## 4 Applications of Piezoelectric Materials in Analytical Chemistry
### Detector for Gas Chromatography  
With gas chromatography, the more sensitive flame ionization detector or thermal conductivity detector is used the most. Mass spectrometry and electron capture have been used to create more sophisticated detection methods. [PERSON] and [PERSON] et al. [115; 116] established a low-cost system of gas chromatography that is based on a piezoelectric crystal detector. The crystals were covered with the liquid that is used in the gas chromatography column. The compounds which are separated are detected by passing them onto the surface of a coated piezoelectric crystal, where the compound disperse within the coating of the crystal, resulting in the change in the piezoelectric crystal's resonance. The frequency shift is used as a response to the detector and is converted to a voltage. Piezoelectric detectors can be used at room temperature or above if the carrier gas is air, nitrogen, or helium. Chromatographs can be used to apply a variety of compounds with boiling temperatures up to 200 \({}^{\circ}\)C. Equation (3) below shows the reaction between the coated piezoelectric crystal and the elution of the compound from the column of gas chromatography:  
\[A=\frac{CW}{\gamma P^{\circ}\,F} \tag{3}\]  
Here, \(A\) represents the area of the response curve, \(W\) represents the total eluent weight, \(\gamma\) represents the eluent activity coefficient, \(P^{\circ}\) represents eluent vapor pressure at the operating temperature, \(F\) represents the flow rate of the carrier gas, and \(C\) represents a constant which is a characteristic of the piezoelectric crystal, detector temperature, and the liquid coating over the crystal. Equation (3) describes several solvent properties of the piezoelectric crystal detectors when they are used in conjunction with the gas chromatographs. When utilizing piezoelectric crystals, the detector's temperature is crucial. When utilized above ambient temperature, the same properties, such as linearity and sensitivity, can be seen. There are variations in the separation between the compounds observed at high  
Figure 22: Amino acid-based detector for the detection of pipe leaks developed by [PERSON] et al. [114]. temperatures because of the respective partition coefficients of the gas components. With the rising temperature, a compound's absolute detector response declines. The result is not as dramatic as predicted. For optimal detector conditions, the detector and column temperature must be maintained as low as possible while still being higher enough for the elution of the compound of interest in a sufficient amount of time [117]. Since the vapor pressure of all chromatographic separation solvents is fixed, the lifetime of the coated crystal is determined by the solvent used and the vapor pressure of the carrier from the column. Crystals coated with polymers and adsorbents have a longer service life. If the crystal detector is damaged in an accident, it can be repaired by cleaning it with a solvent or coating it with a new substance. A detector's lifetime is independent of the performance of the instrument. [PERSON] et al. [118] described a coated piezoelectric crystal's response characteristic as a distribution detector of dissolved vapors in the gas stream. The partition detector theory's authors assume that the detector is attached to the gas chromatograph column's outlet. The response of the peak area is associated with the gas mixture of the sample.