## 4 Applications of Piezoelectric Materials in Analytical Chemistry
### Detector for Liquid Chromatography  
[PERSON] and [PERSON] [124] constructed a worldwide mass detector for liquid chromatography. The effluent of the liquid chromatograph was sprinkled over the surface of the crystal. After the solvent has evaporated, the change in crystal frequency is used to calculate the mass of the remaining solute. Solute deposition and the sampling of liquid flow can be done quickly. Sensitivity was comparable to conventional liquid-liquid chromatography detectors, and nebulization, drying and measurement all took place in just 10 s. A gel transmission chromatograph differential refractometer directed the effluent to a crystal detector. A polystyrene blend was evaluated using chromatography, and the system was tested with butyl rubber. The mass versus retained volume distribution curves was compared with the results that were obtained with the differential refractometer detector. Refractometer detectors have been proven to be inferior to piezoelectric crystal detectors. The benefits listed by [PERSON] and [PERSON] include that the method is non-destructive, highly sensitive, has a mass and universal detector, a wide dynamic measuring range, independent of changes in the chromatograph's pressure temperature or flow rate, any solvent or solute mixture can be useable, volatile impurities do not inhibit it, and it is compatible with the digital response with the digital processing equipment. For effluent detection, [PERSON] and [PERSON] [125] placed a piezoelectric crystal detector immediately in the liquid phase. A large energy loss at the interface of the liquid crystal makes it further difficult to obtain liquid phase adsorption measurements, making it more difficult to sustain crystal vibrations. The piezoelectric crystal's frequency is determined by the liquid phase density over the crystal's surface. Therefore, the change in density induced by the solvent gradient and the solute causes the crystal's resonant frequency to drift. Reference crystals and a coated sample crystal were used to compensate for changes in liquid density. Only one side of each crystal encounters the liquid phase to achieve stable oscillations. The density gradient, flow rate, and temperature effects are removed by using a reference crystal. To regulate the surface adsorption capabilities of the sample crystals in the presence of effluent, long-chain hydrocarbons were used. These coatings could only detect smaller non-polar  
Figure 24: TD-GC-MS system, (**a**) experimental setup (**b**) schematic diagram, created by [PERSON] et al. [120]. molecules; however, advanced surface modification techniques offer more sensitive and rapid detection. Piezoelectric crystals give very good results when they are used in photoacoustic detectors for liquid chromatography. According to the photoacoustic detection principle, when a solute is dissolved in the solvent and irradiated with a suitable light source, the solution expands due to light adsorption. Expansion to a piezoelectric crystal changes crystal resonant frequency. When a laser is utilized to illuminate the sample, photoacoustic detection of the compounds in the static solutions becomes an extremely sensitive approach. Therefore, an analysis of this type of detection system's application in liquid chromatography was analyzed. By combining liquid chromatography with the quartz crystal microbalance, [PERSON] et al. [126] describe a technique for label-free protein analysis, Figure 25. This method uses size omission chromatography to initially separate a mixture of proteins in the physiological buffer solution, allowing for the selection of particular protein fractions, desalination, and subsequent spray-drying over the quartz crystal microbalance for mass analysis. Protein detection and sample fractionation are accomplished simultaneously by creating a continuous interface between the spray device and chromatography column using the flow splitter with precision as low as 100 \(\upmu\)g/mL.