Measurements of Some Volatile Compounds by Means of the Electronic Nose

Using an electronic nose, concentration ranges of volatile fatty acids(VFAs), methane and butane, NH₃, HCl, SO₂ andN₂O have been measured to establish the relation between theconcentration in the liquid or the gas sample and the electronic nosereading. A quantitative sensorial odor perception (SOP) was introduced,based on the average reaction of the twelve available sensors of theelectronic nose. The results of the different compounds showed that thesensors reach a saturation level with increasing concentration. In the lowerconcentration ranges, linearity between concentration and signal outputoccurred. This linear interval was situated for the VFAs between thedetection limit in the range of 5 to 15 g dissolved compound per L distilledwater and the upper limit of 60 g L^-1. For the gases, thedetection limit varied between 6 and 690 volumes of gas per million ofvolumes air (ppmv). The upper limit of the linear interval ranged from 100–3000 ppmv depending on the compound. For the olfactometry reference product n-butanol, with a reported olfactory lower threshold valueof 0.04 ppmv, the electronic nose was less sensitive and gave a detectionlimit around 975 ppmv. The different compounds could be visualized in radarplots, which had a specific profile for each compound. The higher theconcentration of the volatile compounds in the air, the larger the surfaceof the respective radarplot. A discriminant analysis showed clusters ofcompounds such as the VFAs, the non polar gaseous compounds methane andbutane and the other more polar gaseous compounds.