It is an analytical procedure that permits the rapid identification of compounds in complex mixtures that stimulate the olfactory sensilla of an insect. In other words, it can tell you what specific chemicals an insect can smell (and, to some degree, ones it can’t), and it can use odors derived directly from natural sources. This information can be used to discover potentially useful compounds—such as sex pheromones—that alter the behavior of insects.
In the 1950s, it was discovered that the voltage between the tip and the base of an insect’s antenna changed measurably when the antenna was exposed to odors of biological significance for the insect. This voltage is thought to represent the summed potentials of multiple responding olfactory neurons within the antenna, and the amplitude of the voltage roughly corresponds to an insect’s sensitivity to a particular compound. The voltage change does not reliably indicate whether a compound will influence the behavior of an insect or what the behavior might be, and it is quite common for compounds that produce strong antennal responses to have no observable behavioral effect. However, the presence of strong antennal responses (or responses to very low concentrations) indicates a greater probability that a compound will later be found to influence the behavior of an insect. Hence, antennal assays can assist in screening through the hundreds of odors found in the environment of an insect to permit the identification of those most likely to have behavioral activity. This can help in prioritizing compounds for behavioral tests and can greatly speed the identification of compounds that can potentially be used to modify insect behavior in beneficial ways.
Compounds can be exposed to the antenna after being purified or synthesized in a separate process; this procedure for manually exposing an antenna to compounds one at a time is known as an electroantennogram (EAG), and has received wide use in entomological studies since the late 1960s. The GC-EAD takes the EAG to a higher level of sophistication and utility by using the antenna of an insect as a detector for a capillary-column gas chromatograph.
The gas chromatograph (GC) is an apparatus used for separating and determining the identity and relative abundance of compounds present in complex mixtures of volatile and or semi-volatile compounds. Mixtures are flash-evaporated into a stream of inert gas moving through a long (typically 10-100 m), narrow (typically < 1mm) tube (called the “column”) lined with a semi-solid wax or polymer.