2. 1. 1 activated carbon adsorption desorption gas chromatography
This method uses activated carbon for adsorption and carbon disulfide for desorption. The sensitivity of this analytical method is low, and the used carbon disulfide often contains benzene which is not easy to remove. However, this method does not need special pretreatment equipment, and can be analyzed many times in one sampling, especially when the concentration difference of benzene series is large or the concentration is high. It has the advantages of simple method, less reagent consumption and short period. However, the organic solvent used has high purity and needs nitration and purification, which is harmful and does not meet the requirements of cleaner production.
(1) Scope of application This method is suitable for the determination of benzene series in waste gas from pollution sources and ambient air. The detection amount of benzene, toluene, ethylbenzene, xylene and trimethylbenzene by the instrument is at least 0. 1ng. When the sampling volume is 10L, the minimum detection concentration of benzene series is 10μg/m3.
(2) Precautions This method can also accurately characterize and quantify benzene and main benzene series in indoor air, but the following problems need to be paid attention to when using this method:
(1) Because carbon disulfide is used as the analytical solvent of benzene and benzene series, it is necessary to check whether there are impurity peaks in the solvent by chromatography before detection to judge whether the impurity peaks affect the qualitative and quantitative analysis of benzene and benzene series. If there is, it needs to be purified, redistilled and re-examined by chromatography before use. The specific purification method of cARbon disulfide is to add 20 ml of sulfuric acid and 1 ml of formaldehyde to 250 ml of carbon disulfide (ar), fully oscillate, and stand for stratification. Then repeat it many times until carbon disulfide is colorless, then wash it with 20% sodium carbonate solution to neutrality, dry it with anhydrous sodium sulfate, and use it after distillation.
(2) Eliminate the interference factors in the sample. When the humidity in the air reaches, it will condense in the activated carbon tube, which seriously affects the penetration ability and sampling efficiency of activated carbon. When the air humidity is more than 90%, the sampling efficiency of activated carbon still meets the requirements, and other interferences in the air are unknown. Due to the adoption of gas chromatography separation technology, appropriate chromatographic separation conditions can be omitted. When the air sampling pump is used, it should be calibrated with soap film flowmeter to make the error before and after sampling less than 5%. If the flow change before and after sampling is greater than 10%, the analysis result should be suspicious data.
③ Pay attention to the selection of chromatographic conditions and the setting of parameters. The separation efficiency of the target compound on the chromatographic column and the response of the detector to the target compound should be considered in the determination. Zhou Jianzhong et al. found that on HP-5 column, the programmed temperature was 35℃( 1 min) to 95℃, and the heating rate was 10℃/ min. The main components should completely flow out within 6 min, the analysis period is short, and the response value of the detector to benzene series is high.
④ Intermediate concentration test must be carried out before and after sample analysis. If there are 10 samples, the intermediate concentration test is conducted once every 10 sample, and the deviation between the actual value of the intermediate concentration and the value obtained from the curve is ≤ 15%, then the analysis data of the sample is valid.
⑤ Every time a batch of samples are analyzed, the activated carbon blank before and after the adsorption tube must be determined.
⑥ Make a process blank for each sampling (the sampling tube is taken to the site, both ends of the sampling tube are opened, then sealed like a sampling tube without sampling, and analyzed like a sample after being taken to the laboratory, and the analysis result is a process blank).
⑦ When the measured value of activated carbon behind the sampling tube is greater than the previous 25%, the sample should be re-sampled.
⑧ Every time a new batch of activated carbon is used, the desorption efficiency of benzene series in activated carbon should be carried out. When doing desorption efficiency, the final concentration of each compound should be close to the middle concentration of the curve, and the desorption efficiency of each compound should be ≥80%. Desorption efficiency = (measured value-blank value)/actual scalar
Pet-name ruby after sampling, sampling tube for 6 days, benzene series loss is less than 15%, should be desorbed within 6 days, analysis within 10d.
Attending each sampling, samples within 10 and every 10 sample shall be a parallel sample, and the deviation of parallel samples shall be ≤25%.
2. 1.2 gas chromatography of pyrolysis inhalation sample
This method is that after the sample is adsorbed by the adsorbent, the benzene series is desorbed from the adsorbent by heating, and then the benzene series is brought to the chromatographic column by carrier gas for separation and analysis. This method has high sensitivity, no need to use organic reagents and low background value. However, because the sample is injected at one time, when the sample concentration cannot be determined, it sometimes needs to be sampled and analyzed many times. This method has the advantages of low detection limit, improved accuracy and precision, simple and practical operation, and zero pollution to the environment and operators in the daily analysis process, which is worth popularizing.
(1) Scope of application: When the sampling volume is 1L, the minimum detection concentrations of toluene, p-xylene, m-xylene, o-xylene and styrene are1.0×10-3 ~ 2.0×10, respectively. When different models are used, the detection range of this method is different.
(2) Secondary Thermal Desorption Gas Chromatography The principle of secondary thermal desorption gas chromatography is to collect benzene and benzene series in the air with activated carbon, and then adsorb them to Tenax-TA tube after thermal desorption, and then thermally desorb them, and analyze them by gas chromatography to determine the retention time and peak height. The method of direct secondary thermal desorption by activated carbon sampling tube and direct injection by gas chromatograph can accurately analyze benzene series in the air, which is a rapid and non-secondary pollution method. Compared with carbon disulfide desorption gas chromatography, secondary thermal desorption direct injection gas chromatography has the following advantages:
(1) eliminates the influence of carbon disulfide on laboratory environmental pollution, which is beneficial to the construction of green laboratory;
(2) The purification process of carbon disulfide is omitted;
③ Eliminate the pollution of FID collector caused by the massive use of carbon disulfide;
(4) There is no interference from a large number of solvent peaks;
⑤ The operation process of extracting and transferring the analyzed components is fast;
⑥ Reusable activated carbon sampling adsorption tube;
⑦ The repeatability is good, the relative standard deviation is less than 3%, the accuracy is high, and the adsorption and desorption efficiency can reach over 99%;
⑧ The monitoring sensitivity of this method can be increased by 1000 times, the sampling volume can be reduced, and the sampling time can be saved. Therefore, it is an advanced and accurate method to determine benzene series by secondary thermal desorption gas chromatography with activated carbon sampling tube, which is worth quoting.
2.2 Portable gas chromatograph
In addition to these classical determination methods, portable gas chromatograph began to be used for environmental monitoring. The analysis of benzene series in routine environmental samples needs to be sampled by adsorption tube and brought back to the laboratory for determination, which is time-consuming, costly and involves many links. The new portable gas chromatograph has high sensitivity, low cost, simple operation and fast analysis speed, which can meet the monitoring requirements of benzene series in indoor air. Hu et al. studied the application of FFKM30 1 portable gas chromatograph in indoor air field monitoring, and determined the best method setting: column temperature 60℃, pressure 12psi, pumping time 20s, back blowing time 300s. The detection limit of this method is 10ppb (benzene), and the precision is less than 6%, which is the same as that of classic gas. The possible reasons for its deviation from the desktop gas chromatograph are:
(1) The classic gas chromatography continuously samples for 50min, which is the average concentration of 50min, while the portable gas chromatography directly samples, which is the instantaneous concentration;
② The classical gas chromatography was brought back to the laboratory for analysis with a certain time interval, which was easily influenced by other factors, while the portable gas chromatography sampling and field analysis reflected the real situation at that time.
2.3 Other Problems in the Determination of Benzene Series
2.3. 1 packed column and capillary column can be used to separate benzene series. When the packed column is filled with 2.5% DNP and 2.5% Bentane Chromosorb W HP DMCs (80 ~100 mesh), m-xylene and p-xylene can be effectively separated, and the sample injection amount can reach 5 μ l. However, when the sample is sucked in by pyrolysis, the packed column is not universal, and different types of compounds need different fillers, so different packed columns need to be replaced when other compounds are determined at the same time. Capillary columns used for benzene series analysis are generally nonpolar or weakly polar capillary columns (30mm×0.32mm, 30mm×0.25 mm, the thickness of stationary liquid is 0.25~ 1.5μm, DB- 1, DB-5, SE-54). The greater the thickness of the fixed liquid film, the better the separation effect. However, if the sample is inhaled by pyrolysis, the capillary column with large aperture (0.5mm in inner diameter) can allow a large flow of carrier gas, which is beneficial to the sample inhalation by pyrolysis. Because of the versatility of capillary column, benzene series and other types of compounds can be analyzed simultaneously.
2.3.2 BTEX in ambient air can be sampled by summa tank sampling technology or adsorption tube concentration method. The advantage of tank sampling technology is that it can avoid penetration and decomposition during desorption when using adsorbent, but it is not conducive to popularization because of its high cost and complicated operation. Adsorption tube concentration method has become the most widely used sampling method at present because of its advantages such as simple equipment, simple operation and long storage time of samples.
(1) adsorbents At present, the commonly used adsorbents are activated carbon adsorbent, activated carbon adsorbent and Tenax adsorbent. After sampling, benzene series can be transferred from solid adsorbent to gas chromatograph for analysis and determination. Activated carbon is hydrophilic and cannot be used in high humidity environment unless special preventive measures are taken. Tenax is a porous polymer (poly (2,3-diphenyl-p-phenylene ether), which has good adsorption and thermal desorption effects on hydrocarbons with more than 6 carbons. Tenax is hydrophobic, and the osmotic volume will not be affected by humidity when sampling.
(2) When the concentration of components in the outflow gas is 5% of that of the inflow gas, the sampling of solid adsorbent through the volume is considered to be exposed. When measuring reagents, two sampling tubes can be connected in series on the sampling pump, and the pumping volume when the contents in the back tube account for 10% of the total sampling volume is called "osmotic volume". The sampling volume of the assembly should be controlled within the osmotic volume. In order to ensure the accuracy and reliability of the analysis results, whenever the standard sample tube is actually sampled, the permeability volume test must be carried out.
(3) desorbing the solvent
At present, using activated carbon adsorption tube to collect benzene series in the atmosphere, then desorbing with carbon disulfide, and finally waking up for gas chromatography analysis is the most commonly used method for the determination of benzene series in the atmosphere, and it is also recommended by many standards. However, because carbon disulfide contains impurities that are not easy to go out, it has background interference, so it must be purified before use, which is cumbersome and time-consuming, and carbon disulfide has certain toxicity. In order to improve the work efficiency, reduce the workload of the laboratory and reduce the influence of reagents on testers, a capillary gas chromatographic analysis method for benzene series in the atmosphere was established by using dichloromethane re-steaming instead of carbon disulfide as TOEFL machine. The correlation coefficients are all 0.999. The minimum detection amount of the instrument can reach 0. 1ng, and the precision of the method is 1.9%~4.8%. The recovery rate of standard addition is between 8 1.6%~ 109.8%, which can meet the analysis needs of benzene series in the atmosphere.