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Product information

Determination of volatile organic compounds by adsorption tube sampling thermal desorption gas chromatography mass spectrometry

2020-08-07 09:27:58 Practical thermal analysis forty-eight

Method principle

Technical specification for manual monitoring of ambient air quality

A solid adsorbent was used to enrich volatile organic compounds in the ambient air Thermal desorption instrument is also called thermal desorption instrument After separation by gas chromatography, the samples were detected by mass spectrometry. Compared with the standard mass spectrogram of the target to be determined and the retention time, the qualitative analysis was carried out, and the external standard method or internal standard method was used for quantitative analysis.

    

Adsorption tube It is made of stainless steel or glass, with inner diameter of 6mm, filled with carbon pack C, carbon pack B and car box 1000, with length of 13 mm, 25 mm and 13 mm respectively.

Or use other products with the same function.

Focusing tube: stainless steel or glass material, inner diameter not more than 0.9mm, filled with adsorbent type and length the same as adsorption tube. Or use other products with the same function.

    

Aging and preservation of adsorption tube:

The newly purchased adsorption tubes or the adsorption tubes after collecting high concentration samples need to be aged.

The aging temperature was 350 ℃, the flow rate was 40 ml / min, and the aging time was 10-15 min.

The outer end of the tube is sealed with a layer of aluminum foil after the tube is aged. The packed adsorption tube was placed in a dryer containing activated carbon or activated carbon silica gel mixture, and the dryer was placed in the refrigerator without organic reagent, and stored at 4 ℃ for 7 days.

 

Note 1: the aging and preservation method of focusing tube is the same as that of adsorption tube.

Carrier gas: helium, purity 99.999%.

Instruments and equipment

Gas chromatograph: equipped with capillary column split / no split injection port, electronic pressure control of carrier gas, temperature programmed.

The gas chromatograph is equipped with column box cooling device, which can improve the peak shape and sensitivity of volatile target.

Mass spectrometer: electron bombardment (EI) ionization source, which can scan from 35amu to 270amu in one second. It has the functions of NIST mass spectrum library, manual / automatic tuning, data acquisition, quantitative analysis and spectrum library retrieval.

Capillary column: 30 m × 0.25 mm, 1.4 um film thickness (6% nitrile propyl benzene, 94% dimethylpolysiloxane stationary solution). Other equivalent capillary columns can also be used.

Thermal desorption device

Thermal desorption device It should have two-stage desorption function, and the focusing tube part should be able to heat rapidly (at least 40C / sec). At least 50 ~ 150 ℃ should be connected to the gas chromatograph

Heat evenly.

Note 3: the use of thermal desorption device with cold focusing function can reduce the loss of highly volatile target and improve the sensitivity.

    

Aging device

The maximum temperature of the aging device should be above 400 ℃, the maximum carrier gas flow rate should be at least 100ml / min, and the flow rate can be adjusted. The dual channel oil-free sampling pump of the sampler can independently adjust the flow rate and accurately maintain the flow rate within 10 ~ 500ml / min, and the flow error should be within ± 5%.

Calibration flowmeter: it can accurately measure the flow rate within 10 ~ 500ml / min, and the flow accuracy is 2%. Electronic mass flowmeter should be used.

Microsyringes: 5.0, 25.0, 50.0, 100, 250 and 500 UL.

General laboratory instruments and equipment.

sample

Sampling flow and sampling volume

Sampling flow rate: 10 ~ 200ml / min; sampling volume: 2L. When the relative humidity is more than 90%, the sampling volume should be reduced, but it should not be less than 300ml at least.

Samples collected and preserved

Air tightness inspection: connect an adsorption tube (the same specification as the adsorption tube used for sampling, which is only used for air tightness inspection and preset flow rate) to the sampling pump (5.6), open the sampling pump and block the inlet end of the adsorption tube. If the flow rate of the flowmeter returns to zero, the gas path connection of the sampling device has good air tightness, otherwise, the air tightness of the gas path should be checked.

Preset sampling flow: adjust the flow rate to the set value.

Remove the adsorption tube in 6.2.1, connect a new adsorption tube to the sampling pump, and sample according to the airflow direction indicated on the adsorption tube. The collection of ambient air samples shall refer to the relevant provisions of HJ / T194. In the process of sample collection, it is necessary to check and adjust the sampling flow at any time to keep the flow constant. After sampling, the sampling point, time, ambient temperature, atmospheric pressure, flow rate and adsorption tube number were recorded.

After the sample is collected to the outside of the tube, the sample shall be packed in a sealed tube for analysis. Samples that cannot be analyzed immediately shall be stored according to step 4.7 and analyzed within 7 days.

Alternate adsorption tube for monitoring sample penetration.

Field blank sample collection: transport the adsorption tube to the sampling site, open the sealing cap or take it out from the special casing

Collection of alternate adsorption tube: an aging adsorption tube is connected in series behind the adsorption tube. At least one sample should be collected from each batch

Seal both ends of the adsorption tube immediately or put it into a special sleeve, and wrap a layer of aluminum foil on the outside. It is stored with the adsorption tube of the collected samples and brought back to the laboratory for analysis. At least one field blank sample should be taken for each sample collection.

Note 4: temperature and wind speed will affect sample collection. When sampling, the ambient temperature should be less than 40 ℃; when the wind speed is greater than 5.6m/s, the sampling should be conducted

The adsorption tube should be placed perpendicular to the wind direction and the shelter should be placed in the upwind direction.


Analysis steps

Instrument reference conditions

Reference conditions for thermal desorption apparatus

The temperature of transmission line: 130 ℃; initial temperature of adsorption tube: 35 ℃; initial temperature of focusing tube: 35 ℃; desorption temperature of adsorption tube: 325 ℃; desorption time of adsorption tube: 3min; desorption temperature of focusing tube: 325 ℃; desorption time of focusing tube: 5min;

The results showed that the flow rate was 40 ml / min; the aging temperature of the focusing tube was 350 ℃; the dry blowing flow rate was 40 ml / min; the dry blowing time was 2 min.


Reference conditions for gas chromatograph

The temperature of injection port: 200 ℃; carrier gas: helium; split ratio: 5:1; column flow rate (constant current mode): 1.2ml/min; heating program: initial temperature 30 ℃, holding for 3.2min, heating from 11 ℃ / min to 200 ℃ for 3min.

Note 5: in order to eliminate the interference of moisture and overload of detector, the split ratio can be set according to the situation. Some thermal desorption instruments have the function of sample separation, which can be set according to the manufacturer's suggestions or specific conditions.

1.0 UL standard series solution was transferred by micro syringe and injected into the thermal desorption instrument. According to the reference conditions of the instrument, the determination was carried out from low concentration to high concentration, and the calibration curve was drawn.

If the thermal desorption instrument used does not have the function of "standard series for liquid injection preparation", it can be prepared as follows: connect the aged adsorption tube to the injection port of the gas chromatograph packed column, set the injection port temperature as 50 ℃, transfer 1.0ul standard series solution with a micro syringe into the gas chromatograph injection port, pass the carrier gas with a flow rate of 100ml / min for 5min, and quickly remove the adsorption tube, The standard series tubes with target content of 5.00, 10.0, 25.0, 50.0 and 100 ng were prepared.

   

Determination of samples

Put the sample tube into the thermal desorption instrument quickly, and conduct thermal desorption according to the reference condition (7.1) of the instrument. The direction of carrier gas flowing through the adsorption tube should be opposite to that of the gas entering the adsorption tube when sampling. The target compounds in the sample were added into the chromatographic column with the desorption gas for determination. After the analysis, remove the adsorption tube, aging and storage according to step 4.7. If the sample concentration is low, the adsorption tube does not need to be aged.    

Before sampling, 20% of the adsorption tubes should be taken for blank inspection. When the number of samples is less than 10, at least 2 tubes should be taken. The concentration of the target substance equivalent to 2L sampling volume in the blank tube should be less than the detection limit, otherwise it should be aged again.

A blank adsorption tube should be used instead of the sample adsorption tube to determine the system blank before analyzing the sample. The sample can be analyzed only when the system blank is less than the detection limit.

Every 12 hours, a calibration curve should be made in the middle of the concentration check point. The relative error between the measured value of the intermediate concentration check point and the corresponding point of the calibration curve should not exceed 30%.

The detection amount of a single target in the blank sample should be less than 10% of the corresponding detection amount in the sample or equivalent to the detection amount of the blank adsorption tube.


matters needing attention

The residual VOCs in the adsorption tube have great interference on the determination, and strict aging and preservation procedures (4.7) can minimize the interference.


The newly purchased adsorption tubes should be marked with unique code and arrow indicating the direction of sample flow, and the information card of adsorption tube should be established, including the filling or purchasing date of adsorption tube, the maximum allowable temperature and use times.

 Solid thermal desorption