At present, the methods for determining carbon tetrachloride and chloroform in drinking water are mainly determined by the extraction and packing column method of constant temperature water bath in national standard [1], although they can meet the requirements for the detection of carbon tetrachloride and chloroform in drinking water. However, there are potential uncontrollable factors in the measurement process, such as: syringe temperature, headspace bottle temperature, ambient temperature, etc., resulting in low measurement results and high detection limits, which is difficult to meet carbon tetrachloride in bottled pure water And the need for chloroform detection. In this paper, the headspace autosampler is used to find a suitable determination method, and the determination is performed according to the principle of headspace gas chromatography. Compared with the national standard method, the results are satisfactory. The report is as follows.
1 Materials and methods
1.1 Instrument
Agilent 7694E headspace autosampler, Agilent 6890N gas chromatograph with u-ECD detector, HP-5 capillary column (30mm × 0.32mm × 0.25um) METTLER PURE water meter, 10, 20mL headspace vials, Agilent Precision 0.2, 0.5, 1mL syringes.
1.2 Reagent
Chloroform (10, 1.0 μg · mL-1), carbon tetrachloride (10, 1.0 μg · mL-1), standard substance (National Standard Material Center) ascorbic acid (analytical purity).
1.3 Method
1.3.1 Analysis conditions of Agilent 7694E headspace autosampler
Vial: 50 ℃ Loop: 70 ℃ TR.Line: 100 ℃ GC cycle time: 15min VialEQ.time: 5.0min pressuriz.time: 0.10min Loop Fill time: 0.50min Loop EQ.time: 0.5min Inject time: 1.00min
1.3.2 Analysis conditions of Agilent 6890N gas chromatograph
Mode: Constant pressure vaporization chamber temperature: 150 ° C Column pressure: 31.5 psi, nitrogen flow rate: 9.5mL · min-1, average linear velocity: 105cm · s-1, total flow rate: 106mL · min-1, split ratio: 1: 5. Split flow rate: 94.6mL · min-1, makeup flow rate: 20mL · min-1, time: 2.0min, column temperature: 85 ℃, retention time: 3min, detector temperature: 180 ℃, compensator: 60mL · min-1.
1.3.3 Selection of experimental conditions
1) Choose the temperature of the bottle at 40, 50, 60, and 80 ° C to observe the results of dynamic equilibrium. Experiments show that the peak shape is highest at 50 ° C, and the peak shape at 40, 60, and 80 ° C has a downward trend, so 50 ° C is chosen.
2) Choose 60, 70, 80, and 100 ° C for the loop temperature. Observe whether there is any loss of the test object.
3) TR.Line temperature selection choose 60, 70, 80, 90, 100 ℃ to observe the influence of the temperature control of the transmission line on the content of the test object. The experiment proves that the peak shape is highest at 100 ℃, so 100 ℃ is chosen.
4) Select GC-cycle time to choose 10, 15, 20, 30min to observe the change of the content of the test substance. The experiment proves that the peak height is the lowest at 10 minutes, and there is basically no change at 15, 20, and 30 minutes, so 15 minutes is chosen.
5) Vial EQ.time chooses 1, 3, 5, 7, 9min to observe the effect of dynamic balance maintenance time on the change of the content of the test substance. The results showed that the dynamic balance was not reached in 1, 3 minutes, and the peak heights were significantly different. The dynamic balance was reached in 5, 7, 9 minutes, and the peak height did not change much, so 5 minutes was chosen.
6) Loop Fill time choose 10, 20, 30s, 1min and 2min to observe the change of the filling degree of the quantitative loop and the content of the test substance. The results prove that the filling degree of the quantitative loop is not saturated in 10 and 20s, and the peak height is low. 30s, 1min and 2min quantitative ring filling degree saturation, peak height is basically the same. So choose 30s.
7) Choose 10, 20, 30, 40s and 50s for LOOP EQ.time to observe the influence of the balance time of the loop on the content of the test substance. The experiment proves that the analyte in 10 and 20s has not reached the dynamic balance in the quantitative loop, and the peak height is reduced. 30s, 40s, 1min reach the dynamic balance, the peak height is basically the same, so choose 30s.
8) Select 0.5, 1 and 2min for Inject time to observe whether the test substance is quantitatively injected into the chromatographic instrument and the change of the test substance content. The experiment shows: 0.5min is not completely quantitative injection, the peak height is reduced. 1min and 2min are fully quantitatively injected, and the peak height value is basically the same, so 1min is chosen.
1.3.4 Sampling
Take 5mL of bottled purified water sample and add 0.5g of ascorbic acid into a 10mL headspace vial to be tested.
1.3.5 Qualitative and quantitative
Qualitative peak time and external standard method
1.3.6 Standard curve drawing
The standard stock solution was prepared into chloroform containing 0.004, 0.4, 0.8, 1.0, 2.0 and 3.0ng · mL-1 and carbon tetrachloride containing 0.006, 0.012, 0.024, 0.048, 0.15 and 0.30ng · mL-1. The mixed use solution is measured according to the methods of 1.3.1 and 1.3.2, the injection volume is 1mL, the standard curve is drawn with the peak height as the vertical coordinate and the concentration as the horizontal coordinate, and the result is obtained by substituting the measured peak height value into the curve .
2 results
2.1 The stability of the sample
When sampling, first add 0.5g of ascorbic acid in a 20mL headspace bottle and take purified water to the full bottle for sealing. The measurement will be done after 0, 8, 24 and 72h respectively without affecting the results.
2.2 Linearity of the analysis method and minimum detection amount
In order to investigate the linearity of the quantitative response of chloroform and carbon tetrachloride, the calibration curve was measured and the peak height Y and concentration (ng · mL-1) were used as the standard curve. The chloroform was Y = 21.6X-0.0032, r = 0.9996; IV Carbon chloride is Y = 12.7X-0.0016, r = 0.9995, the minimum detection limit calculated by 3 times the signal-to-noise ratio is chloroform 0.0019μg · L-1, carbon tetrachloride 0.0012μg · L-1. The chloroform quantitative lower limit is 0.004 μg · L-1, and the carbon tetrachloride quantitative lower limit is 0.006 μg · L-1.
2.3 Precision and recovery rate of the method
Take 6 parts of the same bottle of purified water and add 0.5g of ascorbic acid respectively, and repeat the operation 6 times according to the operation of items 1.3.1 and 1.3.2. The measurement results showed that the recovery rate of chloroform was between 98% and 104%, and the RSD was between 0.3% and 0.9%. The recovery rate of carbon tetrachloride is between 96.7% and 102%, and the RSD is between 0.5% and 1.2%, as shown in Tables 2 and 3. Table 2 Precision and recovery rate of chloroform (omitted) Table 3 Precision and recovery rate of carbon tetrachloride (omitted)
2.4 Method sensitivity
The sensitivity of chloroform is 18.5 Hz, the sensitivity when quantitatively off-line is 21.6 Hz, the sensitivity of carbon tetrachloride is 10.5 Hz, and the sensitivity when quantitative off-line is 12.7 Hz, with distilled water as the reference, as shown in Table 4 Sensitivity of methyl chloride and carbon tetrachloride (omitted)
2.5 Sample analysis
A bottle of purified water from a certain brand in the market was used to determine the results. The results were 0.0051 μg · L-1 for chloroform and 0.0073 μg · L-1 for carbon tetrachloride. None of them were detected by the national standard method.
3 Discussion
Using the principle of gas chromatography [3] instead of constant temperature water bath extraction with a headspace autosampler, the measurement results more objectively and reliably reflect the actual concentration of the measured substance, solving the problems affecting the detection results in the traditional headspace method [1] Issues such as syringe temperature, whether the headspace bottle has reached dynamic equilibrium, and the effect of ambient temperature on the headspace bottle temperature. More elaborate experimental conditions are discussed, and the experimental results are compared with the national standard method and the reference method [2, 4-5]. This method has the advantages of fast, convenient, good linearity, few influencing factors, high sensitivity, etc. All the methodological indicators meet the analysis requirements and are feasible. Especially suitable for the detection of various types of pure water beverages on the market today, with unique advantages.
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