HJ 644-2013Ambient air - Determination of volatile organic compounds - Sorbent adsorption and thermal desorption / gas chromatography mass spectrometry method (English PDF)

5.2 Mass spectrometer

Electron impact (EI) ionization source. It is capable of scanning from 35 amu to 270 amu in one second; has functions such as NIST mass spectrogram library, manual / automatic tuning, data acquisition, quantitative analysis and spectral library search.

5.3 Capillary column

30 m × 0.25 mm; 1.4 µm in film thickness (6% nitrile propylbenzene, 94% dimethyl polysiloxane stationary liquid). Other equivalent capillary columns can also be used.

5.4 Thermal desorption device

The thermal desorption device shall have a secondary desorption function. The focusing tube part shall be able to heat up quickly (at least 40 °C/sec). The part connecting the thermal desorption device to the gas chromatograph and the gas pipeline in the instrument shall use silanized stainless steel tubes. They can be heated uniformly at least between 50~150 °C.

Note 3.The use of a thermal desorption device with cold focusing function can reduce the loss of extremely volatile targets and improve sensitivity.

5.5 Aging device

The maximum temperature of the aging device shall be above 400 °C. The maximum carrier gas flow can reach at least 100 ml/min. The flow can be adjusted.

5.6 Sampler

Dual-channel oil-free sampling pump. The dual-channel can independently adjust the flow rate and accurately maintain the flow rate within 10~500 ml/min.

The flow rate error shall be within ±5%.

5.7 Calibration flowmeter

It can accurately measure the flow rate within 10~500 ml/min. The flow rate accuracy is 2%. Electronic mass flowmeters should be used.

5.8 Micro syringe

5.0, 25.0, 50.0, 100, 250 and 500 µL.

5.9

Commonly used instruments and equipment in laboratories.

least one on-site blank sample shall be taken for each sample collection.

Note 4.Temperature and wind speed will affect sample collection. When sampling, the ambient temperature shall be less than 40 °C. When the wind speed is greater than 5.6 m/s, during sampling, the adsorption tube shall be placed perpendicular to the wind direction; a bunker shall be placed upwind.

7.1 Reference conditions for instruments

7.1.1 Reference conditions for thermal desorption apparatus

Transmission line temperature. 130 °C; initial temperature of adsorption tube.

35 °C; initial temperature of focusing tube. 35 °C; desorption temperature of adsorption tube. 325 °C; desorption time of adsorption tube. 3 min; desorption temperature of focusing tube. 325 °C; desorption time of focusing tube. 5 min;

primary desorption flow rate. 40 ml/min; focusing tube aging temperature.

350 °C; dry blowing flow rate. 40 ml/min; dry blowing time. 2 min.

7.1.2 Reference conditions for gas chromatograph

Inlet temperature. 200 °C; carrier gas. helium; split ratio. 5.1; column flow (constant flow mode). 1.2 ml/min. Temperature programming. Initial temperature is 30 °C, kept for 3.2 min; the temperature is increased to 200 °C at 11 °C/min for 3 min.

Note 5.In order to eliminate the interference of moisture and the overload of the detector, the split ratio can be set according to the situation. Some thermal desorption apparatus have a sample split function, which can be set according to the manufacturer's recommendations or specific conditions.

7.1.3 Reference conditions for mass spectrometry

Scan mode. full scan; scan range. 35~270 amu; ionization energy. 70 eV;

interface temperature. 280 °C. The remaining parameters are set according to the instrument manual.

Note 6.In order to improve sensitivity, selective ion scanning can also be used for analysis.

Refer to Appendix B for characteristic ion selection.

7.2 Instrument performance check

USE a micro syringe to pipette 1.0 µL of BFB solution (4.3) and directly inject it into the gas chromatograph for analysis. The abundance of BFB key ions obtained by quadrupole mass spectrometry shall meet the standards specified adsorption tube shall be opposite to the direction the gas enters the adsorption tube during sampling. The target in the sample enters the chromatographic column with the desorption gas for determination. After the analysis is completed, remove the adsorption tube and age and store it according to step 4.7.If the sample concentration is low, the adsorption tube does not need to be aged.

7.4.2 Blank test

Analyze on-site blank sample in the same procedures as the sample determination.

8.1 Qualitative analysis

Qualitative analysis is performed by comparing retention time and mass spectrogram.

8.2 Quantitative analysis

Calculate according to the response value of the first characteristic ion of the target. When the first characteristic ion of the target in the sample has interference, the second characteristic ion can be used for quantification; see

Appendix B for details.

8.2.1 Calculation of target mass in adsorption tube

8.2.1.1 External standard method

When using the least square method to draw the calibration curve, the target mass m (ng) in the sample is calculated by the corresponding calibration curve.

8.2.1.2 Internal standard method

When the average relative response factor is used for calibration, the mass m (ng) of the target in the sample is calculated according to formula (5).

Where.

m - The mass of the target in the test portion, ng;

Ax - The response value of the quantitative ion of the target;

6 laboratories respectively measured the ambient air spiked samples with spiked amounts equivalent to 5.0 µg/m3, 12.5 µg/m3 and 35.0 µg/m3.The spike recovery rates are. 56.4%~122%, 53.4%~126%, 76.1%~132%.

See Appendix C for the precision and accuracy results.

10.1

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

10.2

Before each sample analysis, it shall use a blank adsorption tube instead of the sample adsorption tube, to determine the system blank. The sample can be analyzed only after the system blank is less than the detection limit.

10.3

An intermediate concentration check point of the calibration curve shall be made every 12 h. The relative error between the measured value of the intermediate concentration check point and the corresponding point concentration of the calibration curve shall not exceed 30%.

10.4

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

11.1

The residual VOCs in the adsorption tube interfere greatly with the determination. Strict implementation of the aging and storage procedures (4.7) can minimize this interference.

11.2

Newly-purchased adsorption tubes shall be marked with a unique code and an arrow indicating the direction of the sample air flow. An information card of the adsorption tube shall be established, to record information including the filling or purchase date of the adsorption tube, the maximum allowable use temperature and the number of times of use.

HJ/T 400-2007 | GB/T 16157-1996 | HJ 647-2013 | […]