Extending the Golay Equation for Coupling a Gas Chromatograph to a Drift Tube IMS

When it comes to analyzing complex mixtures with highly sensitive ion mobility spectrometers (IMS), a pre-separation of these mixtures is often required due to chemical cross sensitivities and limitations in IMS resolving power. In most cases, gas chromatographic (GC) pre-separation is used. In contrast to typical GC detectors, such as flame ionization detectors or photo ionization detectors, an IMS can add significant dead volume to the system due to its ionization chamber. This dead volume causes memory effects and peak broadening affecting the performance of the chromatographic pre-separation. Therefore, a new but simple model has been developed to estimate the effect of additional detector dead volume and to obtain the optimal operating parameters. This model considers both geometric parameters, such as column length and column diameter, and operating parameters, such as flow rate and temperature of the gas chromatograph and IMS. In addition, the effects induced by the compressibility of the mobile phase were taken into account. In comparison to the commonly used Golay equation our model predicts increased plate heights at low and medium linear velocities. Our model has been experimentally verified using an ultra-high sensitive IMS detector in combination with capillary columns of different lengths and diameters. A UV lamp has been used for ionization due to its good linearity. The columns have been held in isotherm conditions and, additionally, they have been operated with purified air as mobile phase. For each capillary under investigation, a series of measurements with different linear drift velocities has been performed. From these experimental data, the vast difference in plate heights at low linear velocities compared to the expected values from the Golay equation can be confirmed. Despite the simplicity of our model, the predictions are in good accordance with the measurements. Thus, the GC-IMS operating parameters can be easily optimized with respect to maximum gas chromatic pre-separation

Improving ion mobility spectrometer sensitivity through an optimized sample gas flow

The gas flow transporting the sample molecules into and through the reaction region of an ion mobility spectrometer (IMS) has a profound influence on its response, determining both sensitivity and response time. In the past, several key improvements relating to the sample gas flow have been reported, for example the use of a drift gas counter-flow by Baim et al., on-axis sample introduction by St. Louis et al. or side-flow sample introduction by Lee et al. Here, we also aim to optimize the sample gas flow inside an IMS equipped with an extended field switching shutter. By introducing a laminar flow curtain orthogonal to the drift direction through the small field-free reaction region, a significant increase in sensitivity and decrease in response time could be achieved for an already extremely sensitive IMS. The constructed ion source is equipped with both a 10.6 eV UV lamp to initiate direct photo ionization (APPI) and a low-energy X-ray source to initiate chemical ionization (APCI). For an averaging time of just one second during continuous sample introduction, limits of detection in the pptv-range using APPI and in the ppqv-range using chemical ionization (APCI) are reached. For pulsed sample introduction, as in GC-IMS, limits of detection are currently under investigation. However, rise and fall times of less than a second have already been observed for volatile organic compounds at a system temperature of 45 °C. When using APCI, the combination of extremely high sensitivity with fast response times is especially suitable as an ultra-sensitive, orthogonal GC detector for analyzing complex mixtures. When using APPI, the combination of improved linearity with still high sensitivity is especially useful for directly analyzing moderately complex samples

An optimized GC-IMS system with significantly increased sensitivity

Ion mobility spectrometers are extremely fast and sensitive trace gas detectors, but when analyzing complex mixtures, they suffer from disadvantages associated with atmospheric pressure chemical ionization (APCI). This ionization is however necessary for achieving outstanding sensitivity. Thus, for applications requiring the analyses of complex mixtures, a pre-separation e.g. by gas chromatography is required. Here, we present important design considerations for GC-IMS systems. On the one hand, when optimizing the gas chromatograph it is necessary to consider the volume inside the ionization region of the ion mobility spectrometer. This can be accurately represented by adding an additional term to the Golay equation, allowing choosing correct column dimensions and gas flows. On the other hand, when pre-separation is applied, the ion mobility spectrometer just needs to separate co-eluting species and product ions from reactant ions. Thus, a modest resolving power may be sufficient. However, as the eluted analytes are only present for a few seconds, achieving low limits of detection during extremely short averaging times becomes crucial. In fact, extremely high sensitivities that are not practical with a direct inlet due to the very limited dynamic range in IMS become feasible with GC pre-separation. Thus, for our optimized GC-IMS, we optimized our standard IMS with Rp = 100 for significantly increased sensitivity, being able to achieve limits of detections for a measuring time of just one second in the single digit pptv-range and even sub-pptv-range for some compounds

IMS Instrumentation I : Isolated data acquisition for ion mobility spectrometers with grounded ion sources

The drift voltage required for operating ion mobility spectrometers implies high voltage isolation of either the ion source or the detector. Typically, the detector is grounded due to the sensitivity of the small ion currents to interferences and thus higher requirements for signal integrity than the ion source. However, for certain ion sources, such as non-radioactive electron emitters or electrospray ionization sources, or for coupling with other instruments, such as gas or liquid chromatographs, a grounded ion source is beneficial. In this paper, we present an isolated data acquisition interface using a 16 bit, 250 kilosamples per second analog to digital converter and fiber optic transmitters and receivers. All spectra recorded via this new data acquisition interface and with a grounded ion source show the same peak shapes and noise when compared with a grounded detector, allowing additional freedom in design. © 2020, The Author(s)

Influence of Sample Gas Humidity on Product Ion Formation in High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS)

High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) chemically ionize gaseous samples via reactant ions and separate the generated ions by their motion in a neutral gas under the influence of an electric field. Operation at reduced pressures of 10–40 mbar allows for reaching high reduced electric field strengths (E/N) of up to 120 Td. At these high E/N, the generated ions gain the namesake high kinetic energies, leading to a decrease in cluster size of the reactant ions by increasing the reaction rate of collision-induced cluster dissociation of hydrates. In positive ion polarity and in purified air, H3O+(H2O)n, NO+(H2O)n, and O2+•(H2O)p are the most abundant reactant ions. In this work, we investigate the effect of varying sample gas humidity on product ion formation for several model substances. Results show that increasing the sample gas humidity at high E/N of 120 Td shifts product ion formation from a charge transfer dominated reaction system to a proton transfer dominated reaction system. For HiKE-IMS operated at high E/N, the reduction in cluster size of reactant ions allows ionization of analytes with low proton affinity even at high relative humidity in the sample gas of RH = 75% at 303.15 K and 1013.25 hPa. In contrast to conventional IMS, where increasing the sample gas humidity inhibits ionization for various analytes, increasing sample gas humidity in HiKE-IMS operated at 120 Td is actually beneficial for ionization yield of most analytes investigated in this work as it increases the number of H3O+(H2O)n

The origin of isomerization of aniline revealed by high kinetic energy ion mobility spectrometry (HiKE-IMS)

Although aniline is a relatively simple small molecule, the origin of its two peaks observed in ion mobility spectrometry (IMS) has remained under debate for at least 30 years. First hypothesized as a difference in protonation site (amine vs. benzene ring), each ion mobility peak differs by one Dalton when coupled with mass spectrometry where the faster mobility peak is the molecular ion peak, and the slower mobility peak is protonated. To complicate the deconvolution of structures, some previous literature shows the peaks as unresolved and thus proposes these species exist in equilibrium. In this work, we show that when measured with high kinetic energy ion mobility spectrometry (HiKE-IMS), the two peaks observed in spectra of both aniline and all n-fluoroanilines are fully separated (chromatographic resolution from 2-7, Rp > 110) and therefore not in equilibrium. The HiKE-IMS is capable of changing ionization conditions independently of drift region conditions, and our results agree with previous literature showing that ionization source settings (including possible fragmentation at this stage) are the only influence determining the speciation of the two aniline peaks. Finally, when the drift and reactant gas are changed to nitrogen, a third peak appears at high E/N for 2-fluoroaniline and 4-fluoroaniline for the first time in reported literature. As observed by HiKE-IMS-MS, the new third peak is also protonated showing that the para-protonated aniline and resulting fragment ion, molecular ion aniline, can be fully separated in the mobility domain for the first time. The appearance of the third peak is only possible due to the increased separation of the other two peaks within the HiKE-IMS

A compact high resolution ion mobility spectrometer for fast trace gas analysis

Drift tube ion mobility spectrometers (IMS) are widely used for fast trace gas detection in air, but portable compact systems are typically very limited in their resolving power. Decreasing the initial ion packet width improves the resolution, but is generally associated with a reduced signal-to-noise-ratio (SNR) due to the lower number of ions injected into the drift region. In this paper, we present a refined theory of IMS operation which employs a combined approach for the analysis of the ion drift and the subsequent amplification to predict both the resolution and the SNR of the measured ion current peak. This theoretical analysis shows that the SNR is not a function of the initial ion packet width, meaning that compact drift tube IMS with both very high resolution and extremely low limits of detection can be designed. Based on these implications, an optimized combination of a compact drift tube with a length of just 10 cm and a transimpedance amplifier has been constructed with a resolution of 183 measured for the positive reactant ion peak (RIP+), which is sufficient to e.g. separate the RIP+ from the protonated acetone monomer, even though their drift times only differ by a factor of 1.007. Furthermore, the limits of detection (LODs) for acetone are 180 ppt(v) within 1 s of averaging time and 580 pptv within only 100 ms

A High Kinetic Energy Ion Mobility Spectrometer for Operation at Higher Pressures of up to 60 mbar

High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) are usually operated at absolute pressures around 20 mbar in order to reach high reduced electric field strengths of up to 120 Td for influencing reaction kinetics in the reaction region. Such operating points significantly increase the linear range and limit chemical cross sensitivities. Furthermore, HiKE-IMS enables ionization of compounds normally not detectable in ambient pressure IMS, such as benzene, due to additional reaction pathways and fewer clustering reactions. However, operation at higher pressures promises increased sensitivity and smaller instrument size. In this work, we therefore study the theoretical requirements to prevent dielectric breakdown while maintaining high reduced electric field strengths at higher pressures. Furthermore, we experimentally investigate influences of the pressure, discharge currents and applied voltages on the corona ionization source. Based on these results, we present a HiKE-IMS that operates at a pressure of 60 mbar and reduced electric field strengths of up to 105 Td. The corona experiments show shark fin shaped curves for the total charge at the detector with a distinct optimum operating point in the glow discharge region at a corona discharge current of 5 μA. Here, the available charge is maximized while the generation of less-reactive ion species like NOx+ is minimized. With these settings, the reactant ion population, H3O+ and O2+, for ionizing and detecting nonpolar substances like n-hexane is still available even at 60 mbar, achieving a limit of detection of just 5 ppbV for n-hexane

Formation of positive product ions from substances with low proton affinity in high kinetic energy ion mobility spectrometry

Rationale: Ion mobility spectrometry (IMS) instruments are typically equipped with atmospheric pressure chemical ionization (APCI) sources operated at ambient pressure. However, classical APCI-IMS suffers from a limited ionization yield for nonpolar substances with low proton affinity (PA). This is mainly due to ion clustering processes, especially those that involve water molecules, inhibiting the ionization of these substances. Methods: High Kinetic Energy (HiKE)-IMS instruments are operated at decreased pressures and high reduced electric field strengths. As most clustering reactions are inhibited under these conditions, the ionization yield for nonpolar substances with low PA in HiKE-IMS should differ from that in classical APCI-IMS. To gain first insights into the ionization capabilities and limitations of HiKE-IMS, we investigated the ionization of four model substances with low PA in HiKE-IMS using HiKE-IMS-MS as a function of the reduced electric field strength. Results: The four model substances all have proton affinities between those of H2O and (H2O)2 but exhibit different ionization energies, dipole moments, and polarizabilities. As expected, the results show that the ionization yield for these substances differs considerably at low reduced electric field strengths due to ion cluster formation. In contrast, at high reduced electric field strengths, all substances can be ionized via charge and/or proton transfer in HiKE-IMS. Conclusions: Considering the detection of polar substances with high PAs, classical ambient pressure IMS should reach better detection limits than HiKE-IMS. However, considering the detection of nonpolar substances with low PA that are not detected, or only difficult to detect, at ambient pressure, HiKE-IMS would be beneficial

Influence of Reduced Field Strength on Product Ion Formation in High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS)

Classical ion mobility spectrometers (IMS) operated at ambient pressure, often use atmospheric pressure chemical ionization (APCI) sources to ionize organic compounds. In APCI, reactant ions ionize neutral analyte molecules via gas-phase ion–molecule reactions. The positively charged reactant ions in purified, dry air are H3O+, NO+, and O2+•. However, the hydration of reactant ions in classical IMS operated at ambient pressure renders ionization of certain analytes difficult. In contrast to classical IMS operated at ambient pressure, High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) are operated at a decreased pressure of 10–40 mbar, allowing operation at high reduced electric field strengths of up to 120 Td. At such high reduced field strengths, ions reach high effective temperatures causing collision-induced cluster dissociation of the hydrated gas-phase ions, allowing ionization of nonpolar and low proton affinity analytes. The reactant ion population, consisting of H3O+(H2O)n, NO+(H2O)m, and O2+•(H2O)p with an individual abundance that strongly depends on the reduced field strength, differs from the reactant ion population in IMS operated at ambient pressure, which affects the ionization of analyte molecules. In this work, we investigate the influence of reduced field strength on the product ion formation of aromatic hydrocarbons used as model substances. A HiKE-IMS-MS coupling was used to identify the detected ion species. The results show that the analytes form parent cations via charge transfer with NO+(H2O)m and O2+•(H2O)p depending on ionization energy and protonated parent molecules via proton transfer and ligand switching with H3O+(H2O)n mainly depending on proton affinity