[Show abstract][Hide abstract] ABSTRACT: We study nanoporous-carbon (NPC) grown via pulsed laser deposition (PLD) as a sorbent coating on 96.5-MHz surface-acousticwave (SAW) devices to detect trihalomethanes (THMs), regulated byproducts from the chemical treatment of drinking water. Using both insertion-loss and isothermal-response measurements from known quantities of chloroform, the highest vapor pressure THM, we optimize the NPC mass-density at 1.05 ± 0.08 g/cm3 by controlling the background argon pressure during PLD. Precise THM quantities in a chlorobenzene solvent are directly injected into a separation column and detected as the phase-angle shift of the SAW device output compared to the drive signal. Using optimized NPC-coated SAWs, we study the chloroform response as a function of operating temperatures ranging from 10.50°C. Finally, we demonstrate individual responses from complex mixtures of all four THMs, with masses ranging from 10.2000 ng, after gas chromatography separation. Estimates for each THM detection limit using a simple peak-height response evaluation are 4.4 ng for chloroform and 1 ng for bromoform; using an integrated-peak area response analysis improves the detection limits to 0.73 ng for chloroform and 0.003 ng bromoform.
Full-text · Article · Mar 2015 · Journal of The Electrochemical Society
[Show abstract][Hide abstract] ABSTRACT: We have developed a tool we call Two-Dimensional Péclet analysis to help in determining optimum operational parameters for analytical systems that require automated analysis of large numbers of analyte data peaks. In this paper, we derive the technique from moment analysis of the peaks and single-dimensional Péclet theory. Two-Dimensional Péclet analysis allows automated comparison of response peaks with differing shapes and amplitudes to be compared simultaneously in multicomponent mixtures. In addition to peak resolution and fidelity, individual two-dimensional moment components can provide feedback to total mass (zero moment), centroid location in time space (first moment), and two-dimensional spread of data (second moment).
[Show abstract][Hide abstract] ABSTRACT: Fiber-optic gas phase surface plasmon resonance (SPR) detection of several contaminant gases of interest to state-of-health monitoring in high-consequence sealed systems has been demonstrated. These contaminant gases include H<sub>2</sub>, H<sub>2</sub>S, and moisture using a single-ended optical fiber mode. Data demonstrate that results can be obtained and sensitivity is adequate in a dosimetric mode that allows periodic monitoring of system atmospheres. Modeling studies were performed to direct the design of the sensor probe for optimized dimensions and to allow simultaneous monitoring of several constituents with a single sensor fiber. Testing of the system demonstrates the ability to detect 9 Pa partial pressures of H<sub>2</sub> using this technique, <; 0.04 Pa partial pressures of H<sub>2</sub>S, and increases in H<sub>2</sub>O concentration from - 70°C frost point. In addition, a multiple sensor fiber has been demonstrated that allows a single fiber to measure H<sub>2</sub>, H<sub>2</sub>S, and H<sub>2</sub>O without changing the fiber or the analytical system.
[Show abstract][Hide abstract] ABSTRACT: An ion mobility spectrometer does not require a physical aperture grid to prevent premature ion detector response. The last electrodes adjacent to the ion collector (typically the last four or five) have an electrode pitch that is less than the width of the ion swarm and each of the adjacent electrodes is connected to a source of free charge, thereby providing a virtual aperture grid at the end of the drift region that shields the ion collector from the mirror current of the approaching ion swarm. The virtual aperture grid is less complex in assembly and function and is less sensitive to vibrations than the physical aperture grid.
[Show abstract][Hide abstract] ABSTRACT: This paper describes a mass-sensitive microfabricated preconcentrator for use in chemical detection microsystems. The device combines mass sensing and preconcentration to create a smart preconcentrator (SPC) that determines when it has collected sufficient analyte for analysis by a downstream chemical microsystem. The SPC is constructed from a Lorentz-force-actuated pivot-plate resonator with an integrated heater. Subsequent to microfabrication, the SPC is coated with an adsorbent for collection of chemical analytes. The frequency of operation varies inversely with the mass of collected analyte. Such shifts can be measured by a back-EMF in the SPC's drive/transducer line. By using a calibrated vapor system, the limit of detection of the SPC was determined to be less than 50 ppb for dimethyl-methyl-phosphonate (DMMP) (actual limits of detection are omitted due to export control limitations). At 1 ppm of DMMP, 1-s collection was sufficient to trigger analysis in a downstream microsystem; other micropreconcentrators would require an arbitrary collection time, normally set at 1 min or longer. This paper describes the theory of operation, design, fabrication, coating, vapor system testing, and integration of the SPC into microanalytical systems. The theory of operation, which is applicable to other torsional oscillators, is used to predict a shear modulus of silicon (100) of G = 57.0 GPa plusmn2.2 GPa.
No preview · Article · Jan 2009 · Journal of Microelectromechanical Systems
[Show abstract][Hide abstract] ABSTRACT: The mass-sensitive smart preconcentrator (SPC) consists of a Lorentz-Force-actuated MEMS resonator with an integral heater and surface coating for the collection of chemical analytes. Control circuitry is used to drive the SPC to resonance and measure its oscillation frequency. The frequency shift produced by adsorption of analyte on the SPC surface is inversely proportional to the mass of analyte collected. Thus, the SPC can measure when it has collected sufficient analyte for a downstream detection system. The limit of detection (LOD) of the SPC is less than 50 ppb for DMMP (dimethyl-methyl-phosphonate). At 1 ppm, less than 1 second collection of DMMP is sufficient to trigger analysis. An analytical model of operation of the SPC is used to predict the motion of the paddle and the shear modulus of silicon.
No preview · Article · Oct 2008 · ECS Transactions
[Show abstract][Hide abstract] ABSTRACT: Rotationally resolved resonance-enhanced multiphoton ionization (REMPI) spectra of the NO photofragment from nitrobenzene have been observed for the A2Σ+- X2Π (1, 0) transition. These spectra were collected in an atmospheric-pressure nitrogen bath.
[Show abstract][Hide abstract] ABSTRACT: Correlation ion mobility spectrometry (CIMS) uses gating modulation and correlation signal processing to improve IMS instrument performance. Closely spaced ion peaks can be resolved by adding discriminating codes to the gate and matched filtering for the received ion current signal, thereby improving sensitivity and resolution of an ion mobility spectrometer. CIMS can be used to improve the signal-to-noise ratio even for transient chemical samples. CIMS is especially advantageous for small geometry IMS drift tubes that can otherwise have poor resolution due to their small size.
[Show abstract][Hide abstract] ABSTRACT: Ion mobility spectroscopy (IMS) is a technology that is ideally suited for the detection of very low levels of analyte due to its extreme sensitivity and ability to speciate. Detection of common military and industrial explosives using IMS is an ideal application, since IMS can be tailored to be sensitive to compounds that form negative ions such as nitrate-laden explosives. However, realization of a miniaturized IMS-based detection system for explosives has been hampered by limitations in resolution of miniaturized IMS tubes and by the need to preconcentrate explosive samples and then rapidly desorb them creating a transient chemical concentration. We have demonstrated a new gating and data processing technique that takes advantage of pulse compression approaches developed for modern radar systems for decreasing granularity in target identification. We will show that closely spaced peaks can be isolated by adding discriminating codes to the gating signal. We will then employ matched filtering for the received ion current signal greatly improving instrument performance. This scheme is most advantageous to small geometry IMS drift cells that suffer from lack of resolution due to their small size but would improve sensitivity and peak location uncertainty in any geometry IMS tube. Specifically, we have demonstrated a 13 fold increase in signal-to-noise ratio and have effectively decreased the uncertainty in the location of the signal peak by a factor of 4.4 using a 13-bit Barker coding pattern to operate our IMS gating.
[Show abstract][Hide abstract] ABSTRACT: Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. Approved for public release; further dissemination unlimited.
[Show abstract][Hide abstract] ABSTRACT: Measurements of the performance of a miniature, portable 12-mm-diameter, 57-mm-length low-temperature cofired ceramic (LTCC) ion mobility spectrometer drift tube were undertaken to verify models of ion transport and determine the physical shape of the ion "swarms" in the LTCC tube. Simplified two-dimensional Gaussian models of ion swarm shape were fit to measured data to extract geometrical shape parameters. Results indicate that tube-transfer function effects that produce asymmetric ion swarms are minimized in the tube reducing temporal dispersion. Data are presented that illustrate the swarm shape as a function of gate time, electric field magnitude, and total charge in the ion swarm. Characterization and understanding of the ion transport mechanisms and effects that limit the resolution and other performance parameters of miniature IMS drift tubes is essential to the development of practical, robust, portable systems for "first responder" and homeland security missions.
No preview · Article · Sep 2005 · Analytical Chemistry
[Show abstract][Hide abstract] ABSTRACT: We describe the development of a MEMS-based correlation radiometer for remote detection of chemical species. The radiometer utilizes a new type of MEMS programmable diffraction grating called the Polychromator. The Polychromator contains an array of 1024 electrostatically actuated reflective beams that are 10 microns wide by 1 cm long, and have a vertical travel of approximately 2 - 4 microns. The Polychromator grating is used to replace the reference cell of conventional correlation radiometry. Appropriate programming of the deflection profile of the grating array enables the production of any spectral transfer function desired for the correlation measurement. Advantages of this approach to correlation radiometry include the ability to detect multiple chemical species with a compact instrument, the ability to optimize the reference spectra to eliminate chemical interferences, and the ability to produce reference spectra for hazardous and transient species.
No preview · Article · Jan 2004 · Proceedings of SPIE - The International Society for Optical Engineering
[Show abstract][Hide abstract] ABSTRACT: LTCC is applicable to a broad range of micro systems. New LTCC techniques for micro-IMS fabrication have improved function, while simplifying the structure for better manufacturability at lower cost, which is critical to widespread implementation of robust sensing capability. A drift tube is fabricated by rolling unfired glass-ceramic tape with thick film features on both sides, including internal electrodes, external connections, seal rings, a buried heater, and an integral precision resistor network. The tube supports itself mechanically through burnout and firing. The assembly of internal components, including an ionizer, apertures, grids, and a target, is accomplished from the tube ends. The high aspect ratio of LIGA grids accomplishes low obstruction with high axial rigidity. Gas plumbing is also incorporated, and a sacrificial material technique which simplifies the exhaust porting is described. Integral transparent windows in LTCC as they may improve a future IMS are also described. Prototypes have detected ion peaks.
[Show abstract][Hide abstract] ABSTRACT: Material studies of thin films of porous anodized Al2O3 have been undertaken to determine their applicability as sensing films for application on surface acoustic wave sensors. We describe the production of these films including their growth parameters and provide an analysis of their crystal morphology. These films were then exposed to various concentrations of analyte and their surface areas determined using Brunauer-Emmett-Teller-type analysis. Finally, the surface area as a function of anodization potential is provided for the films.
No preview · Article · Jan 2002 · Journal of The Electrochemical Society
[Show abstract][Hide abstract] ABSTRACT: Application of the World Wide Web (WWW) for the transfer of sensor data from remote locations to laboratories and offices is a largely ignored application of the WWW. We have investigated several architectures for this application including simple web server/client architectures and variations of this approach. In addition, we have evaluated several commercial approaches and other techniques that have been investigated and are in the literature. Finally, we have provided conclusions based on the results of our study offering suggestions about the advantages and disadvantages of each of the approaches studied.
[Show abstract][Hide abstract] ABSTRACT: Solid Polymer Electrolytes (SPE) are widely used in batteries and fuel cells because of the high ionic conductivity that can be achieved at room temperature. The ions are usually Li or protons, although other ions can be shown to conduct in these polymer films. There has been very little published work on SPE films used as chemical sensors. The authors have found that thin films of polymers like polyethylene oxide (PEO) are very sensitive to low concentrations of volatile organic compounds (VOCs) such as common solvents. Evidence of a new sensing mechanism involving the percolation of ions through narrow channels of amorphous polymer is presented. They present impedance spectroscopy of PEO films in the frequency range 0.0001 Hz to 1 MHz for different concentrations of VOCs and relative humidity. They find that the measurement frequency is important for distinguishing ionic conductivity from the double layer capacitance and the parasitic capacitance.