Raanan A. Miller

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.

Low-noise MEMS vibration sensor for geophysical applications

The need exists for high-sensitivity, low-noise vibration sensors for various applications, such as geophysical data collection, tracking vehicles, intrusion detectors, and underwater pressure gradient detection. In general, these sensors differ from classical accelerometers in that they require no direct current response, but must have a very low noise floor over a required bandwidth. Theory indicates a capacitive micromachined silicon vibration sensor can have a noise floor on the order of 100 ng//spl radic/Hz over 1 kHz bandwidth, while reducing size and weight tenfold compared to existing magnetic geophones. With early prototypes, we have demonstrated Brownian-limited noise floor at 1.0 /spl mu/g/Hz, orders of magnitude more sensitive than surface micromachined devices such as the industry standard ADXL05.

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.

Dielectric hysteresis from transverse electric fields in lead zirconate titanate thin films

Excellent symmetric dielectric hysteresis is observed from lead zirconate titanate (PZT) thin films using transverse electric fields driven by interdigitated surface electrodes. The 1-μm-thick PZT films with a Zr/Ti ratio of 52/48 are prepared on ZrO2 buffered, 4-in.-diam silicon wafers with a thermally grown SiO2 layer. Both the ZrO2 buffer layer and PZT film are deposited by using a similar sol–gel processing. Remanent polarization of about 20 μC/cm2 with coercive field less than 40 kV/cm is obtained as measured using a triangle wave at 50 Hz. Thicker films are being developed and retention for the transversely polarized state is currently under study. One of the objectives of this study is to develop a large array of d33-driven unimorph sensing elements for a high-resolution acoustic imaging system.

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.

Sensing characteristics of in-plane polarized lead zirconate titanate thin films

The sensing characteristics of in-plane polarized lead zirconate titanate (PZT) thin films were studied and compared with the through-thickness polarized PZT films. The in-plane polarized PZT films were deposited on ZrO2-passivated silicon substrates and had interdigitated electrode systems on the top surface; hence, they can be polarized in the film plane. This in-plane polarization configuration separates the electrode spacing and film thickness as independent variables; thus, the voltage sensitivity can be increased by using wider electrode spacing even for fixed film thickness. The results show that for films with a thickness of 1 μm the voltage sensitivity of in-plane polarized PZT films can be more than 20 times higher than that of the conventional, through-thickness polarized PZT films which were deposited on Pt-buffered silicon substrates.

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.

Application of mobility theory to the interpretation of data generated by linear and RF excited ion mobility spectrometers

Abstract The theory of operation for linear and radio-frequency (RF)-ion mobility spectrometer (IMS) is briefly summarized. The operation of the linear IMS is best described by the continuity equation, and the operation of the RF-IMS is best described by the momentum balance equation. Simple relationships for the RF-IMS show that the compensation field is directly proportional to ion mobility, along with the cube of the dispersion field. When the relationships are applied to the analysis of field-asymmetric ion mobility spectrometry (FAIMS) data collected on a chloride ion, the ion is found to be non-converting (i.e., not clustered with water or nitrogen adducts). For this reason, the chloride ion might be used under controlled conditions to anchor the mobility scale for RF-IMS. Contributions of the electric field to the effective ion temperature influence not only the mobility of the ion, but also its cluster activity.