## 4 Applications of Piezoelectric Materials in Analytical Chemistry
### Detector for Gas Chromatography
The equations developed by them to describe the action of the crystal distribution detector in the gas and liquid chromatography under equilibrium conditions are shown below (Equation (4)),  
\[A_{y}=\frac{mK_{y,x}\ V_{x}\ W_{T}}{F} \tag{4}\]  
where \(A_{y}\) is the area below the peak caused by the \(y\) component, m represents the constant giving the change in crystal frequency caused by a mass increase at the surface, and \(K_{(y,x)}\) is the partition coefficient of gas \(y\) in the liquid \(x\), which describes the following ratio shown by Equation (5):  
\[\frac{W_{y,x}}{W_{y}}=K_{y,x} \tag{5}\]  
\(W_{(y,x)}\) shows the equilibrium mass of the gas \(y\) in one unit of the crystal-coated material \(x\), whereas \(W_{y}\) is the mass of the gas \(y\) in one unit of the gas phase. \(V_{x}\) is the volume of liquid coating \(x\) that is present on the crystal. \(W_{T}\) indicates the mass of the gas contained in the volume of a detector when in equilibrium with a liquid coating, and \(F\) indicates the gas phase flow rate. When squalene-coated crystals were used as a detector, an admirable linear relationship between \(A_{y}\) and an inserted volume of pentane, hexane, and octane was noted. Table 4 shows various coatings used with piezoelectric crystal detectors in gas chromatography. To enable in-line detection in gas chromatography, [PERSON] et al. [119] developed a revolutionary microfluidic film bulk acoustic wave resonator gas sensor (mFBAR) in 2021, Figure 23a. The detecting element FBAR within this detector is contained within a microfluidic flow channel and operates via a desorption or adsorption mechanism. Comparative tests and computer simulations showed that adding an additional mFBAR to the capillary line (flow channel) did not significantly alter the flow or affect separation. In this system (gas chromatography-mFBAR-flame ionization detector (FID)), they showed that the simultaneous measurement of concentration profile within the mobile phase by the flame ionization detector (FID) and the direct measurement of concentration profile over the surface of the solid by mFBAR was possible. The rate of mass transfer could be easily determined by the difference between the maximum peak positions of the solid phase and the mobile phase. Figure 23b displays a 1-dimensional separation of the chromatogram for \(11\) gases utilizing an in-line mFBAR detector. [PERSON] et al. [120], in 2021, created an electronic nose model that is based on the surface acoustic wave to analyze the freshness and quality of the kiwifruit, Figure 24. Piezoelectric properties were displayed by LiNbO\({}_{3}\). Due to the absorption of the volatile organic compounds, the change in frequency varied based on the characteristics of different polymers. We examined the VOCs in the kiwi aroma using a thermal desorption (TD)-gas chromatograph (GC)-mass spectrometer (MS) (TD-GC-MS) system. The kiwifruit started to ripen as the esters' concentrations rose, which was followed by an increase in the concentration and the type of VOCs. As a result, ester aroma was primarily determined using fluoropolymer and polystyrene, which acted as the sensing materials. While kiwifruit's were ripening, polyvinyl butyl, poly-N-vinylpyrrolidone and polyvinyl alcohol were used to extract the acids and alcohols. The frequency shift of 2510 Hz occurred from the unripe stage to the ripe stage, and the frequency shift of 4572 Hz occurred from the ripe stage to the over-ripe stage, which is best for determining the freshness of the kiwifruit and was specifically revealed by the surface acoustic wave chip that is coated with thin polyvinyl alcohol film. This is a promising technique for identification analyses and developing food quality. Figure 23: (**a**) mFBAR developed by [PERSON] et al. [119]; (**b**) 1-dimensional separation of the chromatogram for 11 gases [119]. \begin{table}
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