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Determination of benzene homologues in ambient air by solid adsorption / thermal desorption gas chromatography

2020-08-07 09:44:19 Practical thermal analysis thirty-eight

Determination of Benzene Series in ambient air by solid adsorption/ Thermal desorption instrument is also called thermal desorption instrument -Gas chromatography

Method principle

Benzene homologues in ambient air or indoor air were enriched with filled Tenax sampling tube at room temperature. The sampling tube was connected to a thermal desorption instrument. After heating, the adsorbed components were introduced into a gas chromatograph with a hydrogen flame ionization detector (FID).


Instruments and equipment

Gas chromatograph: equipped with FID detector.

chromatographic column

Capillary column: PEG-20M, 30m × 0.32mm × 1.00um or equivalent capillary column.

 

Thermal desorption device

have Primary desorption or Secondary desorption Function, purchase professional products or make it yourself. The maximum temperature of the thermal desorption unit can reach 300 ℃,

When the temperature reaches the set value, the temperature can be kept constant. After the sampling tube is installed on the thermal desorption instrument, there is no air leakage at both ends of the sampling tube and the whole system. The temperature of the transmission line connected to the gas chromatograph should be maintained above 100 ℃.

The thermal desorption instrument with freeze focusing function is also applicable to this standard.

    

Aging device

   

The temperature was controlled between 200 ℃ and 400 ℃, and a certain nitrogen flow rate was maintained.

The sample collection device has no oil sampling pump, the flow range is 0.01 L / min ~ 0.1 L / min and 0.1 L / min ~ 0.5 L / min, and the flow rate is stable.

   

Sampling tube

The material of the sampling tube is stainless steel or hard glass, filled with Tenax (60 mesh ~ 80 mesh) adsorbent (or other equivalent adsorbent) of no less than 200mg. Both ends are fixed with stainless steel mesh or quartz wool with pores smaller than the particle size of adsorbent to prevent the adsorbent from falling. The adsorbent in the tube should be at least 15mm away from the inlet of the tube, and the length of the adsorbent filled should not exceed the size of the heating zone. The sampling tube can be purchased directly or filled by yourself.

Thermometer: accuracy 0.1 ℃.

Barometer: accuracy 0.01kpa.   

Microinjector: 1 UL ~ 5 pl.

General laboratory instruments and equipment.

    

Preparation of sampling tube

The newly filled sampling tube shall be aged by aging device or thermal desorption instrument with aging function. The aging flow rate is 50ml / min, the temperature is 350 ℃, and the time is 120min. The used sampling tube should be aged at 350 ℃ for more than 30min. After aging, both ends of the sampling tube should be sealed with PTFE cap immediately and stored in a sealed bag or protective tube. The sealed bag or protective tube is stored in the box or dryer containing activated carbon and stored at 4 ℃. The aged sampling tube should be used within two weeks.


The flow rate of the sampler should be calibrated before sampling. At the sampling site, a sampling pipe is connected with the air sampling device to adjust the flow rate of the sampling device. The sampling pipe is only used for regulating the flow rate, and there is no need for sampling analysis. After removing the upper and lower Teflon sampling tube, check the flow rate of the sampling system according to the normal temperature. Air was collected at the flow rate of 10ml / min ~ 200ml / min for 10min ~ 20min. If there are more particles in the atmosphere, the filter head can be connected in front of the sampling pipe. At the same time, the flow rate of the sampler was recorded

Front temperature and air pressure. At 20 ℃, the safe sampling volume of benzene series components in the Tenax TA adsorption tube filled with 200mg was determined

   

Before sampling, record the sampling flow again, remove the sampling tube and seal it with PTFE cap immediately.

After sampling, the two ends of the sampling tube were sealed with Teflon cap immediately. The samples were stored in 4 ℃ away from light and analyzed within 30 days.  

Collection of field blank samples

Transport the aged sampling tube to the sampling site, take off the PTFE cap and re seal it. Do not participate in the sample collection, and store it with the sampling tube of the collected sample. At least one field blank sample should be collected for each sample collection.


Analysis steps

Instrument selection

The gas chromatograph with packed column should be selected when the selected thermal desorption device has only one-stage desorption function.    

The two-stage gas chromatograph with thermal desorption should be selected.

When selecting capillary column, the inner diameter of capillary column should be selected according to the recommended thermal desorption flow rate of two-stage desorbed focusing tube. In general, when the recommended thermal desorption flow rate of the focusing tube is less than 2.0ml/min, the capillary column with inner diameter of 0.25mm can be selected; when the recommended thermal desorption flow rate of the focusing tube is greater than 2.0ml/min, the capillary column with inner diameter of 0.32mm can be selected. The stationary solution was polyethylene glycol, and the capillary column (4.2.2) with film thickness greater than 1.0um could separate the target components of this standard well.

    

Recommended analytical conditions

The reference conditions of the first stage gas chromatography were as follows

Thermal desorption apparatus

Carrier gas flow rate: 50ml / min; valve temperature: 100 ℃; transmission line temperature: 150 ℃; desorption temperature: 250 ℃; desorption time: 3min.

Reference conditions for two stage thermal desorption and capillary column gas chromatography

Thermal desorption apparatus

The results showed that the initial temperature of the tube was 40 ℃ and the initial temperature of the tube was 40 ℃;

The desorption temperature of the sampling tube was 250 ℃, and the desorption time of the sampling tube was 3 min;

Desorption flow rate of sampling tube: 30ml / min; 150 ℃.

Focusing tube

Desorption temperature: 250 ℃; desorption time of focusing tube: 3min; temperature of transmission line: 250 ℃


Capillary column gas chromatography

Column chamber temperature: 80 ℃ constant temperature; column flow rate: 3.0m/min; injection port temperature: 150 ℃; detector temperature: 250 ℃;

Tail gas flow: 30ml / min; hydrogen flow: 40ml / min; air flow: 400ml / min.

   

Calibration curve drawing

Take appropriate amount of standard stock solution (3.2), dilute with methanol (3.1) and fix the volume to 1.00mL. Prepare calibration series with concentration of 5, 10, 20, 50 and 100 UG / ml.

Connect the aged sampling tube to the filling column injection port of other gas chromatographs or a self-made device similar to the filling column injection port of gas chromatography. Set the injection port (device) temperature at 50 ℃, inject 1.0ul standard series solution with syringe, and pass the carrier gas (3.3) with a flow rate of 100ml / min for 5min, and quickly remove the sampling tube, A series of sampling tubes with calibration curves of 5, 10, 20, 50 and 100 ng were obtained by sealing both ends of the sampling tube with a polytetrafluoroethylene cap. The calibration curve series sampling tubes were connected to the thermal desorption instrument in the opposite direction of the gas flow when the standard solution was adsorbed, and the calibration curve was drawn according to the mass and response value of the target component.

Note 1: if the thermal desorption instrument is equipped with liquid reference material injection port, a certain amount of standard solution can be injected directly to draw the calibration curve.

Install the sample tube on the thermal desorption instrument, and adjust the analysis condition (6.2),

After desorption, the target components were separated by gas chromatography and detected by FID. The retention time and corresponding value of chromatographic peak were recorded.

At 20 ℃, the safe sampling volume of benzene series components in the Tenax TA adsorption tube filled with 200mg was determined

 Solid thermal desorption