Bernadette A. Higgins

Towards Enhanced Detection of Chemical Agents: Design and Development of a Microfabricated Preconcentrator

Cascade Avalanche Sorbent Plate ARray (CASPAR), a micromachined hotplate coated with sorbent polymer, has been demonstrated earlier as a selective preconcentrator for chemical agents and explosives. Up to two orders of magnitude increase in sensitivity has been established by incorporating CASPAR as a front end-modification to commercial detectors. Experimental evidence obtained via thermal imaging and fluorescent particle flow tagging suggests that an 8.5 mm times 8.5 mm current hotplate design is sub-optimal due to non-uniformities in the heating and vapor/particle collection profiles. In this study, we discuss the CASPAR design optimization with the aim to enhance the collection efficiency of hazardous chemical vapor while maintaining thermal stability and precise control before injection of a focused pulse of analyte into a detector.

Microfabricated Gas Chromatograph for Trace Analysis

Microfabricated portable gas analyzers with high sensitivity and selectivity offer utility in a variety of critical applications including aviation security, food safety and toxic industrial waste monitoring. An integral component of such analyzers is the gas chromatographic (GC) column which is used for separations of an injected mixture based on the relative sorption of the various analytes in the carrier gas by the stationary phase. In this interim report, we describe our efforts in the design and development of a microfabricated GC column for the trace detection of hazardous chemicals. Specifically, in this work an optimized serpentine layout with a circular cross- sectional profile has been microfabricated. In this work, computational fluid dynamic (CFD) modeling has been employed as a method to aid in the GC column design optimization. Selectivity to hazardous hydrogen bond basic (HBB) analytes (e.g. TNT, GB, VX) was achieved by using an NRL developed hydrogen bond acid (HBA) sorbent polymer HCSFA2 as the stationary phase. HCSFA2 offers a higher partition coefficient than the more commonly used polysiloxanes.

Functionalized Sorbent Membranes for Use with Ion Mobility Spectrometry

Ion mobility spectrometry (IMS) is a technique commonly used for trace detection of hazardous chemicals. The inlet of an IMS typically utilizes a membrane made of generic polymers such as polydimethylsiloxane or polyvinylidene fluoride. These membranes are designed to allow analytes through but protect the detector from dust and keep a controlled relative humidity and pressure. IMS signals can be enhanced using sorbent polymer membranes to concentrate vapors of interest. Specifically, in this work a strong hydrogen bond acid (HBA) sorbent polymer (HCSFA2) was synthesized to reversibly bind with hydrogen bond basic (HBB) analytes. HCSFA2 has suitable thermal stabilities but offers low viscosities above 50degC. To mitigate this problem HCSFA2 was combined with fillers to maintain the membranes physical structure. The HCSFA2 composites were characterized using various techniques including thermogravimetric analysis, optical microscopy, inverse gas chromatography, FTIR, and differential scanning calorimetry. Additionally, data from a membrane interfaced with an ion mobility spectrometer (IMS) is described.

Sorbent Coatings and Processing Techniques for Trace Analysis of Hazardous Materials in Micro/Nano Sensors

The trend towards developing portable instruments for detecting diverse hazardous substances on-site has required the integration of increasingly dense arrays of micro-or nanometer sized sensors. This system complexity evolved as a direct result of the demanding technical specifications to be met by these detectors such as less than six second analysis times, low false alarms and parts per trillion level detections. Sorbent polymers used in conjunction with the proper transducer can enhance sensitivity as well as selectivity to a class of analytes. This paper describes our efforts in designing sorbent polymers with hydrogen bond (hb) acidic groups for trace analysis of hazardous hb bases including chemical agents, toxic industrial chemicals and explosives. Further, we summarize our efforts at developing suitable coating techniques for fragile, micron-sized sensor arrays to improve analytical performance.

Carbosilane polymers with hydrogen bond acidic functionalization for chemical preconcentrator applications

Vapor collection systems, including solid phase microextraction (SPME), require the ability to selectively collect and concentrate a sample from a large volume of air. In the case of SPME, polymers are needed to adhere to the fiber for greater reproducibility and longer lasting fibers. The polymerization of carbosilanes was investigated and produced polymers with molecular weights over 500,000. This polymer class was then functionalized with hexafluoro-2-propanol (HFIP) end groups that will selectively sorb hydrogen bond basic vapors. The results of vapor testing with these polymers utilizing a variety of platforms such as preconcentrators, Surface Acoustic Wave (SAW) sensors, and microcantilevers will be discussed.

Chemoselective Polymers, Nanoparticles, and Nanotubes in Chemical Sensor and Preconcentrator Applications

The functionalization of polymers and nano-materials with 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) groups provides materials suitable for a variety of preconcentrator and sensor applications. These are especially useful in high vapor pressure, hydrogen-bond basic vapor collection. These specific interactions lead to high efficiency collection of basic analytes such as DMMP (organophosphonates), DNT, and TNT (nitroaromatics). The lower vapor pressure analytes such as RDX have a larger dependence on surface interactions without specific (hydrogen bond) interactions. The use of carbosilane polymers with HFIP pendant groups offers dramatic improvements over fluoropolyol (FPOL) and siloxane polymers in sensor and precon applications. The sorbent capacity and thermal stability are both dramatically improved. In this work we will demonstrate the use of Carbon Nanotube (CNT) composites with HFIP polymers as sorbent coatings and evaluate their use as SPME coatings.