## 4 Classification of Measurement Methods
### Combined Methods  
In recent years, especially in the USA and Europe, techniques have been developed that combine traditional gas chromatography analysis with sensory detection. These methods are considered to be very expensive. Gas chromatography-olfactometry (GC-O) is a method based on the separation of odour components from a complex mixture. Suitably trained personnel then smell the eluate using a specially developed connector connected in parallel to conventional detectors to detect the presence of odour compounds [52]. After separation and elution by the chromatography column, every odour compound that can trigger an odour stimulus can be detected by the evaluation unit via a specially developed detection port. A conventional detector (e.g., a flame ionisation detector (FID) or mass spectrometer (MS)) is connected in parallel to this detection port in order to detect the type \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Method** & **Basis of Application** & **Outcome** & **Application** & **Ref.** \\ \hline \multirow{3}{*}{Intensity Scale} & Panellists rate odour & \multicolumn{2}{c}{Assessment of} \\  & intensity on a & \multicolumn{2}{c}{Numerical value} & environmental odours, \\  & numerical scale & (e.g., 0–5 or 0–9) & products, odour emission & \\  & & & analysis & \\ \hline \multirow{3}{*}{\begin{tabular}{c} Hedonic Scale \\ (Pleasantness) \\ \end{tabular} } & Evaluation of odour along & \multirow{3}{*}{Scale, e.g., from \(-\)5} & Olfactory comfort & \multirow{3}{*}{[88; 89]} \\  & the & \multicolumn{2}{c}{to +5} & & \\ \cline{1-1}  & pleasant–unpleasant axis & & & \\ \hline \multirow{3}{*}{\begin{tabular}{c} Threshold Tests \\ (e.g., 3-AFC) \\ \end{tabular} } & Determination of the & \multirow{3}{*}{Detection threshold} & Certified laboratories, \\  & lowest concentration at & (e.g., ouE/m\({}^{3}\), log C) & & olfactory sensitivity \\  & which odour is detectable & (e.g., ouE/m\({}^{3}\), log C) & assessment, normative \\  & & & studies & \\ \hline \multirow{3}{*}{\begin{tabular}{c} Descriptive \\ Profiling \\ \end{tabular} } & Assignment of qualitative & \multirow{3}{*}{Descriptor frequency} & Characterisation of \\  & descriptors from a & & complex odours, food and \\  & predefined lexicon & & environmental analysis & \\ \hline \multirow{3}{*}{\begin{tabular}{c} Reaction Time/ \\ Identification \\ \end{tabular} } & Measurement of response & \multicolumn{2}{c}{Mean reaction time} & Cognitive research, \\  & time and accuracy of & \multicolumn{2}{c}{(s), \% of correct} & neurological testing (e.g., \\  & odour identification & responses & Alzheimer’s, Parkinson’s) & \\ \hline \multirow{3}{*}{\begin{tabular}{c} Olfactory \\ Adaptation \\ \end{tabular} } & Repeated & \multirow{3}{*}{Intensity vs. time} & Assessment of olfactory \\  & exposure—evaluation of & \multicolumn{2}{c}{} & \\ \cline{1-1}  & intensity decreases & \multicolumn{2}{c}{curve} & \\ \cline{1-1}  & over time & & prolonged exposure & \\ \hline \multirow{3}{*}{\begin{tabular}{c} AI/E-nose \\ Assisted \\ Measurements \\ \end{tabular} } & Prediction of perception & \multirow{3}{*}{\
\begin{tabular}{c} Predicted intensity, \\ chemical sensors and AI \\ models \\ \end{tabular} } & \multirow{3}{*}{
\begin{tabular}{c} Panel calibration, \\ real-time measurements \\ \end{tabular} } \\  & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Frequently used quantitative methods for odour measurement by teams of olfactory specialists. of compound perceived by the evaluation unit. In this way, each compound separated by chromatography can be described in terms of the quality of the perceived odour and its intensity (sensory analysis--assessment team), and also qualitatively and quantitatively identified (chemical analysis--conventional detector). The precision and accuracy of the collected data are ensured by dividing the stream leaving the gas chromatograph (at a constant division coefficient--appropriate flow dividers) and directing the precisely divided streams to two different detection methods [101].