Determining Limit of Detection for Odor Sensors

By Fanny Turlure

The detection limit (LOD) is an important specification for any gas concentration measurement system as it characterizes the minimum detectable concentration of a specific target compound in a matrix that the instrument can reliably detect as different from background.

Odor sensors, or e-noses, are intended to detect a wide range of volatile organic compounds (VOC)—in fact Aryballe has executed studies on thousands of target odors across for our customers and partners. These devices are intended to mimic the human sense of smell using sensor arrays and pattern recognition systems. We know that the LOD for the human nose varies based on the specific VOC and often also based on the individual and their personal experiences.

Below is a sample of detection thresholds for a selection of pure molecules based on the human nose (source: INRS and Y. Nagata, Measurement of Odor Threshold by Triangle Odor Bag Method)

Acetic acid Ethyl isobutyrate Styrene Undecanal
Olfactive human detection threshold (ppm) 0.48 0.00002 0.04 0.87

As the LOD for the human nose is highly variable, it should be expected that the LOD for an odor sensor would differ based on the specific VOC as opposed to a universal LOD to represent the device capability. This goes back to the basics of olfaction i.e., the chemistry of odor detection.

VOCs are in fact chemicals that interact with the nose olfactory receptors (ORs) or, in the case of Aryballe’s digital olfaction solutions, with biosensors mimicking these ORs. Such binding is related to the respective physio-chemical characteristics of sensors and VOCs, which generates the unique odor signature, or pattern of interaction. As the association of a VOCs differs by the nature of the peptide, the strength of this interaction provides an additional layer of information on the odor. Aryballe’s sensors offer a more subtle view of binding patterns by going beyond a mere bind/does not bind approach.
To determine the LOD for a particular VOC, there are two primary methods—liquid dilution and gas dilution.
The liquid dilution method can be easily implemented with basic lab equipment and involves preparing 1 mL of each solution at the required dilution in the chosen solvent, which can be mineral oil, triethyl citrate (citroflex), ethanol or water, given their miscibility and norm requirements when they apply. With liquid dilution it’s important to understand to select a diluent that will present the lowest background signal to the system—and to also test the system on the diluent without the sample as a reference to use in post processing of the data.

A gas dilution method is a bit more complex as it requires a system for controlled mixing with clean air and the desired compound in the headspace. Since the VOC is mixed with the ambient air, there is no need to also capture a reference (as it is the same as the system baseline).

Regardless of the dilution method, the set-up of a dilution series to determine the limit of detection needs to be a highly controlled environment (ex. Consistent temperature and humidity) and also incorporate the use of a secondary sensor, such as a PID, to capture the specific concentration levels. To calculate the LOD for a particular target molecule involves the association of the PID measurement and subtraction of the background (in the case of liquid dilution), which is done turnkey as part of our standard Aryballe Suite Detection mode.

In many cases, the question around the LOD for an odor sensor is actually the question of “Can this detect at or below the human threshold?” or in other words “Will this be useful for my application?” Often, we are looking at distinguishing across different types of samples—like flavored beverages or coffee products or detecting malodors in a car. And the usefulness is not just in the detection, but the sensor’s ability to identify and finely discriminate the odors of interest—consistently and objectively.