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The first category involves separating the main organic compounds in the mixed gas using GC, and then performing qualitative and quantitative analysis of each component with detectors such as FID, ECD, PID, ELCD, or others. This is the GC-FID online system that is commonly used in our country; PID systems are also widely employed. Due to significant differences in price and performance, regulatory authorities currently require VOCs monitoring at fixed sources to be carried out using the GC-FID principle. The second category is the Flame Ionization Analysis (FIA) method for determining the total organic carbon (TOC) concentration; it consists of a heated sampling tube, piping, a glass fiber filter, and an FIA analyzer, enabling on-site, real-time analysis to directly and continuously measure the TOC concentration. This method is not widely used for the monitoring and operation of VOCs from fixed sources in China. The third category is: Non-dispersive infrared analysis (NDIR) for the determination of total volatile organic compounds (TOC) concentration. On-site online analysis using a direct interface sampling system. It is mainly used for the determination of the total concentration of gaseous organic compounds containing alkanes. NDIR is primarily a Japanese technology, with agents available in China. The fourth category is: the extraction Fourier-transform infrared method (FTIR) for the determination of gaseous organic and inorganic compounds (HAPs). It is clear that water vapor and carbon dioxide are the most common interferences in the infrared spectrum. It is mainly used for boundary monitoring, traceability, and the so-called \"three-dimensional spatial monitoring\" that is being developed in China at present. It is worth mentioning repeatedly that the EPA has issued a series of standards for instrument performance requirements (Performance Specification, PS). Among these, PS8 serves as the general guideline for online monitoring instruments for VOCs from pollution sources; PS8A outlines the technical requirements for instruments that use the FID principle to monitor total hydrocarbons; PS9 applies to instruments that use gas chromatography to detect VOCs; and PS15 is intended for instruments that use Fourier infrared spectroscopy to measure VOCs. See the table below.