Stanley V. Stepnowski

Stand-off detection of trace explosives via resonant infrared photothermal imaging

We describe a technique for rapid stand-off detection of trace explosives and other analytes of interest. An infrared (IR) laser is directed to a surface of interest, which is viewed using a thermal imager. Resonant absorption by the analyte at specific IR wavelengths selectively heats the analyte, providing a thermal contrast with the substrate. The concept is demonstrated using trinitrotoluene and cyclotrimethylenetrinitramine on transparent, absorbing, and reflecting substrates. Trace explosives have been detected from particles as small as 10 μm.

A sensitive, handheld vapor sensor based on microcantilevers

We report the development of a handheld sensor based on piezoresistive microcantilevers that does not depend on optical detection, yet has high detection sensitivity. The sensor is able to detect vapors from the plastic explosives pentaerythritol tetranitrate and hexahydro-1,3,5-triazine at levels below 10 parts per trillion within few seconds of exposure under ambient conditions. A differential measurement technique has yielded a rugged sensor that is unaffected by vibration and is able to function as a “sniffer.” The microelectromechanical system sensor design allows for the incorporation of hundreds of microcantilevers with suitable coatings in order to achieve sufficient selectivity in the future, and thus could provide an inexpensive, unique platform for the detection of chemical, biological, and explosive materials.

Stand-off detection of trace explosives by infrared photothermal imaging

We have developed a technique for the stand-off detection of trace explosives using infrared photothermal imaging. In this approach, infrared quantum cascade lasers tuned to strong vibrational absorption bands of the explosive particles illuminate a surface of interest, preferentially heating the explosives material. An infrared focal plane array is used to image the surface and detect a small increase in the thermal intensity upon laser illumination. We have demonstrated the technique using TNT and RDX residues at several meters of stand-off distance under laboratory conditions, while operating the lasers below the eye-safe intensity limit. Sensitivity to explosives traces as small as a single grain (~100 ng) of TNT has been demonstrated using an uncooled bolometer array. We show the viability of this approach on a variety of surfaces which transmit, reflect or absorb the infrared laser light and have a range of thermal conductivities. By varying the incident wavelength slightly, we demonstrate selectivity between TNT and RDX. Using a sequence of lasers at different wavelengths, we increase both sensitivity and selectivity while reducing the false alarm rate. At higher energy levels we also show it is possible to generate vapor from solid materials with inherently low vapor pressures.

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.

Infrared microthermography of microfabricated devices.

We report a new experimental apparatus for infrared microthermography applicable to a wide class of samples including semitransparent ones and perforated devices. This setup is particularly well suited for the thermography of microfabricated devices. Traditionally, temperature calibration is performed using calibration hot plates, but this is not applicable to transmissive samples. In this work a custom designed miniature calibration oven in conjunction with spatial filtering is used to obtain accurate static and transient temperature maps of actively heated devices. The procedure does not require prior knowledge of the emissivity. Calibration and image processing algorithms are discussed and analyzed. We show that relatively inexpensive uncooled bolometer arrays can be a suitable detector choice in certain radiometric applications. As an example, we apply this method in the analysis of temperature profiles of an actively heated microfabricated preconcentrator device that incorporates a perforated membrane and is used in trace detection of illicit substances.

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.

Infrared micro-thermography of an actively heated preconcentrator device

We report infrared micro-thermography measurements and analysis of static and transient temperature maps of an actively heated micro-fabricated preconcentrator device that incorporates a dual serpentine platinum heater trace deposited on a perforated polyimide membrane and suspended over a silicon frame. The sorbent coated perforated membrane is used to collect vapors and gases that flow through the preconcentrator. After heating, a concentrated pulse of analyte is released into the detector. Due to its small thermal mass, precise thermal management of the preconcentrator is critical to its performance. The sizes of features, the semi-transparent membrane, the need to flow air through the device, and changes in surface emissivity on a micron scale present many challenges for traditional infrared micro-thermography. We report an improved experimental test-bed. The hardware incorporates a custom-designed miniature calibration oven which, in conjunction with spatial filtering and a simple calibration algorithm, allows accurate temperature maps to be obtained. The test-bed incorporates a micro-bolometer array as the infrared imager. Instrumentation design, calibration and image processing algorithms are discussed and analyzed. The procedure does not require prior knowledge of the emissivity. We show that relatively inexpensive uncooled bolometers arrays can be used in certain radiometric applications. Heating profiles were examined with both uniform and non-uniform air flow through the device. The conclusions from this study provide critical information for optimal integration of the preconcentrator within a detection system, and in the design of the heater trace layout to achieve a more even temperature distribution across the device.