Analysis of multiple volatile organic compounds in water by headspace chromatography - Master's thesis - Dissertation

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Drinking water is easily contaminated by volatile organic compounds such as halogenated hydrocarbons and benzenes that inhibit central nervous system and anesthesia. The Standard for Drinking Water Hygiene lists volatile organic pollutants as water quality monitoring. However, due to the low water content of these substances in drinking (usually in the concentration of μg / L), the determination of these substances requires liquid-liquid extraction, static headspace, purge and trap or solid phase micro-extraction techniques, which is cumbersome. The analysis time is longer and the types of compounds tested are limited. The author studied and established a method for the determination of many common volatile organic compounds in drinking water by headspace-capillary gas chromatography.

Simultaneous detection of 12 volatile organic compounds in drinking water by headspace gas chromatography-hydrogen flame ionization detector was obtained, and satisfactory separation effect and high sensitivity were obtained. Compared with the current standard test method for drinking water [8], the separation effect is good, the operation is simple, the method is sensitive, the organic solvent is not needed, the secondary pollution of the environment and water is avoided, and the requirements of the water quality sanitation standard can be fully satisfied. Simultaneous determination of various trace volatile organic compounds in drinking water.

1 Materials and methods

111 instruments

GC - 2010 Gas Chromatograph (Shimadzu, Japan); Hydrogen Flame Ionization Detector (F ID); TELEDYNE TEKMAR HT3TM Headspace Autosampler System (USA); V IAL LAS Headspace Bottle (USA); AUW220 One-tenth of a million analytical balances (Shimadzu, Japan).

112 reagent

1, 1, 1 - chloroform, dichloromethane, benzene, trichloroethylene, tetrachloroethylene, toluene, 1,2-dichloroethane, ethylbenzene, p-xylene, m-xylene, o-xylene, The styrene standard and methanol are chromatographically pure (Tianjin Guangfu Fine Chemical Research Institute), excellent grade pure sodium chloride (550 ° C for 2 h, placed in a desiccator for storage).

113 Test methods

11311 Chromatographic conditions HP - 5 quartz capillary column ( 30 m × 0125 mm × 0125 μm); column temperature control program: initial temperature is 50 ° C, hold 8 min and then increase to 120 ° C at 10 ° C / min, total running time 15 min; carrier gas flow rate (nitrogen purity 991999%): column pressure 80 KPa; hydrogen flow rate: 40 ml/min; air flow rate: 400 ml / min; inlet temperature: 200 ° C; detector temperature For: 230 ° C, split injection, the split ratio is: 1: 15.

11312 Headspace sampling system condition temperature: furnace temperature is 65 ° C, the temperature of the quantitative tube is 80 ° C, the transmission line temperature is 80 ° C; pressure: transmission line pressure is 80 KPa, headspace bottle pressure is 40 KPa. Time: The high-speed oscillation time is 2 min, the sample equilibration time is 10 min, the charging time is 110 min, and the filling time is

At 110 min, the tube balance time was 115 min, the injection time was 110 min, and the injection volume was 110 ml.

11313 Standard curve drawing: chromatographically pure 1, 1, 1 - trichloroethane, dichloromethane, benzene, trichloroethylene, tetrachloroethylene, toluene, 1,2-dichloroethane, ethylbenzene, p-pair Toluene, m-xylene, o-xylene, and styrene were formulated with methanol to a standard stock solution having concentrations of 63140, 60145, 7134, 64128, 81160, 10118, 60155, 10169, 10147, 8153, 10159, and 12104 mg/ml, respectively. The stock solution was diluted 20 times with methanol before use, and then diluted 50 times with pure water to obtain concentrations of 63140, 60145, 7134, 64128, 81160, 10118, 60155, 10169, 10147, 8153, 10159, 12104 μg/ml, respectively. For the standard intermediate solution, take 10100 ml of each of the above intermediate liquids into a 250 ml volumetric flask, and dilute to the mark with pure water to mix to obtain a mixed standard solution, and prepare 6 series of standard use liquids according to Table 1. Take 15 ml of this series of standard use solution, add to the headspace bottle which has been added with 415 g of sodium chloride, quickly press and mix, and carry out headspace-gas chromatography. According to the retention time qualitative, the standard curve of 12 organic substances was plotted as the peak area and concentration (see Figure 1).

11314 Sample pretreatment A water sample of 1510 ml was placed in a headspace bottle to which 415 g of sodium chloride had been added, and rapidly packed and mixed for headspace-gas chromatography analysis.

11315 Sample determination Determination of 1,1,1-trichloromethane, methylene chloride, benzene, trichloroethylene, tetrachloroethylene, toluene, 1,2-dichloroethane, ethylbenzene in water samples under optimized chromatographic conditions , p-xylene, m-xylene, o-xylene, styrene, qualitative time, peak area quantification, external standard method to calculate the concentration of various organic substances in the sample.

2 Results and discussion

211 Optimization of chromatographic conditions

HP - 5 capillary column, HP - INNOWAX capillary column, DB - 1 capillary column, DB - 1701 capillary column and Rtx -WAX capillary column were compared. The results show that HP - 5 capillary column has the best separation effect. Program control with column temperature for efficient separation in 15 min

The above various volatile organic compounds, and the separation effect and peak shape can meet the requirements of the method, and the results are shown in FIG.

212 selection of headspace conditions

21211 Effect of equilibrium temperature on measurement In the headspace gas chromatography experiment, the gas-liquid equilibrium temperature has the greatest influence on the results [11]. As the equilibrium temperature of the sample increases, the amount of organic matter entering the gas chromatograph increases, thereby improving the analysis. Sensitivity. However, if the temperature is too high, a large amount of water vapor will be generated, and the organic matter will enter the chromatograph together, which lowers the relative concentration of the target compound, and the water vapor will affect the service life of the column. The effects of equilibrium temperature at 50 °C, 55 °C, 60 °C, 65 °C, 70 °C, and 75 °C on the sensitivity were tested at a fixed constant temperature of 15 min.

,

When the sample equilibrium temperature is 65 °C, the water vapor is less, the chromatographic peak is stable, and the sensitivity is high.

21212 Effect of Equilibrium Time on Measurement Volatile organic compounds volatilize from water and it takes time to reach gas-liquid equilibrium. For this reason, the fixed headspace equilibrium temperature is 65 °C, and the equilibrium time is changed to 5, 8, 10, 12, 15 min. The test shows that as the equilibrium time increases, the peak area increases continuously, when the equilibrium time is 10 min. The gas-liquid two phases are basically balanced, and the peak area is close to the highest value. Therefore, the balance time is 10 min.

21213 Effect of Sodium Chloride Concentration on the Determination Sodium chloride has the effect of increasing the ionic strength of the solution and reducing the solubility of organic compounds in water. The relationship between the concentration of sodium chloride and the response of organic matter was tested by taking 1510 ml of the mixed standard solution, fixing the headspace equilibrium temperature (65 ° C) and the constant temperature time (10 min). The test shows that the peak area of ​​the organic matter increases with the increase of the concentration of sodium chloride in the solution. When the concentration of sodium chloride increases to 300 g/L, the peak area does not increase any more, so the concentration of 300 g/L sodium chloride is selected.

213 linear range and detection limit

The peak area of ​​the mixed standard solution series of 12 organic compounds including 1,1,1-trichloromethane was determined under optimized optimal chromatographic conditions, and the linear regression equation and correlation coefficient were obtained. The detection limit of the method is represented by 2 times the noise value of the instrument [12], and the corresponding concentration value is the lowest detection concentration; the quantitative lower limit of the method is represented by 10 times the mean value of the instrument noise, and the corresponding concentration is the lowest quantitative concentration. It can be seen from Table 2 that under the headspace-chromatography conditions, the 12 organic compounds have a wide linear range, good linearity and high sensitivity.

214 Water sample analysis and spike recovery test

Using this method, 38 tap water and groundwater were measured, and 12 organic substances were not detected. Three kinds of mixed standard solutions of high, medium and low concentration were added to the water sample, and the recovery rate and relative standard deviation of each component were measured. The recovery range was 8912%~11013%.

RSD

214% ~ 517%, can meet the methodological requirements

See the results, see Table 3.

3 Summary

Simultaneous detection of 12 volatile organic compounds in drinking water by headspace gas chromatography-hydrogen flame ionization detector was obtained, and satisfactory separation effect and high sensitivity were obtained. Compared with the current standard test method for drinking water [8], the separation effect is good, the operation is simple, the method is sensitive, and no organic solvent is needed, which avoids the secondary pollution of the environment and water, and can fully meet the requirements of water quality sanitation standards, suitable for Simultaneous determination of various trace volatile organic compounds in drinking water.