Development of New Online Monitoring System of Adsorption and Concentration for Atmospheric Volatile Organic Compounds

In order to meet the requirement of monitoring VOCs for qualitative, quantitative analysis at low level, a new online monitoring system of adsorption and concentration was developed and preliminarily tested, which included a sampling gas path system with a dual channel, a ultra-low temperature trap system, a power supply, a control system, and an analysis system with gas chromatography-mass spectrometry/flame ionization detector (GC-MS/FID). A unique technology of two-stage cryogenic enrichment was adopted for the ultra-low temperature trap system, which included two trap tubes of first stage and two focusing tubes of second stage. Each tube was able to reach an ultra-low temperature of -160 ℃ and covered with a heating structure. A sampling gas path system with a dual channel was used as a processing device of front injection for analyzing application of MS and FID, before which sample was separated by GC. Each channel included a trap tube and a focusing tube. VOCs and water were cryogenically trapped in the tube of first stage during sampling and focused in the tube of second stage after the tube of first stage had been heated, but the water was still remained in the tube of first stage because the temperature was less than 100 ℃. All VOCs in the tube of second stage were heated to 170 ℃ and analyzed in the GC-MS/FID by carrier gas, when the twelve-way valve was switched. The valve was switched to the original position in two minutes later, and the gas path was completely purged for 10 min in the opposite direction of the trapping state, when all the tubes were heated to 180 ℃, then the system was going to be in the state of waiting for the sample to be trapped. This technology could adsorb and concentrate 100 kinds of VOCs including C₂-C₁₂ hydrocarbons,oxygenated organic compounds, halogenated hydrocarbons, organic sulfides and so on, as well as conducting online real-time monitoring and analysis. It was easy to get data about the information of VOCs concentration in less 60 min. The system was tested in performance using mixed sample of standard gas of PAMS and TO15 (a total of 108 components). The quantitative standard curve R of 95% of compounds is greater than 0.99, and the relative standard deviation (RSD) of 91% of compounds is less than 5%. The limit of detection (LOD) of benzene can achieve 1×10^-12 mol/mol. The system is applied for testing the VOCs of air, and 76 kinds of VOCs are qualitatively and quantitatively analyzed. In short, the new online monitoring system of adsorption and concentration can meet the online monitoring requirements of volatile organic compounds in the atmosphere.