Configuration and Key Technologies of Gas Chromatography Instruments for Non Methane Total Hydrocarbon Detection

Non methane total hydrocarbons (NMHC) refer to the sum of all volatile hydrocarbons other than methane, mainly including C2 to C8 hydrocarbons. As one of the main precursors of photochemical smog, NMHC can generate oxidants and aerosol particles under sunlight, posing a threat to the environment and human health. Accurately detecting NMHC concentration is of great significance for environmental monitoring and pollution control.

Principle and technical standards of gas chromatography method for detecting non methane total hydrocarbons:

The core principle of non methane total hydrocarbon detection is based on dual column dual detector gas chromatography technology. According to the national environmental protection standards HJ/T 38-1999 and HJ 604-2017, this method directly injects gas samples into a gas chromatograph equipped with a hydrogen flame ionization detector (FID), and measures the content of total hydrocarbons and methane on the total hydrocarbon column and methane column, respectively. The difference between the two is the content of non methane total hydrocarbons. Special attention should be paid to the elimination of oxygen peak interference during the detection process. By replacing the sample with hydrocarbon removal air and measuring the response value of oxygen on the total hydrocarbon column, the interference of oxygen in the sample on the determination of total hydrocarbons can be deducted. The standard detection limit of this method is 0.07 mg/m³ (Calculated by carbon), the quantitative determination concentration range is 0.12-32 mg/m³ Can meet the requirements of environmental monitoring.

Configuration and Key Technologies of Gas Chromatography Instruments for Non Methane Total Hydrocarbon Detection

The main components of a gas chromatograph for non methane total hydrocarbon detection include an injection system, chromatographic column, detector, and data processing system. The injection system usually uses a 1.0 ml quantitative tube injection valve or an airtight glass syringe to ensure the accuracy and reproducibility of the injection volume. Column configuration is key to the method, and there are two commonly used types: packed columns and capillary columns. The packed column usually includes a methane column (filled with GDX-502 or GDX-104 support) and a total hydrocarbon column (filled with silane glass microspheres); Capillary columns include porous layer open tube molecular sieve columns and deactivated capillary empty columns. The detector mainly adopts a hydrogen flame ionization detector, and its working principle is that the gas sample undergoes ionization process after passing through the flame. The high-voltage electrodes at both ends of the flame nozzle generate an electrostatic field, and the positive and negative ions generated by ionization move towards the electrodes to generate current. The current intensity is proportional to the concentration of hydrocarbons. In practical applications, there are four main configuration schemes: ten way valve double column single detector, ten way valve double column double detector, double six way valve single detector, and double six way valve double detector. The dual detector scheme has a faster analysis speed and can simultaneously detect multiple components, but the cost is higher; The single detector scheme has a relatively simple structure and low cost, but the analysis speed is slow.

Non methane total hydrocarbon detection gas chromatography analysis process and operating points

Sample collection is the crucial first step in analysis. According to HJ 604-2017 standard, all glass syringes or airbags can be used as sampling containers. Before sampling, the container should be cleaned with hydrocarbon free air at least 3 times. After sampling, the sample should be stored away from light and analyzed within 8 hours (syringe) or 48 hours (airbag). The chromatographic conditions need to be optimized and controlled. Typical reference conditions include: injection port temperature of 100 ℃, column temperature of 80 ℃, and detector temperature of 200 ℃. The flow rate of the carrier gas (nitrogen) is 15-25 ml/min (packed column) or 8-10 ml/min (capillary column), the flow rate of the combustion gas (hydrogen) is about 30 ml/min, and the flow rate of the auxiliary gas (air) is about 300 ml/min. The calibration process uses a series of standard gases, usually with a calibration series of 5 concentration gradients. Establish a quantitative relationship between concentration and peak area by plotting calibration curves for total hydrocarbons and methane. When performing single point calibration, the calibration gas should be injected at least twice, and the relative deviation of chromatographic response should be 10%. Quality assurance and quality control are key to ensuring data accuracy. This includes transportation of blank sample determination, regular instrument calibration, qualitative confirmation of chromatographic peaks, etc. Other peaks appearing after the total hydrocarbon chromatographic peak should be included in the total hydrocarbon peak area, but the oxygen peak area should be deducted.

The detection of non methane total hydrocarbons plays an important role in environmental air quality monitoring. By monitoring the urban atmosphere, industrial parks, and surrounding pollution sources, the spatial distribution characteristics and variation patterns of NMHC can be grasped, providing scientific basis for air quality assessment and pollution prevention and control. In terms of industrial pollution source monitoring, non methane total hydrocarbon detection technology is widely used in industries that emit VOCs such as petrochemicals, chemicals, coatings, and printing. The monitoring of emissions from fixed pollution sources and unorganized emission monitoring points is an important means for enterprises to meet emission standards and manage pollutant discharge permits.