Kyushu space analysis method for detecting trihalomethane in water
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[Sewage] Total trihalomethane detection method in water [Copy Link]
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First, the method summary
The water sample containing volatile trihalomethane is collected in a sealed bottle and stored at low temperature. Then, it is taken out using a needle and injected into a purge and trap device for concentration. The concentrated sample is separated and measured by introducing a carrier gas into a gas chromatograph, and the total trihalomethane concentration in the water sample is detected using an Electrolytic Conductivity Detector (ELCD) or Hall Detector (HD).
Second, the scope of application
This method applies to tap water, mineral water, drinking water, river water, reservoir water, lake water, and groundwater. It can analyze four types of trihalomethanes: chloroform, bromodichloromethane, dibromochloromethane, and bromoform. The detection limits for these compounds are 0.061 μg/L, 0.048 μg/L, 0.13 μg/L, and 0.27 μg/L, respectively.
Third, interference
(1) The main interference comes from volatile organic compounds in the reagent water. To eliminate this, the reagent water should be heated while passing through nitrogen gas through an organic matter column. The treated reagent water must be stored in a glass bottle and sealed to avoid contamination.
(2) Interference may also come from the purge gas, memory effects from previous high-concentration samples, or organic matter adsorbed on the trapping column. Therefore, before each experiment, the machine should be ventilated, the trapping column and pipeline cleaned at high temperature, and the instrument blank test performed before starting the analysis.
Fourth, equipment
(1) Purge and trap equipment (the capture and desorption modes of the system are shown in Figures 1 and 2):
1. Capturing column: A column filled with TENAX GC, 12" × 1/8", or equivalent.
2. Suggested conditions for capturing and concentrating:
Standby temperature: 35°C
Dry purge time: 11 minutes
Purge flow rate: 40 mL/min
Desorb time: 4 minutes
Desorb temperature: 180°C
Bake purification temperature: 200°C
Bake time: 10 minutes
(2) Gas chromatograph:
1. A gas chromatograph equipped with an electrolytic conductivity detector and temperature control program.
2. An integrator, recorder, or computer for data collection.
3. Recommended operating conditions for the column: DB-5MS, 30m × 0.53mm, 1.5μm (thickness), or equivalent.
Chromatography initial temperature: 30°C
Initial temperature retention time: 6 minutes
Temperature rise rate: 8°C/min
End temperature: 150°C
End temperature retention time: 2 minutes
Carrier gas (He) flow rate: 12 mL/min
Supplemental gas (He) flow rate: 30 mL/min
4. Recommended operating conditions for the electrolytic conductivity detector:
Reaction tank: Nickel 1/16" (outer diameter), 0.02" (inner diameter)
Reaction temperature: 900°C
Base temperature: 200°C
Electrolyte: n-propanol
Electrolyte flow rate: 0.05 mL/min
Reaction gas (Hâ‚‚) flow rate: 100 mL/min
(3) Pure water production equipment: Equipment capable of producing water with resistance greater than 17 MΩ·cm at 25°C.
(4) Analytical balance: Accurate to 0.1 mg.
(5) Heating plate: With stirring function.
(6) Sample injection system: Automatic or manual, with an injection volume of 5 mL.
(7) Refrigerator (or cabinet): Capable of maintaining a temperature of 4°C.
(8) Glass containers and sample bottles (Note 1):
1. Quantitative bottle: Borosilicate glass, 10 mL, with a glass cover.
2. Vial (with Teflon gasket rotating cover): Borosilicate glass, 40 mL.
(9) Water and gas removal devices.
Fifth, reagents
(1) Solvent: Methanol, analytical grade, purity above 99.9%; n-propanol, analytical grade, purity over 99.9%.
(2) Standard products (Notes 2, 3):
1. Trichloromethane, analytical grade, purity above 99.9%.
2. Monobromodichloromethane, analytical grade, purity above 99.9%.
3. Dibromochloromethane, analytical grade, purity above 99.9%.
4. Tribromomethane, analytical grade, purity above 99.9%.
(3) Dechlorination agent for sample preservation: Ascorbic acid, analytical grade, purity above 99.7%.
(4) Reagent water: Take 3L of purified deionized water, boil it, keep it for 15 minutes, then pass nitrogen gas, heat for 1 hour, and store it in a Teflon-sealed glass bottle at about 90°C.
(5) Trihalomethane stock solution:
1. Take about 9.8 mL of methanol. For a 10 mL dosing bottle, let it stand for a few minutes and weigh it. Use a 10 μL injection needle to quickly remove the standard from the sealed bottle and add it to the dosing bottle. Weigh again to determine the amount added. Dilute to the mark with methanol, mix well, and record to 0.1 mg.
2. Pour the standard solution into the bottle and store it in the freezer.
3. The shelf life of this stock solution is 4 weeks.
(6) Intermediate storage solution for trihalomethane:
Add a suitable amount of pure methanol to a 10 mL dosing bottle, add the appropriate volume of the stock solution, dilute with methanol, mix well, and store in the freezer.
(7) Mixed standard solution:
Add pure methanol to a 10 mL dosing bottle. Add the intermediate stock solution in order of decreasing boiling point, dilute to the mark, mix well, and store in the freezer.
(8) Adsorbent: TENAX GC, 60/80 mesh.
(9) Helium: Purity over 99.999%, hydrocarbon content less than 0.5 ppm.
Sixth, sampling and preservation
(1) Sampling:
Add 30 mg of ascorbic acid to a 40 mL sample bottle. If the sample is tap water, let it run for 5 minutes before collecting. Avoid introducing ascorbic acid into the sample and ensure no air bubbles remain. Seal the bottle and store it in a dark place at 4°C.
(2) Sample preservation:
Protect the sample from high temperatures and sunlight. Store it in an environment free of organic solvent gases and at low temperatures. Complete the analysis within 14 days to prevent changes in the sample.
Seventh steps
(1) Set the instrument conditions for the gas chromatograph and purge and trap system as described in the equipment section.
(2) Preparation of the calibration line: Prepare at least 4 different concentrations of the calibration solution to create a calibration curve.
(III) Sample analysis: Inject 5 mL of the sample into the purge and trap system, then combine the chromatogram and signal obtained by the gas chromatograph with the calibration curve for detection and analysis.
Qualitative analysis:
The qualitative determination of trihalomethanes is based on the retention time of each compound. Figure 3 shows the chromatogram from the ELCD standard solution. Retention times may vary slightly depending on the instrument. Analysts should correct them using a mixed standard solution. If necessary, use different polarity columns or analyze using gas chromatography-mass spectrometry.
2. Quantitative analysis:
The amount of trihalomethanes can be determined using the regression equation from the calibration curve.
3. Recovery rate determination:
To assess the recovery rate, perform standard addition. Fill a 5 mL syringe with water, inject the mixed standard solution, and analyze the spiked sample to calculate the recovery rate.
Eighth, processing results:
The total trihalomethane concentration in the sample can be calculated using the formula:
A: Total amount of four trihalomethanes from the calibration curve (μg)
V: Sample injection volume, 5 mL.
Ninth, quality control:
(1) For every 10 samples analyzed, one blank test should be performed. At least one blank test per batch if fewer than 10 samples are tested. Blanks include instrument and reagent blanks.
(2) This method requires reagent, instrument, and field blank experiments. Blank frequency is 10%. Increase frequency if the matrix is different. Blank values should not exceed three times the detection limit for each analyte.
(3) The total trihalomethane recovery rate should be between 75% and 125%.
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