicate difference between the tetra- and penta-coordinate structures, which depends on the environment, is a close reflection of the lower activation barrier of the S_N 2 reaction found in neutral solvent or gas-phase reactions.

Kinetics in DMS: Modeling Clustering and Declustering Reactions

Differential mobility spectrometry (DMS) uses high-frequency oscillating electrical fields to harness the differential mobility of ions for separating complex sample mixtures prior to detection. To increase the resolving power, a dynamic microsolvation environment is often created by introducing solvent vapors. Here, relatively large clusters are formed at low-field conditions which then evaporate to form smaller clusters at high-field conditions. The kinetics of these processes as the electrical field strength oscillates are not well studied. Here, we develop a computational framework to investigate how the different reactions (cluster association, cluster dissociation, and fast conformational changes) behave at different field strengths. We aim to better understand these processes, their effect on experimental outcomes, and whether DMS model accuracy is improved via incorporating their description. We find that cluster association and dissociation reactions for typical ion-solvent pairs are fast compared to the time scale of the varying separation fields usually used. However, low solvent concentration, small dipole moments, and strong ion-solvent binding can result in reaction rates small enough that a lag is observed in the ion's DMS response. This can yield differences of several volts in the compensation voltages required to correct ion trajectories for optimal transmission. We also find that the proposed kinetic approach yields generally better agreement with experiment than using a modified Boltzmann weighting scheme. Thus, this work provides insights into the chemical dynamics occurring within the DMS cell while also increasing the accuracy of dispersion plot predictions.

Gas Chromatography - Ion Mobility Spectrometry as a tool for quick detection of hazardous volatile organic compounds in indoor and ambient air: A university campus case study

Society's concerns about the citizens' exposure to possibly dangerous environments have recently risen; nevertheless, the assessment of indoor air quality still represents a major contemporary challenge. The volatile organic compounds (VOCs) are among the main factors responsible for deteriorating air quality conditions. These analytes are very common in daily-use environments and they can be extremely hazardous to human health, even at trace concentrations levels. For these reasons, their quick detection, identification, and quantification are crucial tasks, especially for indoor and heavily-populated scenarios, where the exposure time is usually quite long. In this work, a Gas Chromatography - Ion Mobility Spectrometry (GC-IMS) device was used for continuous monitoring indoor and ambient air environments at a large-scale, due to its outstanding levels of sensibility, selectivity, analytical flexibility, and almost real-time monitoring capability. A total of 496 spectra were collected from 15 locations of a university campus and posteriorly analysed. Overall, 23 compounds were identified among the 31 detected. Some of them, like Ethanol and 2-Propanol, were reported as being very hazardous to the human organism, especially in indoor environments. The achieved results confirmed the suitability of GC-IMS technology for air quality assessment and monitoring of VOCs and, more importantly, proved how dangerous indoor environments can be in scenarios of continuous exposure.

Significance of Competitive Reactions in an Atmospheric Pressure Chemical Ionization Ion Source: Effect of Solvent

Ionization of organic compounds with different structural and energetic properties including benzene derivatives, polycyclic aromatic hydrocarbons (PAHs), ketones, and polyenes was studied using a commercial atmospheric pressure corona discharge (APCI) ion source on a drift tube ion mobility-quadrupole-time-of-flight mass spectrometer (IM-QTOFMS). It was found that the studied cohort of compounds can be experimentally ionized via protonation, charge transfer, and hydride abstraction leading to formation of [M + H]⁺, [M]^+•, and [M - H]⁺ species, respectively. By experimentally monitoring the product ions and comparing the thermodynamic data for different ionization paths, it was proposed that NO⁺ is one of the main reactant ions (RIs) in the ion source used. Of particular focus in this work were theoretical and experimental studies of the effect of solvents frequently used for analytical applications with this ion source (acetonitrile, methanol, and chloroform) on the ionization mechanisms. In methanol, the studied compounds were observed to be ionized mainly via proton transfer while acetonitrile suppressed the protonation of compounds and enhanced their ionization via charge transfer and hydride abstraction. Use of chloroform as a solvent led to formation of CHCl₂⁺ as an alternative reactant ion (RI) to ionize the analytes via electrophilic substitution. Density functional theory (DFT) was used to study the different paths of ionization. The theoretical and experimental results showed that by using only the absolute thermodynamic data, the real ionization path cannot be determined and the energies of all competing processes such as charge transfer, protonation, and hydride abstraction need to be compared.

Clustering and Nonclustering Modifier Mixtures in Differential Mobility Spectrometry for Multidimensional Liquid Chromatography Ion Mobility-Mass Spectrometry Analysis

Modifiers provide fast and reliable tuning of separation in differential mobility spectrometry (DMS). DMS selectivity for separating isomeric molecules depends on the clustering modifier concentration, which is typically 1.5-3 mol % ratio of isopropanol or ethanol in nitrogen. Low concentrations (0.1%) of isopropanol were found to improve resolution and sensitivity but at the cost of practicality and robustness. Replacing the single-channel DMS pump with a binary high-performance liquid chromatography (HPLC) pump enabled the generation of modifier mixtures at a constant flow rate using an isocratic or gradient mode, and the analytical benefits of the system were investigated considering cyclohexane, n-hexane, or n-octane as nonclustering modifiers and isopropanol or ethanol as clustering modifiers. It was found that clustering and nonclustering modifier mixtures enable optimization of selectivity, resolution, and sensitivity for different positional isomers and diastereoisomers. Data further suggested different ion separation mechanisms depending on the modifier ratios. For 85 analytes, the absolute difference in compensation voltages (CoVs) between pure nitrogen and cyclohexane at 1.5 mol % ratio was below 4 V, demonstrating its potential as a nonclustering modifier. Cyclohexane's nonclustering behavior was further supported by molecular modeling using density functional theory (DFT) and calculated cluster binding energies, showing positive ΔG values. The ability to control analyte CoVs by adjusting modifier concentrations in isocratic and gradient modes is beneficial for optimizing multidimensional LCxDMS-MS. It is fast and effective for manipulating the DMS scanning window size to realize shorter mass spectrometry (MS) acquisition cycle times while maintaining a sufficient number of CoV steps and without compromising DMS separation performance.

Delineation of Isomers by the 13C Shifts in Ion Mobility Spectra

Mass spectrometry (MS) and isotopes were intertwined for a century, with stable isotopes central to many MS identification and quantification protocols. In contrast, the analytical separations including ion mobility spectrometry (IMS) largely ignored isotopes, partly because of insufficient resolution. We recently delineated various halogenated aniline isomers by structurally specific splitting in FAIMS spectra. While this capability hinges on the ¹³C shifts, all preceding studies leveraged ³⁷Cl or ⁸¹Br to enhance the differentiation. However, such abundant heavy isotopes are absent from typical organic compounds. With single I isotope, iodinated organics generate similar isotopic envelopes dominated by the ¹³C atoms. Here, we distinguish the three monoiodoaniline isomers based on the shifts solely for one or two ¹³C atoms. The differentiation may be somewhat improved using multipoint peak position descriptions for more reproducible shifts. The interisomer order of shifts differs from those for chlorinated or brominated analogues, showcasing the specificity of approach. We also investigated the mass scaling of isotopic shifts, encountering divergent trends for different structural families.

Enabling Field Asymmetric Ion Mobility Spectrometry Separation of Fentanyl-Related Compounds Using Controlled Humidity

Due to the widespread abuse of opioids in recent years, the development of quick and reliable methods for analyzing compounds such as fentanyl and its derivatives is increasingly important. Ahead of online mass spectrometric analysis, field asymmetric ion mobility spectrometry (FAIMS) has previously been used for rapid ion prefiltering and demonstrated significantly improved peak capacity with the addition of vapor modifiers to the carrier gas. The application of FAIMS-mass spectrometry (MS) in the analysis of fentanyl and related compounds is presented herein with the use of a water vapor modifier. The inclusion of the water vapor modifier to the FAIMS methodology is made more robust with the incorporation of a humidity sensor. A dramatic improvement in the separation of fentanyl, alfentanil, 4-aminophenyl-1-phenethylpiperidine (4-ANPP), norfentanyl, and heroin has been achieved, and the ability to distinguish the isobars in a mixture, alfentanil and ortho-isopropyl furanyl fentanyl, is demonstrated without lengthy chromatography.

Structurally Informative Isotopic Shifts in Ion Mobility Spectra for Heavier Species

The isotopic molecular envelopes due to stable isotopes for most elements were a staple of mass spectrometry since its origins, often leveraged to identify and quantify compounds. However, all isomers share one MS envelope. As the molecular motion in media also depends on the isotopic composition, separations such as liquid chromatography (LC) and ion mobility spectrometry (IMS) must also feature isotopic envelopes. These were largely not observed because of limited resolution, except for the (structurally uninformative) shifts in LC upon H/D exchange. We recently found the isotopic shifts in FAIMS for small haloanilines (∼130-170 Da) to hinge on the halogen position, opening a novel route to isomer characterization. Here, we extend the capability to heavier species: dibromoanilines (DBAs, ∼250 Da) and tribromoanilines (TBAs, ∼330 Da). The ¹³C shifts for DBAs and TBAs vary across isomers, some changing sign. While ⁸¹Br shifts are less specific, the 2-D ¹³C/⁸¹Br shifts unequivocally differentiate all isomers. The trends for DBAs track those for dichloroanilines, with the ¹³C shift order preserved for most isomers. The peak broadening due to merged isotopomers is also isomer-specific. The absolute shifts for TBAs are smaller than those for lighter haloanilines, but differentiate isomers as well because of compressed uncertainties. These results showcase the feasibility of broadly distinguishing isomers in the more topical ∼200-300 Da range using the isotopic shifts in IMS spectra.

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.

Ultra-high-resolution ion mobility spectrometry-current instrumentation, limitations, and future developments

With recent advances in ionization sources and instrumentation, ion mobility spectrometers (IMS) have transformed from a detector for chemical warfare agents and explosives to a widely used tool in analytical and bioanalytical applications. This increasing measurement task complexity requires higher and higher analytical performance and especially ultra-high resolution. In this review, we will discuss the currently used ion mobility spectrometers able to reach such ultra-high resolution, defined here as a resolving power greater than 200. These instruments are drift tube IMS, traveling wave IMS, trapped IMS, and field asymmetric or differential IMS. The basic operating principles and the resulting effects of experimental parameters on resolving power are explained and compared between the different instruments. This allows understanding the current limitations of resolving power and how ion mobility spectrometers may progress in the future. Graphical abstract.

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.

Modular and reconfigurable gas chromatography / differential mobility spectrometry (GC/DMS) package for detection of volatile organic compounds (VOCs)

Due to the versatility of present day microcontroller boards and open source development environments, new analytical chemistry devices can now be built outside of large industry and instead within smaller individual groups. While there are a wide range of commercial devices available for detecting and identifying volatile organic compounds (VOCs), most of these devices use their own proprietary software and complex custom electronics, making modifications or reconfiguration of the systems challenging. The development of microprocessors for general use, such as the Arduino prototyping platform, now enables custom chemical analysis instrumentation. We have created an example system using commercially available parts, centered around on differential mobility spectrometer (DMS) device. The Modular Reconfigurable Gas Chromatography - Differential Mobility Spectrometry package (MR-GC-DMS) has swappable components allowing it to be quickly reconfigured for specific application purposes as well as broad, generic use. The MR-GC-DMS has a custom user-friendly graphical user interface (GUI) and precisely tuned proportional-integral-derivative controller (PID) feedback control system managing individual temperature-sensitive components. Accurate temperature control programmed into the microcontroller greatly increases repeatability and system performance. Together, this open-source platform enables researchers to quickly combine DMS devices in customized configurations for new chemical sensing applications.

Variables Affecting the Internal Energy of Peptide Ions During Separation by Differential Ion Mobility Spectrometry

Differential ion mobility spectrometry (DIMS) devices separate ions on the basis of differences in ion mobility in low and high electric fields, and can be used as a stand-alone analytical method or as a separation step before further analysis. As with other ion mobility separation techniques, the ability of DIMS separations to retain the structural characteristics of analytes has been of concern. For DIMS separations, this potential loss of ion structure originates from the fact that the separations occur at atmospheric pressure and the ions, during their transit through the device, undergo repeated collisions with the DIMS carrier gas while being accelerated by the electric field. These collisions have the ability to increase the internal energy distribution of the ions, which can cause isomerization or fragmentation. The increase in internal energy of the ions is based on a number of variables, including the dispersion field and characteristics of the carrier gas such as temperature and composition. The effects of these parameters on the intra-DIMS fragmentation of multiply charged ions of the peptides bradykinin (RPPGFSPFR) and GLISH are discussed herein. Furthermore, similarities and differences in the internal energy deposition that occur during collisional activation in tandem mass spectrometry experiments are discussed, as the fragmentation pathways accessed by both are similar. Graphical Abstract ᅟ.

Differential mobility spectrometry/mass spectrometry history, theory, design optimization, simulations, and applications

This review of differential mobility spectrometry focuses primarily on mass spectrometry coupling, starting with the history of the development of this technique in the Soviet Union. Fundamental principles of the separation process are covered, in addition to efforts related to design optimization and advancements in computer simulations. The flexibility of differential mobility spectrometry design features is explored in detail, particularly with regards to separation capability, speed, and ion transmission. 2015 Wiley Periodicals, Inc. Mass Spec Rev 35:687-737, 2016.

High pressure effects in high-field asymmetric waveform ion mobility spectrometry

RATIONALE

High-Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) is an analytical technique based on the principle of non-linear electric field dependence of coefficient of mobility of ions for separation that was originally conceived in the Soviet Union in the early 1980s. Being well developed over the past decades, FAIMS has become an efficient method for the separation and characterization of gas-phase ions at ambient pressure, often in air, to detect trace amounts of chemical species including explosives, toxic chemicals, chemical warfare agents and other compounds. However the resolution of FAIMS and ion separation capability need to be improved for more applications of the technique.

METHODS

The effects of above-ambient pressure varying from 1 to 3 atm on peak position, resolving power, peak width, and peak intensity are investigated theoretically and experimentally using micro-fabricated planar FAIMS in purified air.

RESULTS

Peak positions, varying with pressure in a way as a function of dispersion voltage, could be simplified by expressing both compensation and dispersion fields in Townsend units for E/N, the ratio of electric field intensity (E) to the gas number density (N).

CONCLUSIONS

It is demonstrated that ion Townsend-scale peak positions remain unchanged for a range of pressures investigated, implying that the higher the pressure is, stronger compensation and separation fields are needed within limits of air breakdown field. Increase in pressure is found to separate ions that could not be distinguished in ambient pressure, which could be interpreted as the differentials of ions' peak compensation voltage expanded wider than the dilation of peak widths leading to resolving power enhancement with pressure. Increase in pressure can also result in an increase in peak intensity.

Electrothermal Vaporization Sample Introduction for Spaceflight Water Quality Monitoring via Gas Chromatography-Differential Mobility Spectrometry

In the history of manned spaceflight, environmental monitoring has relied heavily on archival sampling. However, with the construction of the International Space Station (ISS) and the subsequent extension in mission duration up to one year, an enhanced, real-time method for environmental monitoring is necessary. The station air is currently monitored for trace volatile organic compounds (VOCs) using gas chromatography-differential mobility spectrometry (GC-DMS) via the Air Quality Monitor (AQM), while water is analyzed to measure total organic carbon and biocide concentrations using the Total Organic Carbon Analyzer (TOCA) and the Colorimetric Water Quality Monitoring Kit (CWQMK), respectively. As mission scenarios extend beyond low Earth orbit, a convergence in analytical instrumentation to analyze both air and water samples is highly desirable. Since the AQM currently provides quantitative, compound-specific information for air samples and many of the targets in air are also common to water, this platform is a logical starting point for developing a multimatrix monitor. Here, we report on the interfacing of an electrothermal vaporization (ETV) sample introduction unit with a ground-based AQM for monitoring target analytes in water. The results show that each of the compounds tested from water have similar GC-DMS parameters as the compounds tested in air. Moreover, the ETV enabled AQM detection of dimethlsilanediol (DMSD), a compound whose analysis had proven challenging using other sample introduction methods. Analysis of authentic ISS water samples using the ETV-AQM showed that DMSD could be successfully quantified, while the concentrations obtained for the other compounds also agreed well with laboratory results.

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

A new analytical instrument provides enhanced identification of mixture components at trace concentrations through three-dimensional characterization of each discrete peak.

Precise determination of nonlinear function of ion mobility for explosives and drugs at high electric fields for microchip FAIMS

High-field asymmetric waveform ion mobility spectrometry (FAIMS) separates ions by utilizing the characteristics of nonlinear ion mobility at high and low electric fields. Accurate ion discrimination depends on the precise solution of nonlinear relationships and is essential for accurate identification of ion species for applications. So far, all the nonlinear relationships of ion mobility obtained are based at low electric fields (E/N <65 Td). Microchip FAIMS (μ-FAIMS) with small dimensions has high electric field up to E/N = 250 Td, making the approximation methods and conclusions for nonlinear relationships inappropriate for these systems. In this paper, we deduced nonlinear functions based on the first principle and a general model. Furthermore we considered the hydrodynamics of gas flow through microchannels. We then calculated the specific alpha coefficients for cocaine, morphine, HMX, TNT and RDX, respectively, based on their FAIMS spectra measured by μ-FAIMS system at ultra-high fields up to 250 Td. The results show that there is no difference in nonlinear alpha functions obtained by the approximation and new method at low field (<120 Td), but the error induced by using approximation method increases monotonically with the increase in field, and could be as much as 30% at a field of 250 Td.

Description of gas-phase ion/neutral interactions in differential ion mobility spectrometry: CV prediction using calibration runs

Differential ion mobility spectrometry (DMS) coupled to mass spectrometry is increasingly used in both quantitative analyses of biological samples and as a means of removing background interferences for enhanced selectivity and improved quality of mass spectra. However, DMS separation efficiency using dry inert gases often lacks the required selectivity to achieve baseline separation. Polar gas-phase modifiers such as alcohols are therefore frequently employed to improve selectivity via clustering/declustering processes. The choice of an optimal modifier currently relies on trial and error experiments, making method development a tedious activity. It was the goal of this study to establish a means of CV prediction for compounds using a homologous series of alcohols as gas-phase modifiers. This prediction was based on linear regression of compensation voltages of two calibration runs for the alcohols with the lowest and the highest molecular weights and readily available descriptors such as proton affinity and gas phase acidity of the modifier molecules. All experiments were performed on a commercial quadrupole linear ion trap mass spectrometer equipped with a DMS device between electrospray ionization source and entrance quadrupole lens. We evaluated our approach using a homologous series of 4-alkylbenzoic acids and a selection of 23 small molecules of high chemical diversity. Predicted CV values typically deviated from the experimentally determined values by less than 0.5 V. Several test compounds changed their ion mobility behavior for the investigated gas phase modifiers (e.g., from type B to type A) and thus could thus not be evaluated.

Isomer separation of iron oxide cluster cations by ion mobility mass spectrometry

Geometrical structures of iron oxide cluster cations have been analyzed by ion mobility mass spectrometry. The series of (FeO)n(+) and FenOn + 1(+) cluster cations were predominantly observed in a mass spectrum at high ion-injection energy into a drift cell. Arrival time distributions in the ion mobility spectrometry indicate that two structural isomers coexist for the (FeO)n(+) clusters at n ≥ 5. By comparison of experimental collision cross sections determined from the arrival times with theoretical ones, two-dimensional ring and sheet structures were assignable for (FeO)n(+) (n = 3-8). In addition to these isomers, compact three-dimensional structures were also found to be stable at (FeO)n(+) (n ≥ 6). Thus, the two-dimensional and three-dimensional structural isomers coexist for (FeO)n(+) (n = 6-8).

Tandem differential mobility spectrometry in purified air for high-speed selective vapor detection

A tandem ion mobility instrument based on differential mobility spectrometry (DMS) was used to demonstrate selectivity in response through differences in field dependence of mobility for ions in purified air at ambient pressure. The concept of chemical selectivity solely from characteristic dispersion curves or from field dependence of ion mobility was experimentally demonstrated in three steps with mixtures of increasing complexity. In a mixture of four alcohols with carbon numbers four and below, distinct pairs of separation voltage and compensation voltage, applied to the first and second DMS stages, permitted isolation of ions from individual substances without detectable levels of other substances. In a three-component mixture of a ketone, alcohol, and organophosphorus compound, the same level of ion isolation was observed using specific and characteristic separation and compensation voltages on each DMS stage. In the last experiment, the isolation of product ions of individual substances from a mixture of 23 volatile organic compounds from four chemical groups was incomplete though the improvement in the ratio of analyte signal to chemical noise was calculated as 31 for DMMP and 106 for 1-hexanol. These findings demonstrate that chemical information available in dispersion curves can be accessed in response times below 100 ms through a tandem DMS measurement.

Ion mobility spectrometry-mass spectrometry (IMS-MS) of small molecules: separating and assigning structures to ions

The phenomenon of ion mobility (IM), the movement/transport of charged particles under the influence of an electric field, was first observed in the early 20th Century and harnessed later in ion mobility spectrometry (IMS). There have been rapid advances in instrumental design, experimental methods, and theory together with contributions from computational chemistry and gas-phase ion chemistry, which have diversified the range of potential applications of contemporary IMS techniques. Whilst IMS-mass spectrometry (IMS-MS) has recently been recognized for having significant research/applied industrial potential and encompasses multi-/cross-disciplinary areas of science, the applications and impact from decades of research are only now beginning to be utilized for "small molecule" species. This review focuses on the application of IMS-MS to "small molecule" species typically used in drug discovery (100-500 Da) including an assessment of the limitations and possibilities of the technique. Potential future developments in instrumental design, experimental methods, and applications are addressed. The typical application of IMS-MS in relation to small molecules has been to separate species in fairly uniform molecular classes such as mixture analysis, including metabolites. Separation of similar species has historically been challenging using IMS as the resolving power, R, has been low (3-100) and the differences in collision cross-sections that could be measured have been relatively small, so instrument and method development has often focused on increasing resolving power. However, IMS-MS has a range of other potential applications that are examined in this review where it displays unique advantages, including: determination of small molecule structure from drift time, "small molecule" separation in achiral and chiral mixtures, improvement in selectivity, identification of carbohydrate isomers, metabonomics, and for understanding the size and shape of small molecules. This review provides a broad but selective overview of current literature, concentrating on IMS-MS, not solely IMS, and small molecule applications.

Selection and generation of waveforms for differential mobility spectrometry

Devices based on differential mobility spectrometry (DMS) are used in a number of ways, including applications as ion prefilters for API-MS systems, as detectors or selectors in hybrid instruments (GC-DMS, DMS-IMS), and in standalone systems for chemical detection and identification. DMS ion separation is based on the relative difference between high field and low field ion mobility known as the alpha dependence, and requires the application of an intense asymmetric electric field known as the DMS separation field, typically in the megahertz frequency range. DMS performance depends on the waveform and on the magnitude of this separation field. In this paper, we analyze the relationship between separation waveform and DMS resolution and consider feasible separation field generators. We examine ideal and practical DMS separation field waveforms and discuss separation field generator circuit types and their implementations. To facilitate optimization of the generator designs, we present a set of relations that connect ion alpha dependence to DMS separation fields. Using these relationships we evaluate the DMS separation power of common generator types as a function of their waveform parameters. Optimal waveforms for the major types of DMS separation generators are determined for ions with various alpha dependences. These calculations are validated by comparison with experimental data.

Comparison of differential mobility spectrometry and mass spectrometry for gas chromatographic detection of ignitable liquids from fire debris using projected difference resolution

The significance of forensic arson analysis accelerates the applications of new technologies in this area. Based on the previously reported application of differential mobility spectrometry (DMS) as a detection method for gas chromatography (GC) in arson analysis, the performances of DMS and mass spectrometry (MS) were compared using a novel chemometric tool, projected difference resolutions (PDRs). The PDR results show that one-way mass spectra data exhibit higher resolution than DMS data, while total ion chromatograms from GC-DMS show higher resolution than that from GC/MS for differentiating seven kinds of ignitable liquids. Combining the information from both chromatography and spectra, two-way data always have higher resolution than one-way data for these two detection methods, and GC/MS would exhibit better performance than GC-DMS according to the minimum resolution value. To verify the PDR results, a fuzzy rule-building expert system was applied for classifying these seven kinds of ignitable liquids from fire debris based on GC-DMS and GC/MS data, respectively. The prediction accuracies were consistent with PDR results, which proved that PDR is a powerful tool in comparing the performances of different analysis methods for pattern recognition.

Optimum waveforms for differential ion mobility spectrometry (FAIMS)

Differential mobility spectrometry or field asymmetric waveform ion mobility spectrometry (FAIMS) is a new tool for separation and identification of gas-phase ions, particularly in conjunction with mass spectrometry. In FAIMS, ions are filtered by the difference between mobilities in gases (K) at high and low electric field intensity (E) using asymmetric waveforms. An infinite number of possible waveform profiles make maximizing the performance within engineering constraints a major issue for FAIMS technology refinement. Earlier optimizations assumed the non-constant component of mobility to scale as E(2), producing the same result for all ions. Here we show that the optimum profiles are defined by the full series expansion of K(E) that includes terms beyond the first that is proportional to E(2). For many ion/gas pairs, the first two terms have different signs, and the optimum profiles at sufficiently high E in FAIMS may differ substantially from those previously reported, improving the resolving power by up to 2.2 times. This situation arises for some ions in all FAIMS systems, but becomes more common in recent miniaturized devices that employ higher E. With realistic K(E) dependences, the maximum waveform amplitude is not necessarily optimum, and reducing it by up to approximately 20% to 30% is beneficial in some cases. The present findings are particularly relevant to targeted analyses where separation depends on the difference between K(E) functions for specific ions.

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.

The use of dopants in high field asymmetric waveform spectrometry

Ion mobility spectrometry (IMS) is proven core technology for the gas-phase detection of chemical warfare (CW) agents. One disadvantage of IMS technology is that ions of similar mobility cannot readily be resolved, resulting in false alarm responses and a loss of user confidence. High field asymmetric waveform spectrometry (HiFAWS) is an emerging technology for the gas-phase detection of CW agents. Of particular interest is the potential of a HiFAWS-based platform to reduce the number of false alarms by resolving ions that cannot be discriminated using IMS. It has been demonstrated that a water clustering/declustering mechanism can be a dominant process in HiFAWS. Ions that cannot be discriminated in IMS because they possess the same low field mobility value can be resolved using HiFAWS due to differences in the extent of low field ion solvation and high field ion desolvation. When operating in complex environments such as those potentially experienced in military and security arenas, IMS systems commonly employ internal dopants to reduce the number of background responses. It is possible that HiFAWS systems may also require the use of internal dopants for the same reason. It has been demonstrated that dopants employed for use in IMS may not be suitable for use in HiFAWS.