## 4 Classification of Measurement Methods
### Chemical Analysis
To reduce costs, all types of gas sensors with limited selectivity are often used [58; 62; 63]. They allow data to be collected in real time and significantly reduce the cost of research. These can be specific sensors that target individual odorants, e.g., hydrogen sulphide or ammonia [58], or non-specific sensors that respond to the presence of many compounds from the same group, e.g., conductometric or piezoelectric sensors [31; 58; 59]. Non-specificsensors detect the presence of a specific group of chemical compounds in the analysed gas and determine their concentration. The sensors consist of two basic parts: a receptor part (a chemically sensitive layer with high selectivity for the tested compound or group of chemical compounds) and a transducer part (responsible for generating a signal that can be interpreted). The receptor part is dominated by absorption, adsorption, chemisorption or coordination bonds [64]. The following are examples of a group of chemical sensors used in odourimetric measurements (Figure 1). In order to fulfil their task, the sensors for the analysis of odorous substances must be characterised by a low detection limit (close to the detection limit characteristic of the human nose). They should be resistant to the temperature and humidity fluctuations of gases and have good measurement repeatability. Humidity disturbances are an important factor influencing both the sensor resistance and the gas reaction. Water vapour can adsorb competitively on the surface and thus reduce the number of active sites available for adsorption of target molecules (e.g., semiconductor sensors) or interact with the internal electrolyte (electrochemical sensors) [65]. It is also necessary to pay attention to the operating temperature of each sensor. Acoustic surface acoustic wave sensors and polymer sensors are very sensitive to temperature fluctuations, while optical sensors cope well with this problem [66]. In the case of on-site use, the sensors should also be characterised by a short response time and small dimensions. Biosensors are also used. They belong to the group of non-specific sensors. The operating parameters of selected sensors are listed in Table 4 below [64]. Figure 1: Chemical sensors used to detect malodorous compounds. More and more measurement techniques are being developed that are based on a number of different sensors in a single device called an electronic nose [67]. This group of devices also includes those in which chromatographic columns are fitted instead of sensors. The electronic nose is characterised by a fast detection time and the ability to measure several parameters simultaneously [66]. The design of the device is based on the odour receptor cell of the human nose. The device consists of a sampling system, a series of detectors (the so-called sensor matrix) and a system for signal processing and recording. Specialised data analysis techniques, statistical tools and a suitable database of standards are also required to interpret the results obtained with such a device. In an electronic nose system, two essential components must work together synergistically: the sensor array (hardware) and the pattern recognition algorithm (software). Different types of sensors--such as semiconductor, polymer, quartz crystal microbalance or surface acoustic wave sensors--can be used to construct sensor arrays for electronic noses. These sensors can be integrated with both conventional and advanced odour pattern-recognition algorithms. The type of sensors and algorithms used directly affects the sensitivity, selectivity and accuracy of the technology [65]. During analysis, it is crucial to continuously monitor temperature and humidity to prevent sensor degradation [47].