Controlled Odor Mimic Permeation Systems for Olfactory Training and Field Testing

Olfactory research is limited by the lack of methodology for delivering known and reproducible odors during operational testing or training. COMPS provides a convenient fieldable method for controlled odor delivery. COMPS can contain a wide range of odorants and the delivery rates can be varied by adjusting either the bag thickness, the surface area, and/or the polymer type.

To impregnate a substrate with odorant, use a calibrated pipette to add five microliters of neat compound onto a two by two inch cotton gauze pad and fold the pad in half. Then place the gauze into a two by three inch low density polyethylene permeable bag and immediately seal the bag with a heat sealer. To determine the permeation rate of the odorants through the permeable bag, place a newly made COMPS in a weight boat inside a fume hood and use a second clean weight boat to zero an analytical balance.

Transfer the COMPS from the fume hood onto the balance and to record the mass before quickly returning the COMPS to the fume hood. Then calculate the permeation rate from the COMPS by plotting mass versus time and finding the slope of the line. To analyze the headspace by SPME with gas chromatography mass spectrometry, allow the COMPS to equilibrate in the open weight boat in the fume hood for 30 minutes, transferring the COMPS to a non-reactive container, such as a glass jar or an epoxy lined metal container.

For sampling after equilibration, place a lid with a one centimeter hole in the lid onto the one gallon container and insert inappropriate SPME fiber into the hole to extract the analyte of interest. At the end of the extraction period, transfer the fiber into the heated inlet of a gas chromatography mass spectrometer for thermal desorption and analysis. For storage, place a single COMPS in a metalized 3.5 by 4.5 inch barrier bag and heat seal to close, removing as much air as possible from the bag prior to sealing.

If multiple odorants or odorants’delivery rates are being tested in a single experiment, placed the bag and secondary containment to eliminate any possible cross-contamination during transportation and storage. Place replicate multiple barrier bags containing individual COMPS of the same analyte and permeation rate in an outer larger container for storage and transport. Then store the bag in cool ambient or refrigerated conditions.

To set up a basic canine olfactory test, layout multiple lines of at least five identical containers and set up the trials so that each line contains one container with a target COMPS and four with blank COMPS. Positive control lines prepared in the same manner, but with a known target odor, may be used as appropriate for the experiment, training, or testing scenario. An additional negative control or blank line should contain five blank COMPS and no targets.

Next, transfer the permeable bag for each COMPS from the secondary and outer containers into the appropriate trial containers. After 30 minutes of equilibration, start the test. Here, an example of gravimetric results from the permeation of three identical COMPS prepared from five microliters of pentanoic acid on cotton gauze through a three mil low density polyethylene bag are shown.

The addition of regression to the graph reveals that the permeation rate is 37 micrograms per minute for this set of COMPS. The amount of odorant released for a given test can be adjusted by modifying the amount of material in the bag, the surface area of the permeable bag material, or the bag thickness. Here, the variation in vapor pressures across the groups of analytes are compared to the variation in permeation rate after adjusting the bag thickness to the control permeation rates of each analyte, making the rates as similar as possible.

Conversely, adjusting the bag thickness for a single analyte allows the permeation rates to vary by three orders of magnitude. Headspace measurements can be used to better measure the amount of odorant available during a given testing or training scenario. For example, in this representative chromatograph, the piperonal peak areas can be observed to increase with the increasing permeation rate.

Using these three sets of piperonal COMPS in canine trials, a measurement of the limit of detection of the canines for piperonal was estimated. In this analysis, the permeation rate and thus the odor availability, increased as well as the number of canines alerting to the appropriate COMPS. Since COMPS were first developed, we’ve used them to establish active odor signatures for a variety of detection targets, estimate limits of detection, and test canine discrimination capabilities.