Method to Assist in the Preparation of Samples for Trace Detectors

4 July, 2019

Summary

Use thermal imaging to monitor solvent evaporation from samples deposited on chemical trace detection swabs or swipes or to decrease sampling time and assist in the automation of sample processing.

Problem Solved

To prepare test samples for presentation to trace detection equipment, a target analyte; chemical, explosives, TIC, or other analytes of Time Sequence of Droplet Evaporation interest; is dissolved in a solvent (e.g., acetonitrile, methanol, acetone, toluene, alcohol, etc.) and diluted to a very low concentration representative of a detection level of interest (e.g., for a trace explosive sensor, trinitrotoluene (TNT) might be dissolved in a blend of methanol and acetonitrile and diluted to 10 ng/μL). A very small amount of this dilution is then deposited onto a sampling swab to arrive at a precise dose of analyte for testing by the instrument (For instance, 2 μL of a 10 ng/μL dilution will yield 20 ng of analyte on the sampling swab.)

A challenge with this method is determining when the solvent has fully evaporated, leaving only the analyte residue behind. Since the solvent itself may induce a response from the instrument, it is usually important to ensure it has fully dissipated before presenting the sample. However, waiting too long can significantly reduce the sample throughput. Additionally, some analytes are quite volatile themselves, and in such cases the analyte will begin to evaporate after the solution is deposited, reducing the net amount that will be available to the instrument. Furthermore, the solvents are transparent, making these very small droplets difficult to see with the naked eye. Consequently, it is desirable to have a method to monitor the sample preparation to know when the solvent has fully evaporated and the sample is ready for presentation.

Description of Solution

Thermal imaging works well to monitor when the solvent has fully evaporated. As that process takes place, evaporative cooling creates a thermal cold spot on the swab that is clearly visible using a thermal camera. This is evident even when the droplet is present but nearly invisible to the naked eye.

Example of Use

The subsequent figures show the example set up and a sequence of thermal images taken of two swabs dosed with small aliquots of an acetonitrile-based solution. Since the solution itself is transparent, it is difficult to see with the naked eye. However, as the droplet evaporates, the cold spot in the image shrinks in size, but remains in high contrast to the surrounding material due to the distinct temperature difference. Once the droplet evaporates, the cold spot quickly reaches thermal equilibrium with the surrounding material, giving clear visual indication that the sample is fully dried and ready for presentation to the instrument.

Figure 1: Visible image of sampling swabs. The droplets are difficult to see with the naked eye, and almost impossible to see in this photograph (red arrows).

Figure 2: Time sequence of thermal images showing the evaporation of solvent from the swab. Note that it took nearly 8 minutes for the samples to completely dry. (Images acquired with a FLIR® T540 camera).

By Brian O’Dell