What are VOCs? What are the detection methods for VOCs?
“According to the definition of the World Health Organization (WHO), VOCs (volatile organic compounds) are various organic compounds with a boiling point of 50°C to 260°C at room temperature. In my country, VOCs refer to organic compounds with a saturated vapor pressure greater than 70 Pa at room temperature and a boiling point below 260 °C at normal pressure, or all organic compounds with a vapor pressure greater than or equal to 10 Pa and volatile at 20 °C.
What are VOCs
According to the definition of the World Health Organization (WHO), VOCs (volatile organic compounds) are various organic compounds with a boiling point of 50°C to 260°C at room temperature. In my country, VOCs refer to organic compounds with a saturated vapor pressure greater than 70 Pa at room temperature and a boiling point below 260 °C at normal pressure, or all organic compounds with a vapor pressure greater than or equal to 10 Pa and volatile at 20 °C.
As people’s awareness of the harm of VOCs gas increases, the detection methods and detection instruments of VOCs are also constantly being upgraded.
VOCs detection method
The instruments and methods of volatile organic compounds in the air in foreign countries, especially in the United States, are mainly gas chromatography and gas chromatography-mass spectrometry. The sampling methods are mainly container capture method and solid adsorbent sampling method. Adsorbents are divided into activated carbon, carrier (also called carrier) and thermal desorption tube.
Most of the early analytical methods in my country are solid adsorbent adsorption-solvent desorption-gas chromatography. The adsorbent has the effect of enriching air samples. The detection limit of the method is relatively low and the measurement cost is low. Defects such as agent leakage, desorption/desorption efficiency, and secondary pollution. Later, my country’s sampling and analysis methods are gradually in line with international advanced methods.
At present, the most commonly used method at home and abroad is gas chromatography. This method has the advantages of high efficiency, fast speed, wide monitoring range and high sensitivity, and is one of the important means for analyzing VOCs.
For the selection of detectors, the commonly used detectors at home and abroad are: hydrogen flame ionization detector (FID), electron capture detector (ECD), photoionization detector (PID) and mass spectrometry detector (MSD)
The flame ionization detector (FID) is a general-purpose detector that is commonly used in gas chromatography.
It has the advantages of high sensitivity, wide linear range, good stability, and rapid response, and is widely used in the detection of volatile hydrocarbons and many carbon-containing organic compounds; the outstanding advantage of FID is that it responds to almost all organic compounds, especially It is highly sensitive to hydrocarbons and the response is proportional to the number of carbon atoms. It is insensitive to inorganic substances such as H2O, CO2 and CS2, and is insensitive to changes in gas flow rate, pressure and temperature. It has a wide linear range, simple results and easy operation. It has almost zero dead volume and can be directly connected to a capillary column. Therefore, FID has been widely used both in the past packed column period and today when capillary columns are gradually popularized. The disadvantage of FID is that the gas path is relatively impurity, and the gas flow ratio needs to be adjusted repeatedly for ignition during operation or to obtain the best performance, which is troublesome compared with other detectors.
Electron capture detectors (ECDs) are the most sensitive gas chromatographic detectors and the earliest selective detectors.
It only responds to those compounds that can capture electrons, such as halogenated hydrocarbons, and compounds containing heteroatoms such as N, O, and S. Because of its high sensitivity and good selectivity, it is also the most widely used radioactive ionization detector. It is widely used in biology, medicine, pesticide, environmental protection, metal chelate and weather tracking and other fields. Although ECD has the characteristics of high sensitivity, its shortcomings are also obvious: first of all, there are many things to pay attention to in daily operation. Therefore, it is often mistaken for a detector with the worst operation; at the same time, it is difficult and time-consuming to establish a chromatographic analysis method with ECD, such as: sample pretreatment, chromatographic column preparation and aging, system cleanliness, solvent and utensils used, etc. There are special necessary requirements.
The mass spectrometer detector (MSD) is a mass, general-purpose detector that responds to all ionizable compounds suitable for GC detection.
Due to the advantages of mass spectrometry, such as higher sensitivity, stronger qualitative ability, and the ability to provide relative molecular mass and structural information, which improves the qualitative limitations of gas chromatography, gas chromatography-mass spectrometry (GC/MS) is now more and more widely used. Applied to the detection of VOCs in the environment. The advantage of MSD is that it measures accurately and can respond to compounds that are suitable for GC detection and capable of calligraphy and painting; its disadvantage is that the MSD data processing workload is very large, and it must work with a computer system to work effectively, and the cost is high.
Photo Ionization Detector (PID), PID (Photo Ionization Detector) Photo Ionization Detector is a sensor mainly used to monitor VOCs in the atmosphere.
The principle is that the organic gas will ionize the gas under the excitation of the ultraviolet light source. The PID uses a UV (ultraviolet) lamp. The organic matter is ionized under the excitation of the ultraviolet lamp, and the ionized “fragments” have positive and negative charges. An electric current is thus generated between the two electrodes. The detector amplifies the current, and it can Display the concentration of VOCs gas through the instrumentation equipment. The advantages of the photoion detector are that it is non-destructive, the sensor is small in size, and can be used in a wide range of environments. With the improvement of sensor technology, its detection accuracy is not inferior to that of large-scale detection instruments.
The PID-AH sensor of British Alphasense has a resolution of 1ppb and a sensitivity of over 20mv/ppb, which meets and realizes the precise detection and monitoring needs of atmospheric VOCs.
At present, the technology of PID-AH has been very mature, and it has been applied in a large number of industrial projects.
As far as PID is concerned, its applications mainly include: mainly for oil refining industry monitoring, emergency treatment of hazardous chemical leakage, definition of leakage danger area, safety monitoring of oil tank and oil station, monitoring of organic matter discharge purification efficiency, etc.