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Correct detection of toxic and harmful substances is extremely important in emergency response to various chemical accidents. For those toxic and harmful chemicals whose identity is difficult to determine after a chemical accident, identifying the type of poison is even more meaningful. According to the information provided in the 2001 edition of the Jane’s Nuclear, Biological, and Chemical Defense Yearbook, there are currently 10 main techniques for the rapid on-site detection of toxic and harmful substances worldwide. These include: ionization/ion mobility spectrometry technology. The detector used in ion mobility spectrometry is a type of continuously operating detector; it uses an air pump to collect samples from the environment, and the pollutants collected are ionized by a weak electric field within the detector. The ionization of gaseous toxins can be achieved under atmospheric pressure. Almost all toxic and harmful substances can be ionized using methods such as proton detachment, charge detachment, dissociative charge detachment, or negative ion reactions like ionization spectroscopy. The World Environmental Cooperation Organization has developed and produced a handheld APD-2000 type ion mobility spectrometer using this technology. Flame photometry detection technology: Flame photometry detection technology is based on the principle of hydrogen flame combustion; the flame is capable of breaking down any toxic or harmful substances present in the air. Toxic and harmful substances containing phosphorus and sulfur produce hydrogen phosphite (HPO) and elemental sulfur respectively. When the flame temperature is increased, phosphorus and sulfur emit light of specific wavelengths. This light is transmitted through a more ideal filter; the light emitted by phosphorus and sulfur is directed to a photomultiplier tube, which generates an electrical signal similar to that of the substance in question. This electrical signal is directly related to the concentration of phosphorus and sulfur compounds present in the air. It can be seen that any compound containing phosphorus and sulfur can be detected using flame photometry. Flame photometry is highly sensitive, allowing the instrument to directly sample and analyze ambient air. However, the drawback of this method is that interference and false alarms occur as long as phosphorus and sulfur are present in the ambient air. To reduce interference during detection, using a flame photometric detector from gas chromatography technology in the manufacture of instruments will **decrease the occurrence of false alarms**. Infrared spectroscopy is a technique that involves measuring the intensity of infrared light absorbed by a sample within a specific wavelength range (4000–200 cm). The wavelengths of the infrared absorption bands exhibit distinct characteristics, and each molecule has a unique infrared spectrum. Infrared spectroscopy can be used to analyze the characteristic peaks of molecular structures, thereby detecting unknown toxic and harmful substances. To date, two infrared spectroscopy techniques have been applied in field rapid detection instruments: one is photoacoustic infrared spectroscopy. The photoacoustic infrared detector uses the photoacoustic effect to monitor and measure vapors of toxic and harmful substances; when a gas absorbs infrared radiation, it causes a rise in temperature, which in turn leads to the expansion of the gas. If the intensity of the infrared radiation is adjusted, the sample will expand and contract. If audio is designed, a microphone can be used to transmit sound signals. Photoacoustic infrared gas detectors use different filters to selectively transmit specific light wavelengths absorbed by the toxic and harmful substances being monitored, and employ longer-wavelength signals to identify unknown compounds. When there are no toxic or harmful substances in the atmospheric sample, no infrared absorption peaks at specific wavelengths appear, and therefore no audio signal can be detected. When toxic and harmful substances are present in the atmospheric sample, audio signals are generated by adjusting the absorption of infrared light. If the sample continuously absorbs infrared light of different wavelengths, the selectivity will **increase**. In other words, when light of several wavelengths passes through the sample sequentially, it is possible to identify which toxin is present among the contaminants. However, the photoacoustic detector is highly sensitive to external vibrations and humidity; therefore, the operating environment is one of the crucial factors when calibrating the detector, as this is necessary to achieve high selectivity. Electrochemical detection technology: Electrochemical detectors measure the potential changes in solutions or films that have absorbed toxic and harmful substances. The inhibition of cholinesterase by toxic and harmful substances is the most typical example; in a solution containing a known amount of cholinesterase, if such toxic and harmful substances are present, the percentage of cholinesterase inhibition is proportional to the concentration of those substances. This relationship can be used to determine the concentration of the toxic and harmful substances. There is also an electrochemical detector used to measure the resistance value of films; the resistance value increases in films that absorb toxic and harmful substances. Therefore, they are not as sensitive as ion mobility spectroscopy and flame photometry, and they are also more affected by environmental factors. Changes in temperature can alter the rate of reactions as well as the equilibrium points of different reactions, thereby affecting their sensitivity and selectivity. Wet chemical detection technology: Wet chemical detection technology is actually what we commonly refer to as colorimetry. To determine whether toxic and harmful substances are present, it is sufficient to observe the color change in the colorimetric tube or test strip upon contact with such substances. The method used is as follows: when it is known from other alarm devices that toxic or harmful substances may be present, a colorimetric tube or test strip is then used for detection. They are also used to detect contaminated drinking water sources. The most common application of this technology is in test papers, which are made from a certain crystalline dye or chromogenic reagent dissolved in a solution. Test paper is commonly used to detect suspicious droplets or liquids on object surfaces. For gaseous or vapor-form toxic and harmful substances, colorimetric tubes are commonly used for detection. A colorimetric tube is made up of a glass tube, silica gel, and reaction reagents adsorbed on it. When in use, the two ends of the glass tube are broken off and inserted into a vacuum pump, allowing the substance to be tested to pass through the silica gel inside the tube; if toxic or harmful substances are present, the reaction reagents in the tube will change color. Surface acoustic wave technology: Surface acoustic wave technology detectors use a small piezoelectric quartz crystal coated with a special polymer to distinguish absorbed toxic and harmful substances from the air. Surface acoustic wave quartz crystals are an important component of high-performance oscillation circuits, and several such crystals make up the chemical sensor array within detector devices. The fundamental frequency of surface acoustic waves is 275 MHz. Each polymer is specifically designed for the determination of a particular compound. For example, one type of polymer will preferentially adsorb water, while another type of polymer will preferentially adsorb different types of toxic and harmful substances. This selective absorption by the polymer can change the resonance frequency of surface acoustic waves. There is a neural network algorithm designed to process frequency changes from surface acoustic wave arrays in order to determine the type and concentration of toxic and harmful substances; it is also possible to upgrade this neural network algorithm when new types of toxins emerge. Many surface acoustic wave detectors are equipped with ultra-condensed tubes to reduce interference from environmental pollutants and to increase sensitivity. The surface acoustic wave portable Type II detector, developed using microsensors and surface acoustic wave technology, is now available on the market. Photoionization detection technology: A photoionization detector relies on ultraviolet light with sufficient energy to ionize toxic and harmful substances present in the photon stream. If toxic and harmful substances are present in the gas stream, they are ionized, and then the photoionization detector records the ratio of the amount of ions generated in the gas sample (the concentration of toxic and harmful substances) to the voltage. Perkin-Elmer has developed two types of photoionization detectors: one is the handheld MINIRAE PLUS detector manufactured using photoionization detection technology, and the other handheld photoionization detector is the Photovac2020 model. Sensor array technology (electronic nose): The sensor array device is composed of several different chemical sensors arranged together. These sensors utilize polymer conductors, metal oxides, bulk acoustic waves, and surface acoustic waves for real-time monitoring; the various sensors used must respond quickly, and they must be reversible when exposed to toxic and harmful substances. This technology is often used in instruments known as electronic noses; a portable detector developed using sensor array technology is the EEEVeneeze 5000 electronic nose. Thermoelectric conduction technology: Thermoelectric conduction detectors utilize semiconductor devices based on the thermal conductivity of metal oxides. This device is used to measure the changes in thermal conductivity caused by toxic and harmful gas substances adsorbed on the surface of metal oxides. In other words, when the gas under test is adsorbed on the surface of the metal oxide, the resistance and conductivity across the metal film in the measurement system change. In the actual measurement process, the final result is obtained by determining the difference between the electrical signal values of atmospheric pollutants and those of a \"clean\" or \"background atmosphere\". Furthermore, different pollutants have varying thermal conductivities, allowing this technique to be used to detect certain unknown toxic and harmful substances. Flame ionization detection technology: The flame ionization detector is a commonly used detector that is employed to measure volatile carbon-containing compounds when they burn in a hydrogen-oxygen flame. When carbon-containing compounds burn, the baseline ion current generated by the flame increases, and the compounds begin to be detected. Flame ionization technology is very simple and does not require complex separation techniques. For example, the identification of compounds by gas chromatography usually involves comparing retention times, or identification is carried out using retention indices under standard conditions. Perkin-Elmer has manufactured a handheld micro flame ionization detector with a detection limit of 0.1×10‑6 to 50000×10‑6.