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Determination of volatile organic compounds in waste gas by gas chromatography / solid phase desorption mass spectrometry

2020-08-07 09:53:28 Practical thermal analysis fifty-two

The volatile organic compounds (VOCs) in the waste gas from fixed pollution sources are directly collected by an adsorption tube filled with appropriate adsorbent (or the gas in the air bag is collected first, and then the gas in the air bag is collected into the solid adsorption tube), and the adsorption tube is placed in the Thermal desorption instrument is also called thermal desorption instrument The desorbed gas was separated by gas chromatography and detected by mass spectrometry. The gas was qualitatively determined according to retention time, mass spectrum or characteristic ions, and quantified by internal standard method or external standard method.

   

Adsorbents: Tenax gr (specific surface area: 35m? / g), carbo pack B (specific surface area: 100M? / g), carbon pack C (specific surface area: 10M? / g), carbon box 1000 (specific surface area: 1200m? / g), or other equivalent adsorbents, such as domestic GDX-502 (specific surface area: 170m? / g), gdx-201 (specific surface area: 510m? / g), GDX-101 (specific surface area: 330m? / g)

Etc. The particle size of commonly used adsorbents is 60 ~ 80 mesh.    

Adsorption tube: stainless steel, inner diameter 5mm.    

Combination 1 adsorption tube, containing Tenax GR and carbon pack B, with length of 30 and 25 mm respectively.

The adsorption length of carboxeb was 1 000, 2 and 10 mm respectively.   

Three adsorption tubes with carbon pack C, carbon pack B and carbon box 1000 were assembled, and the lengths were 13 mm, 25 mm and 13 mm respectively. Or use other products with the same function.

Thermal desorption device: it can conduct two-stage thermal desorption on the adsorption sampling tube, and bring the desorption gas into the gas chromatography with carrier gas. The desorption temperature, desorption time and flow rate can be adjusted, and the cold trap can realize rapid temperature rise. Semiconductor refrigeration is usually used in cold trap. Silylated stainless steel pipe should be used in the connection part of thermal desorption device and gas chromatography and gas pipeline in the instrument, and it can be heated evenly at least between 50-150c.

Adsorption tube aging device: the maximum temperature should be above 400 ℃, and the maximum carrier gas flow rate should be at least 100ml / min,

The flow rate is adjustable.

 

Preparation of adsorption sampling tube

After aging, both ends of the adsorption sampling tube shall be sealed with sealing cap immediately and stored in airtight sealing bag or sealed box.

Sealed bags or sealed boxes are stored in boxes or desiccators containing activated carbon and stored at 4 ℃.

If necessary, add a certain amount of substitute standard (generally the middle concentration of calibration curve) into the aged adsorption tube before sampling. See 8.2.2 for loading method

   

Sample collection

Adsorption tube sampling

Samples were collected according to the sampling requirements of HJ / t397 using adsorption tube sampling system (as shown in Figure 1).    

For the adsorption sampling tube using multi-layer adsorbent, the gas inlet end of the adsorption sampling tube should be weak adsorbent (small specific surface area), and the outlet end should be strong adsorbent (large specific surface area). For the stainless steel adsorption sampling tube with an outer diameter of 6 mm, the recommended sampling flow rate is 20 ~ 50 ml / min. At least 300ml of gas was produced from each sample. If the VOCs above C 6 were monitored by the combined 1 Adsorption sampling tube, the gas recovery of the sample could reach 2L. The temperature of the waste gas is high and the moisture content is more than 2%. The safe sampling volume of the target compound can not meet the requirement of 300 ml of sample gas. When the adsorption efficiency of the adsorption sampling tube is affected, the adsorption sampling tube should be cooled

(0-5 ℃).    

Sampling penetration test sample collection: after the adsorption sampling tube, an adsorption sampling tube is connected in series and samples are taken at the same time.

At least one adsorption sampling tube in series shall be collected for each batch of samples to monitor whether the sampling is penetrated.   

Note 1: the main interfering substance of combination 2 and 3 adsorption sampling tubes is water vapor. If there is water vapor interference, one can be added in front of the adsorption sampling tube

Ice water bath small impact water collector or semiconductor refrigeration dehumidification device. The water in the water collector should be analyzed with hj639 at the same time, and the content should be included in the sample.

Take the sealed adsorption sampling tube to the sampling site, open the sealing cap of the same quality adsorption tube at the same time to contact the ambient air at the site, close the sealing cap of the blank adsorption tube in the whole procedure during sampling, and close the sealing cap at the same time when the same quality adsorption tube contacts the ambient air at the end of sampling, and processes and measures according to the same operation steps as the sample, which is used to check whether the whole process from sample collection to analysis is correct Contaminated.

   

Sample preservation

After sampling, seal both ends of the sampling tube with a sealing cap immediately, store it in 4 ℃ away from light, and analyze within 7 days.

   

Analysis steps

Reference conditions for instrumental analysis

Reference conditions for thermal desorption apparatus

The initial temperature of adsorption tube: room temperature; the initial temperature of focusing cold trap: room temperature

Dry blowing flow rate: 30ml / min; dry blowing time: 2min; adsorption tube desorption temperature: 270 ℃; adsorption sampling tube desorption time: 3min; desorption flow rate: 30ml / min; focusing cold trap temperature: - 3 ℃; focusing cold trap desorption temperature: 300 ℃; cold trap desorption time: 3min; transmission line temperature: 120 ℃.

General cold hydrazine with graphite carbon black as filler.

The desorption time and desorption temperature can be adjusted according to the different adsorption conditions.

Note 2: target compound 100-1000ng, split ratio 30.4:1, setting: initial temperature of adsorption sampling tube: room temperature; initial temperature of focusing cold trap: 1

The results showed that: room temperature; dry blowing flow rate: 48ml / min; dry blowing time: 1min; combination 1 Adsorption sampling tube desorption temperature: 270 ℃; adsorption sampling tube desorption time: 5min; primary focusing cold trap flow rate: 30ml / min, primary shunt flow rate: 18ML / min; focusing cold trap temperature: - 3 ℃; secondary focusing cold trap desorption temperature: 300 ℃; cold trap desorption time: 3min; secondary shunt flow rate: 18ML / min, Column flow rate: 1ml / min; transmission line temperature: 120 ℃.

   

Reference conditions for capillary column gas chromatograph

The temperature of injection port: 200 ℃; column flow rate (constant flow mode): 1.5ml/min; heating program: initial temperature 35 ℃, holding for 5min, heating to 140 ℃ at the speed of 6 ℃ / min, rising to 220 ℃ at the speed of 15 ℃ / min, and holding at 220 ℃ for 3min.


Standard sample loading

Install the aged adsorption tube on the loading platform of thermal desorption standard sample (note that the inlet end of the adsorption tube faces the syringe), inject 1.0ul mixed standard solution (8.2.1) into the blank adsorption tube with micro syringe, add 1.0ul 50pg/ml internal standard solution (diluted with 5.2), purge the adsorption tube with 50ml / min N2 for 2min, and remove the adsorption tube quickly, The two ends of the adsorption tube were sealed with a sealing cap to obtain calibration series adsorption tubes with contents of 5.00, 10.0, 20.0, 50.0 and 100 ng. The internal standard content of each adsorption tube was 50 ng.    

Note 4: if there is no standard loading platform, the standard sample can be loaded in the following way: connect the aged adsorption tube to the injection port of the packed column of gas chromatograph, set the injection port temperature at 50 ℃, inject 1.0ul mixed standard solution (8.2.1) and internal standard solution (diluted with 5.2) with micro syringe, fill the carrier gas with 50ml / min for 2min, and remove the adsorption tube quickly, The two ends of the adsorption tube are sealed with a sealing cap to obtain a calibration series of adsorption tubes.

   

Calibration curve drawing

According to the reference condition (8.1), the calibration curve was analyzed from low concentration to high concentration. According to the target / internal standard mass ratio and the target / internal standard characteristic mass ion peak area (or peak height) ratio, the calibration curve was drawn by the least square or relative response factor.

   

 Solid thermal desorption