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6.1 Instrument Setup
Set up the TOC analyzer (4.1) and recorder (4.2) according to the instruction manual; select the appropriate sensitivity, measurement range, total carbon combustion tube temperature, and carrier gas flow rate. Power on the instrument and preheat for 2 hours until the output of the infrared analyzer and the baseline on the recorder stabilize.
6.2 Interference Elimination
When the content of common coexisting ions in the water sample exceeds the allowable interference value (1.3), it will affect the absorption of infrared rays. In this case, the water sample must be diluted with carbon dioxide-free distilled water (3.1) until the content of the coexisting ions is lower than their allowable interference concentration (1.3) before analysis.
6.3 Sample Injection
6.3.1 Differential Determination Method: Before testing, the acidified water sample should be neutralized to neutral with sodium hydroxide solution. Using a 50.00 μL microsyringe (4.3), accurately pipette 20.9 μg of the mixed water sample and inject it sequentially into the total carbon combustion tube and the inorganic carbon reaction tube. Measure the peak height of the corresponding absorption peaks on the recorder.
6.3.2 Direct Determination Method: Transfer approximately 25 mL of acidified water sample into a 50 mL beaker and stir vigorously on a magnetic stirrer for several minutes, or introduce carbon dioxide-free nitrogen gas into the beaker to remove inorganic carbon. Pipe 20 μL of the water sample after inorganic carbon removal into the total carbon combustion tube and measure the peak height of the absorption peaks on the recorder.
6.4 Blank Test: Perform a blank test following the steps described in section 6.3, using 20.0 μL of water (3.1) instead of the sample. 6.5 Calibration
Plotting the Calibration Curve:
Add 0.00, 0.50, 1.50, 3.00, 4.50, 6.00, and 7.50 mL of organic carbon standard solution (3.6) and inorganic carbon standard solution (3.8) to a set of seven 10 mL stoppered colorimetric tubes, respectively. Dilute to the mark with distilled water (3.1) and mix well. Prepare a series of organic and inorganic carbon standard solutions with concentrations of 0.0, 4.0, 12.0, 24.0, 36.0, 48.0, and 60.0 mg/L. Then proceed as described in 6.3. Subtract the peak height of the blank test from the measured peak height of the standard series solutions to obtain the corrected absorption peak height. Plot the organic and inorganic carbon calibration curves using the standard series solution concentrations and the corresponding corrected absorption peak heights. Alternatively, the linear regression equation for the calibration curve can be calculated using a linear regression method.
7.1 Calculation Methods
7.1.1 Differential Determination Method
Based on the peak height of the absorption peak of the tested sample, subtract the correction value of the peak height of the blank test absorption peak. The total carbon (TC, mg/L) and inorganic carbon (IC, mg/L) values are obtained from the calibration curve or calculated using the calibration curve regression equation. The difference between total carbon and inorganic carbon is the concentration of total organic carbon (TOC, mg/L) in the sample:
TOC (mg/L) = TC (mg/L) - IC (mg/L)
7.1.2 Direct Determination Method
Based on the peak height of the tested sample, subtract the correction value of the peak height of the blank test absorption peak. The total carbon (TC, mg/L) value is obtained from the calibration curve or calculated using the calibration curve regression equation. The concentration of total organic carbon (TOC, mg/L) in the sample is:
TOC (mg/L) = TC (mg/L)
The injection volume is 20.0 μL, and the results are expressed to one decimal place.
7.2 Precision and Accuracy
7.2.1 The arithmetic mean of the results of parallel double-sample determinations (relative deviation less than 10%) is taken as the final result.
7.2.2 The results of determinations of a standardized dispensing solution containing 10.8 mg/L TOC by four laboratories according to the procedure in section 6.3 are as follows:
7.2.2.1 Repeatability: Intra-laboratory relative standard deviation is 2.1%.
7.2.2.2 Reproducibility: Inter-laboratory relative standard deviation is 2.9%.
7.2.2.3 Accuracy: Relative error is 1.9%.
7.2.3 The results of determinations of a standardized dispensing solution containing 39.8 mg/L TOC by four laboratories according to the procedure in section 6.3 are as follows:
7.2.3.1 Repeatability: Intra-laboratory relative standard deviation is 0.8%.
7.2.3.2 Reproducibility: Experimental space relative standard deviation is 0.8%.
7.2.3.3 Accuracy: Relative error is 4.3%.
Appendix A General Explanation of this Standard
(Reference)
A1 Replace the carbon dioxide absorbent, catalyst in the high-temperature combustion tube, and decomposer in the low-temperature reaction tube periodically, as specified in the instrument manufacturer's instructions.
A2 According to literature reports, when the inorganic carbon content in surface water is much higher than the total organic carbon content, it will affect the accuracy of organic carbon determination. Recovery results (95.9%–103.6%) from synthetic samples containing both inorganic and organic carbon (where the ratio of inorganic carbon to total organic carbon is similar to that in some surface waters in northern and southern my country, generally several times) indicate that the impact on accuracy of determining total organic carbon in surface water using the difference method is acceptable.
A3 The direct method for determining total organic carbon involves aerating the water sample after acidification (pH < 4) to decompose various carbonates into carbon dioxide, which is then removed. The aerated sample is then injected into the high-temperature combustion tube for direct determination of total organic carbon. However, due to the loss of volatile organic compounds in the water sample during aeration, measurement errors may occur. Therefore, the measurement result only includes non-ablable organic carbon.
Additional Notes: This standard was proposed by the Standards Division of the Science and Technology Standards Department of the State Environmental Protection Administration.
This standard was drafted by the Wuhan Environmental Monitoring Center Station.
The main drafter of this standard is Rao Chunxi.
The China National Environmental Monitoring Centre is entrusted with interpreting this standard.
