Miniature Chemical Laboratories|
(ÁChemLab) Gas Phase System Development
As part of a large internal Sandia investment, a miniature, integrated chemical laboratory (ÁChemLab) is being developed that uses microfabrication to provide faster response, smaller size, and an ability to utilize multiple analysis channels for enhanced versatility and chemical discrimination. One goal of this program is to demonstrate (by 9/99) an autonomous chemical analyzer the size of a palm-top computer that incorporates a gas phase analysis system for detecting chemical warfare (CW) agents and a liquid phase analysis system for detecting explosives. This document focuses on the gas phase system development where the goal is to demonstrate detection of nerve agents such as sarin and soman (detection levels of 1 ppb) and of blister agents such as mustard gas (detection levels of 10 ppb). This detection should be demonstrated with low false alarm rates and in the presence of large backgrounds (>1000-fold higher concentrations) of potentially interfering compounds such as gasoline, diesel, and industrial solvent vapors. A complete analysis, including sample collection, separation, detection, and data analysis, should be completed in as little as one minute.
Figure 1: Schematic of the baseline design of the ÁChemLab gas phase chemical analysis system.
ÁChemLab systems should be able to provide fast analyses with high sensitivity and effective chemical discrimination using small, potentially low-cost systems. These performance capabilities will make ÁChemLab useful for a wide variety of applications. Some of the potential applications include:
- detection of explosives and chemical weapons (counterterrorism, nonproliferation)
- on-line monitoring of industrial and environmental remediation processes
- rapid biomedical screening
- low cost monitoring for air, water, and food safety
- industrial hygiene detection of workplace chemicals
The baseline design for our gas phase ÁChemLab system is shown in Figure 1. Improved sensitivity and selectivity are achieved by using a cascaded approach where each channel includes a sample collector/concentrator, a gas chromatographic (GC) separator, and a chemically selective surface acoustic wave (SAW) array detector. The sample collector/concentrator includes a thin film porous adsorbent to selectively collect chemical anolytes to be detected and a heater for thermal desorption of the collected anolyte into a concentrated pulse. The GC separator includes a long flow channel (current components have one meter long columns in a one cm2 area--see Figure 2). By coating the flow channels with a stationary phase material, anolytes are separated in time based on differences in partitioning into this stationary phase as they move through the column. An example of separation using a microfabricated GC column is shown in Figure 3. Finally, detection is achieved using an array of surface acoustic wave (SAW) sensors acting as sensitive mass detectors (subpicogram mass changes can be detected). By coating the SAW devices with chemically distinct thin film materials, a unique pattern of responses for different chemical anolytes can be used to provide chemically selective detection. At the current time, commercially available miniature gas valves and diaphragm pumps are used to direct gas flows and to pull gas through the analysis channels.
Figure 2: Picture of current prototype components for the sample collector/preconcentrator, gas chromotographic separation column, and surface acoustic wave sensor array
detector. Components shown in a snow pea pod for scale.
Prototypes of all three components (shown in Figure 2) have been developed and demonstrated and current work is focused on integrating these into a complete analysis system. The size of the components shown indicates one of the advantages of microfabrication. These small components (all three components fit in an area less than that of a US dime) allow miniature systems incorporating multiple analysis channels to be produced. These multiple analysis channels are used to improve discrimination by allowing the simultaneous use of more than one channel to identify an anolyte. They can also increase versatility by allowing different channels to be tailored to detect different chemical anolytes. For example, for CW agent detection, one channel might be optimized for detection of phosphonate nerve agents while another is tailored to blister agents like mustard gas.
Figure 3: Rapid separation of dimethyl methyl phosphonate (DMMP) a simulant for a nerve agent such as sarin, from three components of gasoline, a potentially interfering chemical background. This separation was achieved using a microfabricated GC column that has one meter of column in an area of only 1 cm2.
Batch microfabrication also provides other advantages for the ÁChemLab. First, it provides several performance advantages. For the sample collector/concentrator, the low heat capacity of the microfabricated thermal desorption stage allows it to be heated rapidly with low power. The rapid heating provides sharp chemical pulses that provide improved temporal separation in the GC column. This small size also offers the opportunity for low power heating of the GC and SAW components for either temperature control or temperature ramping. Miniaturization of the SAW devices actually results in a significant increase in their mass sensitivity. In addition, the very low dead volume of the components can result in improved GC separations (the current design has a dead volume in the gas chamber over the SAW device of only 1.5 ÁL). The use of microfabrication also allows integration and improved packaging. For example, we are currently working on integrating the SAW drive electronics on-chip with the SAW sensors allowing a single chip measuring less than 5 mm on a side to be used as the detector. Finally, batch microfabrication can produce components at low cost, potentially opening up a variety of new markets where current chemical analysis system are cost prohibitive.
Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy.
Last modified: August 23, 1999
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