An olfactometer or "electronic nose" is able to vary a plurality of operating parameters during a test cycle in parallel, in accordance with a measurement protocol. This measurement protocol, and correspondingly the operating parameters to be varied, the values to be set for those parameters, and the timing of the variation in these values is tailored to most effectively distinguish between likely candidates in a particular testing scenario. A characterisation library is then used to match the results of the measurement protocol to the best target in the characterisation library. Test protocols and/or characterisation libraries may be downloaded from a remote server on demand, and certain activities may be carried out either locally or remotely. The gas measurement system may include an ultraviolet light source, a heat source, a controllable voltage source and a chemical gas filter.