Determination of toluene in air by thermal desorption analyzer
Toluene in the air was collected with activated carbon, and then injected into the sample after thermal desorption. The sample was separated by polyethylene glycol 6000 column and detected by flame ionization detector. The retention time was used for qualitative analysis and peak height was used for quantification.
Activated carbon tube: a 150 mm long glass tube with an inner diameter of 3.5-4.0 mm and an outer diameter of about 6 mm is used to load 100 mg of 20-40 mesh coconut shell activated carbon. Both ends are fixed with a small amount of glass wool. This tube can be used in a short time. It can also be filled with 250mm long glass tube with the same inner diameter, which can be used for long-term preservation. The activated carbon should be treated with nitrogen at 300 ~ 350 ℃ for 3 ~ 4H before piping. After the pipe is installed, the nitrogen is used to blow at the same temperature for 2 ~ 3min, or not treated before the pipe is installed. After the pipe is installed, the nitrogen is used to blow at the above temperature for 5 ~ 10min, and it is used or sealed for storage in a short time.
Sampling pump, 0 ~ 1L / min.
Syringe, 100ml, 1ml.
Micro syringe, 10pl, 1pl.
Thermal desorption unit ： Domestic thermal desorption It is mainly composed of heater, temperature controller, thermometer and gas flow controller. The temperature control range is 100 ~ 350 ℃, the desorption gas is nitrogen, and the flow control range is 50 ~ 100ml / min. The structure of the thermal desorption device used should be such that the activated carbon tube can be easily inserted into the heater, and the gas passing through the pipe is preheated first, and the activated carbon is heated evenly.
Gas chromatograph, hydrogen flame ionization detector, 1ng toluene gives signal-to-noise ratio not less than 3:1
Chromatographic column: stainless steel column with length of 2M and inner diameter of 4mm.
Polyethylene glycol 6000:6201 red support = 5:100.
Column temperature: 90 ℃;
Temperature of vaporizer: 120 ℃;
Detection room temperature: 150 ℃;
Carrier gas (nitrogen): 69ml / min.
Open the activated carbon tube at the sampling site, with the aperture of at least 2mm at both ends, place it vertically, and extract 1L air at the speed of 0.1 ~ 0.2l/min. After sampling, the two ends of the tube were covered with plastic caps and brought back to the laboratory for analysis.
Control test: the activated carbon tubes which were brought to the site but not sampled were analyzed at the same time as the control.
Sample treatment: after sampling, plastic cap should be set. If the sample cannot be analyzed immediately, the carbon tube can be refrigerated in the refrigerator.
Drawing of standard curve: accurately extract a certain amount of toluene (mass of 1 PL toluene is 0.8669 mg at 20 ℃) with micro syringe, inject into 100 ml syringe to prepare toluene standard gas of certain concentration, take a certain amount of toluene standard gas, dilute with clean air into 0.02, 0.05, 0.1, 0.2, 0.4 pg / UL toluene standard gas, respectively take 1 ml sample, measure the retention time and peak height, each concentration Repeat for 3 times, take the average value of peak height, plot the peak height with toluene content, and draw the standard curve. Retention time is a qualitative indicator.
Determination: connect the sample tube with a 100ml syringe, and use nitrogen to desorb at 300 ~ 350 ℃ at a speed of 50 ~ 60ml / min with nitrogen. The desorption volume is 100ml. Take 1ml of desorbed gas for injection, qualitative analysis with retention time and quantitative analysis by peak height
Due to the different thermal desorption instruments, the desorption temperature can be slightly different. Generally, toluene can get higher desorption efficiency at 350 ℃.
When benzene, toluene and xylene coexist, they should also be desorbed at 350 ℃.
In the application of thermal desorption method, the desorption efficiency of the tested organic compounds at the desorption temperature should be tested first, and it can be used when the desorption is nearly complete.
If the desorption efficiency is low, but the result is stable, it should be corrected in the quantitative calculation with standard gas.