Establishing the limit of detection and the resolution limits of odorous sources in the environment for an array of metal oxide gas sensors

Odour identification with gas sensor arrays relies on two main parameters: the limit of detection, defined as the concentration of the analyte corresponding to the lowest detectable signal for the sensors, and the limit of resolution, defined for pair-wise comparisons of vapours, as the concentration levels below which a vapour can no longer be reliably recognised from another one on the basis of its response pattern. For real-life environmental odours, however, it is no longer possible to express such limits in "concentration" terms, because they are complex gas mixtures composed of hundreds of various compounds. The paper tries to assess both limits for odorous sources in the environment firstly by finding an equivalence of the odour concentration, expressed in odorous unit per cubic meter (ou/m3), and the concentration of a reference gas, here the ethanol, and secondly by working with a statistical metric, based on the magnitude and standard deviations along linear projections of clustered response data. The methodology is illustrated for a metal oxide gas sensor array and the odour generated by urban waste composting facilities, which is compared to the background air and to ethanol vapour.