Erkinjon G. Nazarov

A MEMS radio-frequency ion mobility spectrometer for chemical vapor detection

A first-of-a-kind micro-electro-mechanical systems (MEMS) radio-frequency ion mobility spectrometer (rf-IMS) with a miniature drift tube of total volume 0.6 cm 3 has been fabricated and tested. The spectrometer has detection limits in the parts per billion (ppb) and the ability to identify chemicals such as isomers of xylene not resolved in conventional time-of-flight ion mobility spectrometry. Spectrometer operation with a miniature 10.6 eV (l ¼ 116:5 nm) UV photodischarge lamp and a 1 mCi radioactive ionization source has been demonstrated. The resultant spectra with both these ionization sources are similar, with several additional peaks evident for the radioactive source. The effect of varying the carrier gas flow rate on the resultant spectra has been investigated and optimal flow conditions are found at flow rates between 2 and 3 l/min. The rf-IMS has been interfaced to a mass spectrometer (MS) and rf-IMS spectral peaks have been confirmed. The rf-IMS/MS configuration illustrates another use for the rf-IMS as a pre-filter for atmospheric pressure chemical ionization (APCI) mass spectrometry applications. # 2001 Elsevier Science B.V. All rights reserved.

A novel micromachined high-field asymmetric waveform-ion mobility spectrometer

Abstract The fabrication and characterization of a novel micromachined high-field asymmetric waveform-ion mobility spectrometer (FA-IMS) is described. The spectrometer has a 3×1×0.2 cm3 rectangular drift tube and a planar electrode configuration. The planar configuration permits simple construction using microfabrication technology where electrodes and insulating regions are made with deposited metal films on glass substrates. The spectrometer is characterized using organic vapors (including acetone, benzene, and toluene) at ambient pressure and with air as the drift gas. Ions are created in air at ambient pressure using photo-ionization with a 10.6 eV photo discharge lamp (λ=116.5 nm). The micromachined FA-IMS exhibited behavior consistent with conventional FA-IMS designs where compensation voltage was effective in discriminating between ion species in high-field radio-frequency (RF) regimes. Excellent resolution of benzene and acetone ions in mixtures illustrates an advantage of the FA-IMS over low-field ion mobility spectrometry. Detection of toluene at concentrations as low as 100 ppb has been demonstrated. Improvements in detection limits, by as much as 100×, are anticipated with improved ionization source designs. The ability to transport both positive and negative ions simultaneously through the FA-IMS drift tube is demonstrated here for the first time. Ion intensity is found to be proportional to sample concentration, although clusters of sample ions and neutrals at high concentrations illustrate the need for a drift region which is kept free of sample neutrals. Micromachining promises cost, size, and power reductions enabling both laboratory and field instruments.

Chemical effects in the separation process of a differential mobility/mass spectrometer system.

In differential mobility spectrometry (also referred to as high-field asymmetric waveform ion mobility spectrometry), ions are separated on the basis of the difference in their mobility under high and low electric fields. The addition of polar modifiers to the gas transporting the ions through a differential mobility spectrometer enhances the formation of clusters in a field-dependent way and thus amplifies the high- and low-field mobility difference, resulting in increased peak capacity and separation power. Observations of the increase in mobility field dependence are consistent with a cluster formation model, also referred to as the dynamic cluster-decluster model. The uniqueness of chemical interactions that occur between an ion and cluster-forming neutrals increases the selectivity of the separation, and the depression of low-field mobility relative to high-field mobility increases the compensation voltage and peak capacity. The effect of a polar modifier on the peak capacity across a broad range of chemicals has been investigated. We discuss the theoretical underpinnings which explain the observed effects. In contrast to the result with a polar modifier, we find that using mixtures of inert gases as the transport gas improves the resolution by reducing the peak width but has very little effect on the peak capacity or selectivity. The inert gas helium does not cluster and thus does not reduce low-field mobility relative to high-field mobility. The observed changes in the differential mobility alpha parameter exhibited by different classes of compounds when the transport gas contains a polar modifier or has a significant fraction of inert gas can be explained on the basis of the physical mechanisms involved in the separation processes.

Chemical standards in ion mobility spectrometry

Positive ion mobility spectra for three compounds (2,4-dimethylpyridine (2,4-DMP, commonly called 2,4-lutidine), dimethyl methylphosphonate (DMMP) and 2,6-di-t-butyl pyridine (2,6-DtBP)) have been studied in air at ambient pressure over the temperature range 37–250 ◦ C with (H2O)nH + as the reactant ion. All three compounds yield a protonated molecule but only 2,4dimethylpyridine and dimethyl methylphosphonate produced proton-bound dimers. The reduced mobilities (K0) of protonated molecules for 2,4-dimethylpyridine and DMMP increase significantly with increasing temperature over the whole temperature range indicating changes in ion composition or interactions; however, K0 for the protonated molecule of 2,6-di-t-butyl pyridine was almost invariant with temperature. The K0 values for the proton-bound dimers of 2,4-dimethylpyridine and DMMP also showed little dependence on temperature, but could be obtained only over an experimentally smaller and lower temperature range and at elevated concentrations. Chemical standards will be helpful as mobility spectra from laboratories worldwide are compared with increased precision and 2,6-di-t-butyl pyridine may be a suitable compound for use in standardizing reduced mobilities. The effect of thermal expansion of the drift tube length on the calculation of reduced mobilities is emphasized.

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.

Detection of biological and chemical agents using differential mobility spectrometry (DMS) technology

With international concern growing over the potential for chemical and biological terrorism, there is an urgent need for a sensor that can quickly and accurately detect chemical and biological agents. Such a sensor needs to be portable, robust, and sensitive, with fast sample analysis time. We will demonstrate the use of a micromachined differential mobility spectrometer (DMS) with these characteristics that can detect multiple agents simultaneously on a time scale of seconds. In this study, we have demonstrated the ability of the DMS to detect Bacillus subtilis spores, a surrogate for Bacillus anthracis spores, the causative agent of anthrax. Pyrolysis was used as the sample introduction method to volatilize the spores before introducing material into the DMS. Additionally, we examined the effect of pyrolysis on B. subtilis spores suspended in sterile water using SDS-PAGE. These experiments showed that the spores must be heated at 650/spl deg/C or greater for 5 s or at 550/spl deg/C for at least 10 s to be fragmented into particles considerably smaller than 10 kDa, which the DMS can detect. Several major biomarkers can be easily distinguished above the background of the sterile water in which the spores are suspended, and we hypothesize that additional biomarkers could be liberated by further optimizing conditions. The DMS also has shown promise as a detector for chemical weapon agents, and we have demonstrated the ability of the DMS to detect nerve and blister agent simulants at clinically relevant levels.

Analysis of a drift tube at ambient pressure: Models and precise measurements in ion mobility spectrometry

Mobility spectra for positive ions, created from a 63Ni foil in purified air at ambient pressure (660 Torr) with 0.15 ppm moisture, were obtained with a drift tube with a discrete drift ring design at 250 °C as electric fields for components were individually and independently varied. Peak area, peak width, baseline intensity, drift times, and reduced mobilities (Ko) were used to measure the function and performance of each component and findings were interpreted using a model for the transport of thermalized ions in weak electric fields at ambient pressure. Transit times and intensities for ions in drift tubes at ambient pressure can be understood through a detailed knowledge of the fields local to a component and derivations from theory of ion transport. Prolonged ion residence in the drift region resulted in ion transformations even for highly purified gases of low moisture at high temperature. These findings suggest that mobility spectra may be obtained with uniformly high quality and reproducibility ...

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.

Micro-machined planar field asymmetric ion mobility spectrometer as a gas chromatographic detector.

A planar high field asymmetric waveform ion mobility spectrometer (PFAIMS) with a micro-machined drift tube was characterized as a detector for capillary gas chromatography. The performance of the PFAIMS was compared directly to that of a flame ionization detector (FID) for the separation of a ketone mixture from butanone to decanone. Effluent from the column was continuously sampled by the detector and mobility scans could be obtained throughout the chromatographic analysis providing chemical inforrmation in mobility scans orthogonal to retention time. Limits of detection were approximately I ng for measurement of positive ions and were comparable or slightly better than those for the FID. Direct comparison of calibration curves for the FAIMS and the FID was possible over four orders of magnitude with a semi-log plot. The concentration dependence of the PFAIMS mobility scans showed the dependence between ion intensity and ion clustering, evident in other mobility spectrometers and atmospheric pressure ionization technologies. Ions were identified using mass spectrometry as the protonated monomer and the proton bound dimer of the ketones. Residence time for column effluent in the PFAIMS was calculated as approximately 1 ms and a 36% increase in extra-column broadening versus the FID occurred with the PFAIMS.

Monitoring volatile organic compounds in ambient air inside and outside buildings with the use of a radio-frequency-based ion-mobility analyzer with a micromachined drift tube

A radio-frequency-based ion-mobility analyzer with a micromachined drift tube was operated continuously to monitor volatile organic compounds (VOCs) in ambient air inside a building and in an open space near the union of I-10 and I-25 at Las Cruces, New Mexico. Air was drawn directly, without enrichment or preseparation, through the analyzer, which was regulated to 35 °C. The ion source was a photo-discharge lamp at 10.6 eV, providing a preliminary level of selectivity in response to chemicals with low ionization potentials. The compensation voltage was scanned continuously from −40 to +20 V at rates of 60 V/s, providing profiles of ions obtained from VOCs in air. Solvents were detected at 1-ppm levels as fugitive emissions from other experiments under way in the laboratory from 8:00 a.m. to 6:00 p.m. However, patterns in VOC levels from 1 to 5 ppb between 6:00 p.m. and 7:00 a.m. and on weekends was attributed to air exchange between ambient air and the ventilation system of the building. The mobility analyzer results were consistent with VOCs from traffic on major city thoroughfare adjacent to the building. In-field studies near two interstate highways demonstrated that analyzer response could be correlated to traffic patterns and exhibited diurnal trends. These findings demonstrate the concept and practice of micromachined mobility analyzers as continuous monitors for VOCs as airborne vapors in buildings and on site.