## 4 Classification of Measurement Methods
### Chemical Analysis  
If the gas contains a type of odorant, chemical analysis often allows a clear correlation between its concentration and the odour sensations using the Weber-Fechner law (Equation (3)) or [PERSON] law (Equation (4)) [58; 59]. \[I=Cln\frac{B}{B_{0}} \tag{3}\]where \(I\)--odour intensity; \(B\)--odorous substance concentration; \(B_{0}\)--olfactory detection threshold of the odorous substance and \(C\)--empirical constant characteristic of the odorous substance. \[I=a\cdot B^{n} \tag{4}\]  
where \(I\)--odour intensity; \(B\)--odorous substance concentration and \(a\), \(n\)--empirical constants characteristic of a particular odorous substance. On the other hand, it is believed that the chemical analysis of a mixture of different types of odorous substances is theoretically possible but poses too many problems of interpretation to be used on a large scale to determine odour experiences. If the composition of an odorant mixture is determined qualitatively and the concentration of the individual components is specified, it is difficult to predict the intensity of the odour of such a mixture. This is due to olfactory interactions between the gas components, such as synergism, neutralisation or masking of the odour. The intensity of the odour of the mixture usually deviates completely from the predictions made on the basis of the odour thresholds of the individual odorous substances and the values of their concentrations in the tested gas. If the intensity of the odour is higher than expected, we speak of a synergy of odours. The perceptibility of the odour is enhanced by the mutual interaction of two or more gas components with each other. If the odour intensity of the mixture is lower than expected, we speak of a neutralisation (or compensation) of the odour. In such cases, we are dealing with a reduction in the perceptibility of the odour or its disappearance as a result of the mutual interaction of two or more odorous substances. Masking refers to a distinct type of interaction where an unpleasant odour is replaced by a pleasant one. The introduced ingredient blocks the perception of an unpleasant odour in favour of a pleasant odour [58]. Not only the type of compound, but also its concentration influences the interactions. Not only odour-causing compounds can be involved, but also those that potentially do not cause odour sensations (when isolated). Their mechanism is not yet fully known, hence the problem with interpreting the results obtained in this way. Another problem is the correct determination of the composition of the mixture in terms of quality. It is easy to ignore an important component of such a mixture if its concentration is low (it can be categorised as insignificant background). We do not always have such sensitive research equipment to determine very low concentrations in a gas mixture. It is worth remembering that even very small amounts of an odorant can make a significant contribution to the olfactory experience. This poses a technical problem. In addition, the determination of low concentrations is often associated with higher analysis costs, so that another problem arises--an economic one. However, gas deodorisation requires classical chemical analysis, which requires knowledge of the gas composition. The analysis techniques can be very different. Chromatography with various types of detectors, gas sensors and electronic noses is most commonly used. Mixture separation techniques, which include chromatography, are used in combination with different types of detectors, e.g., based on spectroscopy, potentiometry, conductometry or polarography. Most commonly used are one-dimensional chromatography techniques, e.g., gas chromatography (GC) in combination with mass spectrometry (MS) or more accurate multidimensional chromatography techniques, e.g., MDGC, also in combination with MS [57; 58; 60; 61]. With these techniques, the composition of the gas can be determined both quantitatively and qualitatively.