What do dogs detect

For explosives, there are three possible types of signature a dog can key on (1) the explosive itself (2) a contaminant or minor constituent of the explosive common to most batches of that explosive and (3) a decomposition product of that explosive. 

One theory advanced about the canine detection of explosive is that the dog actually identifies the specific explosive molecule. This line of reasoning concludes that training on a pure sample of a single explosive compound should enable a dog to detect any target containing that explosive, regardless of the presence of other materials in the vapor headspace. This does not appear to be the case. 

Analysis of a group of smokeless powders of known composition showed that the main components of their headspace vapor were acetone, toluene and limo-nene the concentration of nitroglycerin (NG) was relatively low. A series of 

Analysis of the headspace of a group of TNT samples from different sources showed the principal vapor component to be 2,4-dinitrotoluene, with smaller amounts of the other dinitrotoluene isomers, and 1,3-dinitrobenzene [8]. Controlled tests in the olfaction chamber showed that most of the dogs principally responded to 2,4-dinitrotoluene as smelling like TNT and mostly ignored the other compounds [9]. 

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