Non methane hydrocarbons (NMHC) typically refer to all volatile hydrocarbons other than methane, also known as non methane total hydrocarbons. When the concentration of NMHC in the atmosphere exceeds a certain level, it is harmful to human health and can also produce photochemical smog, posing a threat to the environment and humans.
The factory boundary concentration standard for non methane total hydrocarbons in China's Comprehensive Emission Standards for Air Pollutants (B1627-1996) is 5mg/m³According to the fourth edition of the "Air and Waste Gas Monitoring Methods", Kexiao Company adopts a process optimization scheme of ten way valve single injection, dual column parallel single FID detection, which meets the detection requirements of total hydrocarbons and non methane total hydrocarbons, with a minimum detection concentration of 0.05mg/m³.

Non methane total hydrocarbon chromatographyFeatures:

Single FID configuration eliminates errors caused by differences in FID sensitivity due to dual FID.

Double packed columns are connected in parallel, and a ten way valve is used for injection to achieve a single injection, while simultaneously analyzing total hydrocarbons and methane.

2.Experimental apparatus

GC1690Gas chromatograph (Hangzhou Kexiao Chemical Instrument Equipment Co., Ltd.);

JO-4（Dalian);

3.Test procedure

3.1Sample processing procedure

The sample processing process shall refer to Method 1 of the "Air and Waste Gas Monitoring Methods" (Fourth Edition). Use a 100ml glass syringe, seal the syringe after on-site sampling.

3.2Chromatographic conditions

Column temperature: 50 ℃
Detector temperature: 150 ℃
Injector temperature: 150 ℃
Injection volume: 1ml
Column flow rate: 20ml/min

3.3qualitative analysis

When the sample enters the total hydrocarbon analysis column, only one total hydrocarbon peak appears, and other components in the sample cannot be separated; The sample enters the methane analysis column, which can separate methane from other components.

1.4quantitative analysis

Using methane as the standard gas, quantitative analysis was conducted using the area external standard method, and the results were calculated based on methane.

4.experimental results

4.1System Applicability Test

Analysis was conducted using 5 μ mol/mol (VN) methane standard gas, and the results are shown in Table 1

4.2standard curve

Using a 100ml glass syringe and high-purity nitrogen gas dilution method, different concentrations of 5/10/20/50/100 μ mol/mol methane standard gas were prepared. Appropriate amounts of gas standard gas were injected for analysis, and the peak area and corresponding standard gas concentration were used as standard curves.

According to the above chromatographic conditions, the following standard curve is obtained:4.3Dehydrogenation air analysis

Due to the positive interference of O2 response in FID on total hydrocarbon analysis, it is necessary to use dehydrogenated air as a blank, and the actual total hydrocarbon of the sample needs to be deducted from the air

Interference. The analysis of hydrocarbon removal air is shown in Figure 4. The quantitative results are shown in Table 2

4.4Air sample analysis

Use a 10ml glass syringe to aspirate the air in the test chamber, such as gas chromatography for analysis. The spectra are shown in Figure 5 and the repeatability analysis is shown in Table 3 for three times

The quantitative repeatability of total hydrocarbons is 2.04%, and the quantitative repeatability of methane is 1.74%, which meets the requirements of quantitative detection methods. Non methane total hydrocarbons=total hydrocarbons - air blank - methane=4.60-1.74-2.40=0.45 μ mol/mol, or 0.32/m³

1.Conclusion

The method described in this article can meet the testing requirements for total hydrocarbons and non methane total hydrocarbons in the air, which meets national standards. The lowest detection concentration can reach 0.05mg/m³.