Vapor Generation for Sensor Calibration
Overview and Detection Applications
The need to detect very low levels of illicit substances (chemical and biological agents and explosives) has become, after September 11, 2001, a priority for federal, state and local government agencies. Systems capable of detecting minute amounts of the above materials are required in airports, border crossings, federal buildings, ports, embassies, and high security areas. Already there are tens of thousands trace detectors deployed in such areas. Explosives represent one important class of illicit substances with military explosives (e.g. TNT, RDX, PETN, HMX) being an important subclass that is currently targeted by various trace detection methods. Trace detection - detection of very small amounts of explosives - identifies people or things that have come in contact with explosives. Trace detection methods have been implemented in a variety of instruments ranging from hand held and portable to bench top or portals. Some of the most used detection methods are described below.
Detection methods identify the signature from vapors emanated by explosives. The main issue is that the vapor pressure or concentration of high explosives is fairly low at ambient temperatures. The concentration of explosive vapors in 25°C air ranges between 1 part per thousand to 1 part per trillion or lower. Thus the detection instrument either has to sample large volumes of air or have a high sensitivity, with the later option being preferred. Besides trace detection by canines, detection methods can be classified as: separation methods (gas chromatography - GC, high performance liquid chromatography - HPLC, capillary electrophoresis - CE), ion detection methods (mass spectroscopy - MS, ion mobility spectrometry - IMS), vibrational spectrometric methods (infrared absorption, Raman scattering, etc.), UV/visible methods (fluorescing polymers, color reactions), immunochemical sensors, or electrochemical sensors. Multiple sensors can be incorporated in a single "electronic nose" instrument. From the analysis of the systems that are currently available on the market, it appears that the IMS trace instruments are very commonly used and can be applied to a wide range of systems (from hand held to portals).
The Need of a Vapor Generator
Out of the various methods described, vapor detection emerged as the most practical and preferable detection method, especially after the latest improvements in sensitivity. The existing systems that produce vapor of known explosive concentrations are based on extracting the vapors from solid explosives. These systems are fairly large and with little perspective for miniaturization and with very small dynamic range. MicroFab’s system can be easily reduced in size and manufactured as a modular component to be included in trace detection systems for periodic auto calibrations.
By creating explosive vapors of known concentration, the vapor generator provides the means to verify the detection limit of the systems in the field and their recalibration. IMS is one of the most popular technologies employed in vapor trace detection, but it is sensitive to variations in pressure due to weather or altitude. The vapor generator can be used to recalibrate IMS systems at various operating / environment conditions.
Another area of use of the vapor generator is the comparison between various instruments. Currently, the only information available in terms of the sensitivity of an instrument is from the manufacturer. Each manufacturer uses different methods to determine and to report the sensitivity of their instruments. To be able to compare trace detectors from various manufacturers, benchmark instruments and test procedures are necessary.
The continuous research and development for the improvement of the detection limit requires a vapor source of very low concentration. It is desired that such a vapor source is portable, because a large number of the vapor trace detectors deployed in the field are fixed. Existing technologies are not very precise and cannot be easily miniaturized. NIST has used data from MicroFab’s microdispensers to evaluate the potential range provided by a vapor generator employing ink-jet microdispensers for several explosives (RDX, TNT and PETN) and has shown that the concentration can be varied almost continuously from 0 to hundredths of parts per trillion (v/v). This range covers not only the current detection limits, but will also cover future improvements in sensitivity of newly developed detectors. Because the vapor levels within the range can be precisely controlled, the proposed vapor generator will be capable of quantifying the intermediary steps in the development of new detection methods or improvement of existing ones.
Advantages of the Vapor Generator for Sensor Calibration
For the HPLC based detection systems there are standards available from AccuStandard and Absolute Standards. To a certain extent, these standards can be used for the vapor trace detectors by pipetting known volumes of such solutions onto substrates and then use these samples. The issues for this approach are related to the precision of the dispensing and the dynamic range.
Another option consists of particle standards. But, these do not work very well for vapor detectors with very low detection limits. In NIST’s view, vapor generators have significant advantages over solid particle sampling, especially in the context of increased sensitivity. Also, systems using explosive particles to generate the vapors are fairly complicated and cannot be miniaturized.
The ink-jet based vapor generaton system brings the following major advantages:
- High Precision: Ink-jetting produces highly repeatable drops that can create larger volume by accretion.
- Continuous Variation: The very small size of the individual drops (20-200 picoliters) produces, from the perspective of this application, almost continuous variation of the total (accumulated) amount.
- Range of Concentration: The dynamic range of a vapor generator based on ink-jet microdispensers extends from almost zero (equivalent of several drops) to several thousands of parts per trillion. The low-end resolution can be further increased by using more dilute solutions containing the substances of interest.
- Data Driven: The piezoelectric dispensers are electrically driven and they can be controlled from data files. This makes the ink-jet vapor generator easily adaptable for automatic testing and possibly for automated calibration.
- Multiple Solutions / Explosives: An ink-jet vapor generator can be easily adaptable for multiple solutions. The cartridges containing the solutions can be all loaded in the system and the operator (or the automatic calibration program) can select between them.
- Size: The proposed vapor generator can be developed in a portable format that is required by the fixed systems in the field. The system can be further miniaturized and possibly made as a modular component for incorporation in the vapor trace detectors.
- Automatic Calibration: A modular component of the vapor generator can be incorporated in the vapor trace detectors. Due to its data drive nature, the ink-jet vapor generator can be integrated with the detector part for automatic calibrations.
MicroFab's Experience in Vapor Generation
MicroFab has extensive experience working with ink-jet based microdispensing systems and microdispensing devices. One of such systems developed under a National Institute of Health grant employed microdispensing devices to quantify the olfaction threshold of humans. Because the olfaction brain center is one of the first affected by neurodegenerative diseases like Alzheimer's, an increase in the olfaction threshold is a possible early diagnostic method. The developed digital olfactometer used piezoelectric dispensers manufactured by MicroFab to produce 100 picoliter drops of a phenethyl alcohol (rose) fragrance. These drops were deposited onto
a heater that vaporized the solution and presented the vapor cloud to the patient's nose.
MicroFab's prototype vapor generator. Top: Overall view. Bottom: Detail of the generator subassembly.
The first prototype of a vapor calibration system was built by MicroFab and shipped to NIST early in 2005. The system has been under evaluation with very encouraging results. NIST has used the system to create vapors of three high explosives: RDX, TNT, PETN. Preliminary results were presented at
ISIMS 2005.
MicroFab's prototype vapor generator as installed at NIST.
Handheld Calibrator
The figure to the right shows a portable vapor generator concept that is suitable for on site calibration of explosives detectors. It features a graphical user interface, preprogrammed test routines, data storage for test results, and replaceable cartridges that would allow a wide range of test vapor types from a single or two channel system. The fundamental approach is technically very similar to the olfactometer and the vapor calibration system at NIST. Because of the digital nature of the delivery system, the vapor cloud generated could be either constant concentration, or a pulse of a known number of molecules.
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