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 and N2O have been measured to establish the relation between the concentration in the liquid or the gas sample and the electronic nose reading. A quantitative sensorial odor perception (SOP) was introduced, based on the average reaction of the twelve available sensors of the electronic nose. The results of the different compounds showed that the sensors reach a saturation level with increasing concentration. In the lower concentration ranges, linearity between concentration and signal output occurred. This linear interval was situated for the VFAs between the detection limit in the range of 5 to 15 g dissolved compound per L distilled water and the upper limit of 60 g L-1. For the gases, the detection Limit varied between 6 and 690 volumes of gas per million of volumes 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 value of 0.04 ppmv, the electronic nose was less sensitive and gave a detection limit around 975 ppmv. The different compounds could be visualized in radar plots, which had a specific profile for each compound. The higher the concentration of the volatile compounds in the air, the larger the surface of the respective radarplot. A discriminant analysis showed clusters of compounds such as the VFAs, the non polar gaseous compounds methane and butane and the other more polar gaseous compounds.