# An Improved Dictionary-Based Method for Gas Identification with Electronic Nose
## 2 Materials and Methods  
### Experimental Setup  
In recent years, metal-oxide-semiconductor (MOS) gas sensors have attracted extensive academic attention for their great sensitivity, fast response capability, low cost, ease of fabrication, and ease of integration into portable devices. Among them, tungsten oxide has been extensively researched due to its excellent chemical, physical stability, and good structural controllability [34]. A composite gas sensor was proposed based on one-dimensional W18O49 nanorods (NRs) and two-dimensional Ti3C2T\({}_{\mathrm{x}}\) Mxene flakes [35]. The W18O49/Ti3C2T\({}_{\mathrm{x}}\) composite structure on the surface of the flakes had a high response to low concentrations of acetone and long-term stability, making it an ideal choice. Elsewhere, researchers have formed porous SnO\({}_{2}\) nanosheets formed by direct calcination, which imbues the sensor with ideal selectivity, excellent repeatability, and cycling stability [36]. A new 2D/2D-based portable real-time acetone concentration integrated detection system has also proposed, which has the characteristics of the real-time monitoring of acetone concentration and low energy consumption [37].  
Therefore, in this paper, metal oxide semiconductor gas sensors composed of WO\({}_{3}\) + CeO\({}_{2}\), WO\({}_{3}\) + Au, and WO\({}_{3}\) + AZO materials were used as the experimental equipment. This paper used a laboratory-made electronic nose system. The experimental devices are shown in Figure 1a,b, whilst the experiment and the measurement circuit are shown in Figure 2a,b, respectively. The electronic nose system includes a single-chip microcomputer, a display screen, a heating circuit, a Bluetooth module, and a sensor array consisting of three different gas sensors.  
The sensor's composite material gas sensing probe detects the change in the gas sample concentration and the voltage signal changes caused by the resistance change of the material. The working current of the electronic nose system is 10-80 mA, the output voltage of the test circuit is 0-5 V, and the output voltage of the heating circuit is adjustable (from 5 V to 10 V). The array is placed in a 20-L airbox, and the heating voltage of each gas sensor is adjusted to the optimum operating voltage. The data generated are transmitted to the PC through the Bluetooth module and then controlled by the PC. The entire experimental testing procedure is as follows: (1) The chemical analyte of the required concentration is  
Figure 1: The laboratory equipment and background. (**a**) Gas testing laboratory. (**b**) Homemade electronic nose.  
injected into the gas box, and is evaporated into gas by contacting the evaporating dish; (2) the test gas is kept in the gas box for about 240 s to test the steady-state response of gas; and (3) exhausting the gas for about 300 s to make it easier to test for other types of gas.  
As shown in Table 1, the electronic nose system tested three chemical analytes. The measured gases were xylene, acetone, and formaldehyde. For the convenience of expression, the three gases are marked as three separate categories. In this paper, all gases were obtained by evaporating the liquid with different concentrations. Therefore, the concentration is the mass of solute in parts per million of the total solution mass, and ppm is the unit of concentration. The entire experiment lasted for two consecutive months: each test was repeated 60 times, and the number of samples in every category was 300, therefore the measured dataset was 900 in total.  
Figure 2: A schematic diagram of the experiment and measurement circuit. (**a**) Schematic diagram of the experiment; (**b**) measurement circuit.