Maybe it would be simpler to train a dog.

From Bristol University(UK) academics have found, for the first time, smells from healthy faeces and people with infectious diarrhoea differ significantly in their chemical composition and could be used to diagnose quickly diseases such as Clostridium difficile (C. Diff.). It is hoped the discovery of these chemical profiles will lead to the development of a device capable of rapid diagnosis at the bedside, saving both time and money. Rather than trying to develop a "sniffometer" it would be simpler to train dogs to detect the difference. If dogs can smell drugs and explosive materials why not bugs?