## 4 Classification of Measurement Methods
### Combined Methods
The popular combination with a mass spectrometer as a conventional detector (GC-MS/O) enables not only the evaluation of odour compounds, but also their identification based on information about the mass spectrum. Simultaneous sensory evaluation makes it possible to determine whether a particular compound, separately from the others present in the mixture, is odorously active (at a certain concentration in the sample extract that is above the sensory detection limit) [102]. Assessors may be asked to indicate the time of odour perception (indicating the beginning or the end) and to describe the hedonic tone or intensity, e.g., using an odour intensity scale. Only a portion of the volatiles present in the matrix can really contribute to the overall odour perception and not all odorants contribute equally to the odour profile of the mixture, therefore a large peak area in GC does not necessarily correspond to a high odour intensity [101]. The use of combined techniques facilitates the solution to this problem. The separation of chemical and sensory analysis also encounters the problem of the low sensitivity of the analytical instruments to the human nose. Important odorants may be present in the matrix in trace amounts, and other odorants present in the matrix may complicate the evaluation, e.g., by co-elution (peak overlap) [52]. The simultaneous combination of sensory and analytical techniques therefore enables a more in-depth analysis of the results obtained and can improve accuracy. The GC-MS/O method is used, for example, to determine the effectiveness of different gas deodorisation systems and to develop a suitable and cost-effective strategy [103; 104]. Gas chromatography-mass spectrometry-olfactometry (GC-MS/O) compounds are modifiable, and there are already known combinations of multidimensional gas chromatography with mass spectrometry and olfactory (MDGC-MS/O). If the composition of the matrix is very complex (the complexity of the matrix leads to multiple peak overlaps), one-dimensional gas chromatography may be inadequate. The principle of multidimensional gas chromatography techniques is based on the use of sequentially connected capillary columns, which are characterised by different selectivity towards the chemical compounds present in the sample mixture. The choice of column combination is closely related to the composition of the matrix and the purpose of the analysis. Usually, the first column (the so-called first dimension) is non-polar and the second (the second dimension) is polar [101], although the \"reverse column configuration\" is also used [105]. Unseparated sample fractions leaving the first column are transferred to the second column using a device that supports the heartcut or modulation process [106]. MDGC-MS/O are used to analyse complex mixtures of odorous substances and to identify odour-active compounds. This shifts the evaluation of odour emission from purely olfactometric evaluation to the characterisation of the volatile components responsible for the odour nuisance. MDGC-MS/O has been used to identify odorous substances in livestock farms [107]. However, it should be noted that in combined methods based on chromatographic techniques, volatile compounds are evaluated separately. The aspect of interactions between the odorous substances in the mixture is omitted here. Therefore, an integration of the obtained data with the sensory properties of the whole gas is necessary to understand the olfactometric phenomenon. A similar approach to the problem is proposed by the aforementioned e-noses. E-noses are a promising alternative for monitoring and analysing gases in real time. However, unlike gas chromatography, which separates compounds based on their chemical properties, e-noses use sensor arrays to recognise compounds through pattern recognition. Here, the individual sensors only react to certain compounds/groups of compounds. Only the signals collected from the individual sensors provide a comprehensive picture of the gas being tested.