Determination of benzene, toluene, and xylene in indoor air

I.1capillary gas chromatography

　　I.1.1Related standards and basis

This method is mainly based onGB11737Standard Method for Hygienic Inspection of Benzene, Toluene, and Xylene in the Atmosphere of Residential Areas-Gas Chromatography Method.

　　I.1.2principle

Benzene, toluene, and xylene in the air are collected using activated carbon tubes and then extracted using carbon disulfide. Using a hydrogen flame ionization detectorgas chromatographAnalysis, qualitative retention time, quantitative peak height (peak area).

　　I.1.3Measurement range

The sampling amount is20LWhen, use1mLCarbon disulfide extraction, injection1μLThe determination range of benzene is0.025~20 mg/m3Toluene is0.05~20 mg/m3Xylene is0.1~20 mg/m3.

　　I.1.4Reagents and materials

　　I.1.4.1Benzene: chromatographically pure.

　　I.1.4.2Toluene: chromatographically pure.

　　I.1.4.3Xylene: chromatographically pure.

　　I.1.4.4Carbon disulfide: analytically pure, requires purification treatment to ensure no impurities in chromatographic analysis.

Purification method of carbon disulfide: using carbon disulfide5%Extract the concentrated sulfuric acid formaldehyde solution repeatedly until the sulfuric acid is colorless, wash the carbon disulfide with distilled water until neutral, dry with anhydrous sodium sulfate, re distill, and store in the refrigerator for later use.

　　I.1.4.5Coconut shell activated carbon:20~40Purpose: Used for installing activated carbon sampling tubes.

　　I.1.4.6Pure nitrogen:99.99%.

　　I.1.5Instruments and equipment

　　I.1.5.1Activated carbon sampling tube: use long150mm, inner diameter3.5~4.0 mmThe glass tube is inserted100mgCoconut shell activated carbon, fixed at both ends with a small amount of glass wool. After installing the tube, use pure nitrogen gas again300 ℃~350Blow at ℃ temperature conditions5~10 minThen put on plastic caps and seal both ends of the pipe tightly. This tube can be stored in a dryer5dIf the glass tube is melted and sealed, it can remain stable for three months.

　　I.1.5.2Air sampler. Corrected.

　　I.1.5.3Syringe:1mLAfter correction.

　　I.1.5.4Micro injector:1μL，10μLAfter correction.

　　I.1.5.5Test tube with stopper scale:2mL.

　　I.1.5.6gas chromatographHydrogen flame ionization detector.

　　I.1.5.7Chromatography column: non-polar quartz capillary column.

　　I.1.6Sampling and sample preservation

Open the activated carbon tube at the sampling location, with a minimum aperture at both ends2mmConnect vertically to the inlet of the air sampler to0.5L/minSpeed of extraction25Lair After sampling, put plastic caps on both ends of the tube and record the temperature and atmospheric pressure at the time of sampling. Samples can be preserved5d.

　　I.1.7Analysis Steps

　　I.1.7.1Chromatographic analysis conditions: As chromatographic analysis conditions often vary due to different experimental conditions, chromatographic analysis conditions that can analyze benzene, toluene, and xylene should be developed based on the model and performance of the gas chromatograph used.

　　I.1.7.2Draw standard curve and determine calculation factors: Under the same conditions as sample analysis, draw standard curve and determine calculation factors.

In the5.0mLIn a volumetric flask, first add a small amount of carbon disulfide and use1μLAccurately take a certain amount of benzene, toluene, and xylene with a microsyringe（20At ℃,1μLBenzene heavy0.8787mg, toluene heavy0.8669mgOrtho, meta, and para xylene are respectively heavy0.8802、0.8642、0.8611mg）Inject into volumetric flasks separately, add carbon disulfide to the mark, and prepare a reserve solution of a certain concentration. Before use, take a certain amount of reserve solution and dilute it step by step with carbon disulfide to obtain the following contents of benzene, toluene, and xylene:0.5、1.0、2.0、4μg /mLThe standard solution. take1μLInject standard solution, measure retention time and peak height (peak area). Repeat each concentration3Next, take the average peak height (peak area). Using the content of benzene, toluene, and xylene (μ g/mL)g /mL）Draw a standard curve with the horizontal axis as the horizontal axis and the average peak height (peak area) as the vertical axis. And calculate the slope of the regression line, taking the reciprocal of the slopeBsCalculate the factor for sample determination.

　　I.1.7.3Sample analysis: Pour the activated carbon from the sampling tube into a stoppered graduated test tube and add1.0mLCarbon disulfide, plug the tube tightly, place it1hOccasionally shaking. take1μLSample injection, qualitative analysis using retention time, and quantitative analysis using peak height (peak area). Perform three analyses on each sample and calculate the average peak height (peak area). At the same time, take an unsampled activated carbon tube and operate it simultaneously with the sample tube to measure the average peak height (peak area) of the blank tube.

　　I.1.8result calculation

　　I.1.8.1Sample volume according to4.7.7Convert to the sampling volume under standard conditions.

　　I.1.8.2The concentrations of benzene, toluene, and xylene in the air are calculated using the following formula:

In the formula:

　　c--The concentration of benzene, toluene, and xylene in the air,mg/m3；

　　h--The average peak height (peak area) of the sample;

　　h′--Peak height (peak area) of blank tube;

　　Bs--causeI.1.7.2The calculated factors obtained;

　　Es--The efficiency of extracting carbon disulfide determined by experiments;

　　V0--Sampling volume under standard conditions,L.

　　I.1.9Method characteristics

　　I.1.9.1Detection lower limit: The sampling amount is10LWhen, use1mLCarbon disulfide extraction, injection1μLAt that time, the detection limits for benzene, toluene, and xylene were respectively0.025 mg/m3、0.05 mg/m3and0.1 mg/m3.

　　I.1.9.2Linear range:106.

　　I.1.9.3Precision: The concentration of benzene is8.78and21.9μg /mLThe relative standard deviation of repeated measurements for liquid samples7%and5%The concentration of toluene is17.3and43.3μg/mLThe relative standard deviations for repeated measurements of liquid samples are as follows:5%and4%The concentration of xylene is35.2and87.9μg/mLThe relative standard deviation of repeated measurements for liquid samples is5%and7%.

　　I.1.9.4Accuracy: for benzene content0.5，21.1and200μgThe recovery rates are as follows:95%，94%and91%The toluene content is0.5，41.6and500μgThe recovery rates are as follows:99%，99%and93%The content of xylene is0.5，34.4and500μgThe recovery rates are as follows:101%，100%and90%.

　　I.1.10Interference and exclusion

The amount of water vapor or mist in the air is too large, which seriously affects the penetration capacity and sampling efficiency of activated carbon when it condenses in the carbon tube. The air humidity is90%At that time, the sampling efficiency of the activated carbon tube still met the requirements. The interference of other pollutants in the air can be eliminated by using gas chromatography separation technology and selecting appropriate chromatographic separation conditions.

　　I.2Determination of toluene, xylene, and styrene--gas chromatography

　　I.2.1Related standards and basis

This method is mainly based onGB/T 14677The Air Quality Determination of toluene, xylene, and styrene Gas Chromatography Method.

　　I.2.2principle

Fill with fillingTenax-GCThe sampling tube is used to enrich toluene and xylene in indoor air at room temperature. After connecting the sampling tube to the gas chromatography analysis system, the adsorbed components are fully introduced into the gas chromatograph equipped with a hydrogen flame ionization detector for analysis by heating. Within a certain concentration range, the content of toluene, xylene, and styrene is directly proportional to the peak area (or peak height).

　　I.2.3detection limit

Baseline noise during heating desorption analysis5The detection limits for toluene, p-xylene, m-xylene, o-xylene, and styrene in this standard method are calculated based on the corresponding component quantities of the peak height 1.0～2.0 ng, press1LSampling volume calculation, the detection limits for each component are as follows: 1.0×10-3～2.0×10-3 mg/m3.

　　I.2.4Reagents and materials

　　I.2.4.1Carrier gas and auxiliary gas

　　I.2.4.1.1Carrier gas: nitrogen, purity99.99%Using equipment5AMolecular sieve and activated carbon purification tube purification.

　　I.2.4.1.2Combustion gas: hydrogen, purity99.9%.

　　I.2.4.1.3Gas aid: air.

　　I.2.4.2Reagents used for preparing standard samples

　　I.2.4.2.1Toluene（CH3C6H5）Paraxylene[（CH3）2C6H4]Ortho xylene[（CH3）2C6H4]All are chromatographically pure.

　　I.2.4.2.2carbon disulfide(CS2)Analytical pure (toxic), with no interfering components determined by chromatography If there is interference, it needs to be distilled again using a full glass still and collected46℃ fraction.

　　I.2.4.3Sampling tube

The material of the sampling tube is hard glass, long15cm, inner diameter4mm, wall thickness0.5mmOn one side, there is a grinding port that can be connected to the needle of the syringe, and it is filled inside0.5gTenax-GCOr it may have an equivalent adsorbent, with a small amount of quartz cotton filling at both ends for fixation. The two ends of the tube are respectively plugged with silicone rubber plugs and stainless steel needles (sealed with silicone rubber plugs in front of the needles). The newly filled sampling tube needs to be200Aging under nitrogen gas at ℃ conditions30min(Nitrogen flow rate)100mL/min）. Before each sampling, the sampling tube needs to be heated and treated with nitrogen gas, and checked by chromatography for any residual impurities. After each processing, the total storage time before and after sampling should not exceed two days and should be stored in the dark.

　　I.2.5instrument

　　I.2.5.1Gas chromatograph.

　　I.2.5.2Recorder.

　　I.2.5.3Detector: Hydrogen flame ionization detector.

　　I.2.5.4chromatographic column

　　I.2.5.4.1Column type: packed column.

　　I.2.5.4.2Characteristics of chromatographic column: Made of hard glass, long2m, inner diameter 3～4 mm.

　　I.2.5.4.3Carrier: 80～100 eyeChromsorb G·DMCS.

　　I.2.5.4.4stationary liquid

　　I.2.5.4.4.1Name and Chemical Properties: Organic Soap Soil-34(Benrane)High operating temperature200℃； Dinonyl phthalate（DNP）High operating temperature160℃.

　　I.2.5.4.4.2Liquid phase loading capacity:DNP 2.5%，Benrane 2.5%.

　　I.2.5.4.4.3Method of applying fixative: Weigh organic bentonite0.525gandDNP0.378gPlace it in a round bottom flask and add60mLBenzene, in90Reflux in a ℃ water bath3hAdd the carrier again15gContinue to reflux2hAfterwards, transfer the stationary phase to a culture dish, bake under an infrared lamp while shaking to a loose state, and then let it sit for baking2hAfterwards, the column can be installed.

　　I.2.5.4.5Preparation of chromatographic column

　　I.2.5.4.5.1The filling method of chromatographic column: plug the tail end of the chromatographic column (connected to the detector) with quartz wool, connect it to a vacuum pump, and connect the other end of the column to a funnel through a hose. After starting the vacuum pump, slowly load the stationary phase into the chromatographic column through the funnel, tap the chromatographic column lightly while loading to make the filling uniform. After filling, plug the other end of the chromatographic column with quartz wool.

　　I.2.5.4.5.2Aging of chromatography column: Fill the packed chromatography column in150℃ to20～30 mL/minNitrogen gas supply, continuous aging24h.

　　I.2.5.4.6Column efficiency and separation degree: Under given conditions, the separation degree of the chromatographic peak of the tested component is greater than1.0.

　　I.2.5.5Heating desorption device

　　I.2.5.5.1Preparation of heating tube: in a length of12cmThe inner diameter can be inserted into the glass tube of the sampling tube, and then wrapped with a layer of aluminum foil and a layer of glass ribbon in sequence. The thermocouple wire with a glass fiber sleeve is fixed, and the temperature measuring point is placed in the middle of the tube0.5cmWrap heating wire evenly with spacing（1kWThe straight cut part of the electric furnace wire is covered with a glass fiber sleeve and wrapped with a layer of glass fiber ribbon.

　　I.2.5.5.2Heating tube power supply temperature controller: The output power is greater than the heating tube power, and the output voltage is not less than60VContinuously adjustable within the range, with a temperature control upper limit not lower than300A temperature controller with a temperature of ℃.

　　I.2.5.6Gas path conversion system: using a gas path conversion valve for chromatography, connect the gas inlet to the carrier gas source, connect one side of the gas outlet to the original carrier gas inlet of the chromatograph, and connect the other side1～2mThe other end of the PTFE tube is connected to the ventilation needle. The gas path conversion valve can control the direct flow of carrier gas into the chromatographic column or allow it to enter the chromatographic column through the sampling tube via the external gas path.

　　I.2.5.7Sampling instrument: capable of simultaneously measuring extraction flow rate and pressure, with a sampling flow rate adjustment range of0～1.5LThe air sampler.

　　I.2.6sample

　　I.2.6.1Collection of gas samples

Remove the silicone rubber plugs and closed needles on both sides of the heated sampling tube, and connect the tube to the needle on the same side as the sampler0.2～0.6 L/minFlow collection of air10～20 min.

　　I.2.6.2Preservation of samples

After sampling, seal both sides of the sampling tube with a silicone rubber stopper and a closed needle, store in the dark, and analyze as soon as possible.

　　I.2.6.3blank experiment

Perform blank analysis results by analyzing the same batch of sampling tubes that have not been sampled but have been processed simultaneously.

　　I.2.7analysis operation

　　I.2.7.1Adjustment of analytical instruments

Carrier gas path: internal gas path (without sampling tube);

Gasification chamber temperature:150℃；

Column temperature: constant temperature75℃；

Detector temperature:150℃；

Carrier gas flow rate: 85～95 mL/min；

Hydrogen flow rate:60mL/min；

Air velocity:500mL/min.

　　I.2.7.2calibration

　　I.2.7.2.1Quantitative method: External standard method.

　　I.2.7.2.2Standard Sample

　　I.2.7.2.2.1Usage frequency: The standard sample used to determine the correction factor is periodically reused, and the length of the cycle depends on the stability of the instrument. Generally, it can be used for determination5Calibrate once after each sample.

　　I.2.7.2.2.2Preparation of standard samples

　　I.2.7.2.2.2.1Storage solution: Take toluene, p-xylene, m-xylene, o-xylene, and styrene separately1.00mLand0.30mLa surname2Only equipped with90mLPurified carbon disulfide100mLDilute the volumetric flask with carbon disulfide to the mark. This reserve solution is available4It can be stored for one month at ℃.

　　I.2.7.2.2.2.2Series standard solution: Prepare two concentration stock solutions separately with purified carbon disulfide10Dilute step by step, prepare a series of standard solutions, and the concentrations at each point are shown in the tableI.2.1.

TableI.2.1Series standard sample concentration

The sequence name0 1 2 3 4 5 6 2Stock solution 1Stock solution

Composition and concentration μg/mLtoluene0 2.61 8.70 26.1 87.0 261 870

P-xylene0 2.58 8.60 25.8 86.0 258 860

Meta xylene0 2.61 8.70 26.1 87.0 261 870

O-xylene0 2.64 8.80 26.4 88.0 264 880

Styrene0 2.73 9.10 27.3 91.0 273 910

　　I.2.7.2.2.3Instrument stability test: The temperature and gas flow rate of each part are stable and normal, and two consecutive standard samples with similar content to the measured components of the sample are introduced2μLThe relative deviation of its peak area (or peak height) is not greater than5%It is considered that the instrument is in a stable state.

　　I.2.7.3analysis

　　I.2.7.3.1Desorption operation of actual sample sampling tube: Connect the sampling tube to the analysis device system, switch the carrier gas steering valve to allow the carrier gas to enter the chromatograph injection port through the sampling tube via the external gas path. After the pre column pressure returns to normal, start the heating control device of the sampling tube and record the analysis time (press the button on the chromatographic processing machine at this time)STARTKey and chromatographic baseline. The heating of the heating tube is controlled by a temperature controller to increase its temperature30sRising internally200After all the adsorbed components in the tube have flowed out, stop heating the sampling tube, switch the carrier gas conversion valve (press the stop recording button when using the chromatography processor to enter the calculation working state), replace the sampling tube and prepare for the next analysis.

　　I.2.7.3.2Standard sample analysis: Heat treated sampling tubes are analyzed according toI.2.7.3.1The method and conditions are connected to the system and injected with a microsyringe from the silicone rubber stopper on the back of the sampling tube2μLStandard sample, according toI.2.7.3.1Analyze the method. Due to the full amount of adsorbed components and the wide applicability of this method, the working curve of a series of standard samples with a large concentration range should be tested. Based on the magnitude of the curve correlation coefficient and analysis needs, the working curve range should be determined. When measuring actual samples, the calibration period for the working curve should be determined based on the stability of the instrument using standard samples.

　　I.2.7.3.3qualitative analysis

Qualitative retention time is used.

　　I.2.7.3.4quantitative analysis

Calculation of component concentration in gas samples:

In the formula:

　　C--The concentration of the measured component in the gas,mg/ m3；

　　Cs--The amount of tested components added to the standard sample,ng；

　　hi--After deducting the blank, measure the peak height (or peak area) of the sample components;

　　hs--Peak height (or peak area) of standard sample components;

　　Vnd--Converted to the sampling volume under standard conditions,L.

　　I.2.8Precision and Accuracy

The content analyzed by five laboratories is: toluene261mg/LParaxylene258mg/LMeta xylene261mg/LOrtho benzene264mg/LStyrene273mg/LThe precision and accuracy of the unified sample are detailed in the tableI.2.2.

TableI.2.2Unified sample precision and accuracy parameters

Ingredient result parameters toluene P-Xylene xylene O-Xylene STYRENE

Uniform sample content,mg/L 261 258 261 264 273

Repetitive standard deviation,mg/L 3.35 2.19 4.92 3.80 3.00

Coefficient of variation for repeatability,% 1.3 0.9 1.9 1.5 1.1

Reproducibility standard deviation,mg/L 9.54 7.75 5.92 9.66 6.59

Reproducibility coefficient of variation,% 3.7 3.1 2.3 3.7 2.5

Spiked recovery rate,% 99.0 98.4 99.7 98.3 97.8

Average adsorption efficiency,% 99.7 99.9 99.8 99.8 99.8