Kent B. Pfeifer

Mass-Sensitive Microfabricated Chemical Preconcentrator

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 SPCs 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.

Porous Al2 O 3 Nanogeometry Sensor Films: Growth and Analysis

Material studies of thin films of porous anodized Al 2 O 3 have been undertaken to determined 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 area determined using Brunauer-Emmett-Teller- type analysis. Finally, the surface area as a function of anodization potential is provided for the films.

Portable acoustic wave sensor systems

A portable acoustic wave chemical sensor system that has the unique advantage of providing two independent responses, doubling the amount of information provided by the sensor, is described. These sensors utilize surface acoustic wave (SAW) devices coated with viscoelastic polymers that absorb a wide variety of volatile organic species, including chlorinated hydrocarbons (CHCs). A comparison of the relative magnitudes of these two responses, specifically the wave velocity and the wave attenuation can be used to discriminate between different isolated chemical species. This allows species identification and quantification using a single SAW sensor. Tests of this portable acoustic wave sensor (PAWS) system using polymer-coated SAW devices show rapid, reversible detection of gas phase species, rapid reestablishment of sensor baseline using an activated carbon scrubber, and discrimination of species based on a comparison of the attenuation and velocity responses.<<ETX>>

Characteristics and Mechanisms in Ion-Conducting Polymer Films as Chemical Sensors Polyethyleneoxide

Solid polymer electrolytes 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 solid polymer electrolyte films used as chemical sensors, We 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. We will present impedance spectroscopy of PEO films in the frequency range 0.0001 Hz to I MHz for different concentrations of VOCs and relative humidity. We find that the measurement frequency is important for distinguishing ionic conductivity from the double layer capacitance and the parasitic capacitance.

A MEMS-based correlation radiometer

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.

Viologen polymer-coated impedance sensors for midrange humidity levels and other volatile organic compounds

The authors have demonstrated a sensor based on an interdigitated electrode platform coated with a conductive viologen polymer that has excellent moisture sensitivity, response time, and low power consumption. Work has been done to explain the limits of sensitivity and characterize the response of the sensor in low-humidity environments and also characterize the response of the device to various organic interferents such as alcohols and organophosphonates. Using lumped-circuit models, the nature of the response is explained and design parameters are isolated that will allow future performance improvements. In addition, the temperature dependence of moisture sensitivity is presented.

Signal-to-Noise and Resolution Enhancement in Ion Mobility Spectrometry Using Correlation Gating Techniques: Barker Codes

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.

Two-dimensional patterns for optical alignment.

Two-dimensional patterns that have properties suited for optical alignment have been constructed from one-dimensional binary Barker codes. Applications include automated alignment of masks with patterns in photolithography.

Surface Plasmon Sensing of Gas Phase Contaminants Using a Single-Ended Multiregion Optical Fiber

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 H2, H2S, 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 H2 using this technique, <; 0.04 Pa partial pressures of H2S, and increases in H2O concentration from - 70°C frost point. In addition, a multiple sensor fiber has been demonstrated that allows a single fiber to measure H2, H2S, and H2O without changing the fiber or the analytical system.

Functionalized nanoelectrode arrays for in-situ identification and quantification of regulated chemicals in water.

The nanoelectrode arrays for in-situ identification and quantification of chemicals in water progressed in four major directions. 1) We developed and engineered three nanoelectrode array designs which operate in a portable field mode or as a distributed sensor network for water systems. 2) To replace the fragile glass electrochemical cells used in the lab, we designed and engineered field-ready sampling heads combining the arrays with a high-speed potentiostat. 3) To utilize these arrays in a portable system we designed and engineered a light-weight high-speed potentiostat with pulse widths from 2 μsec to 100 msec or greater. 4) Finally, we developed the parameters for an analytical method in low-conductivity solutions for Pb(II) detection, with initial studies for the analysis of As(III) and As(V) analysis in natural water sources.