In the process of monitoring ambient air, non methane total hydrocarbons are often used as a monitoring indicator. In fact, the material components of air pollution are complex, including various hydrocarbons, oxygenates, halogenated hydrocarbons, benzene, etc. In addition to NMHC, VOC and TVOC are also frequently mentioned but easily confused concepts, but their meanings and monitoring methods are different.
Definition
Non methane hydrocarbon:Non methane total hydrocarbon refers to the collection of all gaseous hydrocarbon compounds except methane, including alkanes, olefins, aromatic hydrocarbons, etc. In addition, HJ/T 38-2017 also stipulates the definition of total hydrocarbon: it refers to the total amount of gaseous organic compounds responding on the hydrogen flame ionization detector of the gas chromatograph under the test specified in this standard.
VOC：VOC represents volatile organic compounds. In the general sense, VOC refers to volatile organic compounds, but in the sense of environmental protection, it refers to a kind of active volatile organic compounds, that is, the kind of volatile organic compounds that will cause harm.
There are many specific definitions of VOC. For example, the American ASTM D3960-98 standard defines VOC as any organic compound that can participate in atmospheric photochemical reaction. The United States Federal Environmental Protection Agency (EPA) defines volatile organic compounds as any carbon compounds participating in atmospheric photochemical reaction except CO, CO2, H2CO3, metal carbides, metal carbonates and ammonium carbonate.
TVOC: Researchers generally refer to various VOCs as TVOC, which represents total volatile organic compounds. The World Health Organization defines total volatile organic compounds as the general term of volatile organic compounds whose melting point is lower than room temperature and boiling point is between 50 and 260 ℃.
Determination of non methane total hydrocarbon
Determination of Total Hydrocarbons, Methane and Non methane Total Hydrocarbons in Exhaust Gas from Stationary Pollution Sources - Gas Chromatography (HJ/T 38-2017) specifies the method of using gas chromatograph to determine the content of non methane total hydrocarbons. The principle is: directly inject the gas sample into the gas chromatograph with hydrogen flame ionization detector, and measure the content of total hydrocarbon and methane on the total hydrocarbon column and methane column respectively. The difference between the two is the content of non methane total hydrocarbon. At the same time, the sample is replaced by hydrocarbon removing air, and the response value of oxygen on the total hydrocarbon column is measured to deduct the interference of oxygen in the sample on the determination of total hydrocarbon.
As shown in the figure above, connect the gas production system to collect the sample gas, and then use gas chromatography to analyze the gas composition. The specific steps are described in detail in the national standard, and the impact of the type of gas bag on the chromatographic results is mainly discussed here.
Air Collection Bag
There are various materials of common air bags on the market, including aluminum foil and Teflon. Aluminum foil gas bag is usually made of metal, which is light tight and suitable for storing some gases that need to be light shielded. Teflon gas bags are made of fluoropolymer series plastics, and can be subdivided into FEP, PVF, PVDF and other different types.
The Air Bag Method for Sampling Volatile Organic Compounds from Exhaust Gas of Fixed Pollution Sources (HJ732-2014) specifies the method for manually collecting volatile organic compounds (VOCs) in the exhaust gas of fixed pollution sources with a temperature lower than 150 ℃ using fluoropolymer film air bags such as polyfluoroethylene (PVF).
This standard is applicable to the sampling of non methane total hydrocarbons and some VOCs in the exhaust gas of fixed pollution sources. Many experimenters are not clear about the requirements for air bags in the actual application process. When using aluminum foil air bag for gas collection, the blank value will be too high.
Because the fluoropolymer film air bag is inert, has low absorption and gas permeability, does not release interfering substances, and has a low background value, Teflon gas bags are generally recommended as non methane total hydrocarbons. The most recommended is the FEP bag, which has the lowest background value. Secondly, the following table shows the comparison of VOC gas recovery rate of three common fluoropolymer airbags. It can be seen that FEP can also maintain a high recovery level.
In addition to the gas flow bag, other components, such as the valve of the gas flow bag, the connecting pipe connecting the gas flow device, and the lining of the sampling pipe, are also recommended to use PTFE to ensure the accuracy of the test results; The injection analysis shall be carried out within 8h as far as possible.
It is not recommended to reuse the gas bag. If reuse is required, a blank test should be conducted before each sampling. If the determination result of total hydrocarbon is higher than the detection limit, the gas bag needs to be replaced.
Therefore, in addition to the experimental method itself, you should also pay attention to the consumables, instruments and other products used during the experiment. Maybe there are slightly different products, which will have a great impact on the experimental results, but we don't know it at ordinary times.
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