Increasing Signal-to-Noise Ratio in Gas Chromatography - Electroantennography Using a Deans Switch Effluent Chopper

Abstract

Gas-chromatography-electroantennographic detection (GC-EAD) is a technique used in the identification of volatile organic compounds (VOCs), such as pheromones and plant host odors, which are physiologically relevant to insects. Although pheromones often elicit large EAD responses, other behaviorally relevant odors may elicit responses that are difficult to discern from noise. Lock-in amplification has long been used to reduce noise in a wide range of applications. Its utility when incorporated with GC-EAD was demonstrated previosuly by chopping (or pulsing) effluent-laden air that flowed over an insect antenna. This method had the disadvantage that it stimulated noise-inducing mechanoreceptors and, in some cases, disturbed the electrochemical interfaces in a preparation, limiting its performance. Here, the chopping function necessary for lock-in amplification was implemented directly on the GC effluent using a simple Deans switch. The technique was applied to excised antennae from female Heliothis virescens responding to phenethyl alcohol, a common VOC emitted by plants. Phenethyl alcohol was always visible and quantifiable on the flame ionization detector (FID) chromatogram, allowing the timing and amount of stimulus delivered to the antennal preparation to be measured. In our new chopper EAG configuration, the antennal preparation was shielded from air currents in the room, further reducing noise. A dose-response model in combination with a Markov-chain monte-carlo (MCMC) method for Bayesian inference was used to estimate and compare performance in terms of error rates involved in the detection of insect responses to GC peaks visible on an FID detector. Our experiments showed that the predicted single-trial phenethyl alcohol detection limit on female H. virescens antennae (at a 5.0% expected error rate) was 140,330 pg using traditional EAG recording methods, compared to 2.6-6.3 pg (5th to the 95th percentile) using Deans switch-enabled lock-in amplification, corresponding to a 10.4-12.7 dB increase in signal-to-noise ratio.

Using two confluent capillary columns for improved gas chromatography‐electroantennographic detection (GC‐EAD)

“…The confluent configuration of two capillary columns described here, modified from the system described by Cork et al (), provides an elegant means of introducing make‐up gas when splitting GC effluent for GC‐EAD experiments, and allows for easy switching between stationary phases. This configuration of columns could also be combined with recent advances in techniques for effluent chopping (Zhou et al, ; Myrick & Baker, ) for a particularly sensitive EAD system. Given the benefits of running samples on different phases, this twin column method warrants greater consideration and uptake by other GC‐EAD practitioners, particularly when the samples contain mixtures of polar and non‐polar compounds (as is the case for many floral scents).…”

Induction Coil Heating Improves the Efficiency of Insect Olfactory Studies

“…Electroantennography (EAG) is a technique for measuring the difference in electrical potential across an insect antenna when exposed to a given odorant (Syntech, 2015), and has been commonly used in studies of chemical ecology and synthetic repellents. Although large EAG responses can be obtained from compounds such as insect pheromones, compounds only available in small amounts or those with low volatility result in small signals compared to background noise (Myrick and Baker, 2018). Increasing the amount of compound delivered to antennae is typically achieved by holding a treated filter paper within a sealed application pipette for up to 24 h to increase the vapor phase concentration (Stelinski et al, 2003), while a recent method employed a shortened pathway of the odor source to the antenna to prevent adsorptive loss and increased delivery efficiency (Ng et al, 2017).…”

Olfaction in Tephritidae: a balance between detection and discrimination

“…To improve the signal to noise ratio, the GC system was equipped with a D-Swafer Dean’s switch (PerkinElmer, Inc., USA) at the column output, as described previously 21 . The device allows for the GC signal to be chopped with the alternate emission of the effluent to both the EAG 3 and FID detectors 21,25 . The chop frequency was regulated to 1 Hz with Labview software (National Instruments, France) and a digital module (NI 9472, National Instruments, Nanterre, France).…”