Infrared Thermal Desorption Methods And Systems

patent description

The invention described here consists of a number of systems that incorporate broad spectrum infrared radiation and thermal energy storage for high-temperature thermal desorption of trace chemicals. This thermal desorption process can happen both from wipe-collected sample or directly from target surfaces for subsequent chemical analysis by typical detection schemes (e.g., mass spectrometry (MS}, ion mobility spectrometry (IMS), molecular sensing, or colorimetric). These infrared thermal desorption (IRTD) systems generate inherent temporally discrete and rapid (on the order of seconds to tens of seconds) ramped heating profiles that effectively desorb chemical species at their optimal temperatures. More specifically, volatile species desorb at lower temperatures early in the emission interval, while refractory inorganic and nonvolatile species desorb at elevated temperatures achieved late in the emission interval.

These capabilities are uniquely positioned to benefit many fields interested in trace contraband detection (explosives, narcotics, chemical warfare agents, etc.) both for security screening and in the field. This is vital for the defense sector, homeland security, customs and border patrol, transportation security, law enforcement, and forensic science community. This need, in concert with the ongoing threat posed by explosive-based terrorist attacks, continues to drive the technical advancement of detection instrumentation. Of the explosive threats, homemade explosives from propellants, pyrotechnics, and powders are among the most abused materials for formulating improvised devices. These low explosives consist of nonvolatile refractory inorganic oxidizers. Widely deployed commercial chemical detection systems (~50,000 units deployed worldwide), which often incorporate the thermal desorption of wipe-collected analytes, are currently not capable of achieving the temperatures necessary for the desorption of these nonvolatile species. These commercial systems utilize resistive heating of thermal masses at a constant temperature, however these configurations have difficulty with compounds of vastly different chemical properties such as volatility, vapor pressure, and melting and boiling points. Current systems designed for organic explosive and narcotic compounds operate at temperatures insufficient to thermal desorb low vapor pressure refractory salts, most notably chlorate and perchlorate salts. Comparable difficulties arise with significantly raising the steady-state temperature of thermal desorption, specifically the thermal decomposition or degradation of labile species. In addition to screening applications, the IRTD's capability to vaporize, for subsequent detection, both organic and inorganic species provides a single unique tool to the forensic community for characterizing these pyrotechnics, propel1ants, and powders.

The initial infrared thermal desorber identified above (has been publicly disclosed) consists of a heater housing, a twin tube near-infrared emitter (approximated as blackbody at 2200 °C­ filament temperatures), and a glass-mica ceramic insulator base plate (the location of wipe insertion and impinging infrared irradiation). This system was operated with a polytetrafluoroethylene (PTFE)-coated fiberglass weave wipe material (commercially available). The infrared heater sits in the housing and emits in a downward direction, directly at the wipe materials and base plate. In this configuration the wipe material directly experiences the infrared irradiation. For the applications described above, the target lists of compounds are large and include a wide variety of species. Developing a radiative heating source that targeted all their spectral absorption wavelengths would be difficult, if not impossible. This invention takes advantage of the high infrared transmission of the wipe material (PTFE-coated fiberglass) and thermal absorption and energy storage properties of the base plate (impingement location: glass­ mica). As the infrared emission interval (order of seconds) is initiated, the radiation transmits through the wipe material (which holds the swipe collected target species), impinges the glass­ mica base plate where a sufficient fraction is absorbed subsequently heating the glass-mica, which then in turn heats the wipe material and analytes by conduction and convection. The glass­ mica (and similar materials) provide a unique combination of infrared absorption and heat retention without itself melting or decomposing. The glass-mica exhibits a rapid heating profile that provides the above described capabilities. Similarly, the glass-mica rapidly cools under the typical gas flows that extract the vaporized analytes following the end of the emission interval. This system, as demonstrated, is limited only by the thermal properties of the PTFE-coating on the wipe materials - it will melt if the desorber is allowed to get too hot. However, other wipe materials with similar infrared transmission properties and higher operating temperatures could also be used as alternatives. This limitation also led to further systems described below.

The remaining systems (have not been publicly disclosed) incorporate these core properties for additional capabilities. In the second system, indirect infrared thermal desorption, the geometry is rearranged such that the infrared emitted sits below a much thin(ner) energy storage plate (currently glass-mica) and emits upward, impinging the thermal energy storage plate. Here, the wipe-collected sample is inserted on top of the rapidly heated glass-mica energy storage plate. In this configuration the conduction through the heated glass-mica material plays a critical role in the temporal response of the device. Ongoing measurements have demonstrated that the prototypes of this system can achieve similar heating ramps and elevated temperatures as the original system. However, unlike the original configuration, the infrared transmission properties of the wipe materials are no longer restricted, therefore solving the problems described above. This configuration can use other commercially available wipe materials (Nomex, muslin, etc.) as well as additional wipe materials. This configuration also fully separates the near infrared lamp from the vaporized samples, eliminating the potential for contamination, fouling, etc. as well as any exposure of the user to the infrared irradiation.

A similar "indirect" configuration can also be used for the direct sampling of target surfaces (packages, luggage, vehicles, cargo, etc.) in a semi-remote sampling probe configuration. The infrared emitter remains isolated from the environment by a thermal energy storage plate. This plate is housed within a cavity that is directly open to the ambient environment. The emission intervals and infrared power levels can be manipulated to provide the heating profiles and durations desired for the sample materials and thermal properties. The sampling unit hovers over the target surface during the emission interval providing a heated gas pocket in contact with the hot glass-mica plate and target surface, thermally desorbing trace residues and particles of interest. This thermal energy storage plate is vital to the capabilities of the described system. Given the strong infrared radiation absorption of common everyday materials (e.g., many plastics), direct exposure to the infrared irradiation would lead to excessive heating and melting of select materials being interrogated. The described system uniquely allows for rapid and temporally discrete heating periods that converts infrared energy to thermal energy for conductive and convective heat transfer to target surfaces. The generated vapors and aerosols are then transported through a connecting transfer line to the detection scheme (MS, IMS, molecular sensing, etc.). This provides real-time sampling and detection over large areas, as well as high throughput screening. This system may also be useful for the direct sampling of large and onerous pieces of evidence in forensic applications.

Finally, all the systems described can also be configured with alternative broad-spectrum infrared emitters with varying spectral distributions, response times, etc.; alternative thermal energy storage materials with targeted properties; alternative wipe materials; and alternative geometries.