Comparative Study On Dissolved Organic Carbon And Isotopes In Water Samples By High Temperature Oxidation And Wet Oxidation

The study of DOC and its isotopes in water is of great significance for understanding the photosynthesis of phytoplankton,the metabolism of organisms and the activities of bacteria in water.Domestic research methods on DOC have matured,but research methods on DOC carbon isotopes are rarely reported.In this paper,five different compounds(caffeine,glucose,phthalic acid,potassium acetate,and humic acid)were selected according to the degree of oxidation difficulty to configure the DOC standard solution.The content of DOC standard solution and its ?13C value were determined by high temperature oxidation method and wet chemical oxidation method respectively.Determine the optimal experimental conditions of high-temperature oxidation method,and make the determination,and then compare with the results of the wet chemical oxidation method.The results of this study are as follows:(1)By changing the temperature of the oxidation tube,the time of passing oxygen,and the flow rate of the carrier gas,the high temperature oxidation method was used to test the carbon conversion rate and ?13C value of the five compound DOC standard solutions of caffeine,glucose,phthalic acid,potassium acetate,and humic acid.When the carrier gas flow rate is 80 mL/min,the temperature of the oxidation tube is 850?,and the oxygen ventilation time is 20 s(oxygen ventilation flow rate is 10mL/min),the deviation between the measured ?13C value and the reference value is-0.82‰?0.11‰ The standard deviation is between 0.01‰?0.03‰;the carbon conversion rate of the DOC standard solution of the five compounds is 98.60%?106.20%,and the standard deviation is between 0.01%?0.05%.(2)The high-temperature oxidation method can convert all caffeine,glucose,phthalic acid,and potassium acetate in DOC standard solutions of different concentrations to CO2.At the same time,there is no obvious isotope fractionation of carbon isotopes during the conversion process.Since humic acid is a substance that is more difficult to be oxidized,the high-temperature oxidation method has a lower conversion efficiency(<20%)of carbon in the DOC standard solution of humic acid,which also leads to greater carbon isotope fractionation during the conversion process.(3)When testing the content and ?13C value of the DOC solution with a concentration of less than 1mg/L by the high temperature oxidation method,there is a large sample blank interference,and a blank correction of the DOC content and ?13C value is required to make it closer to the reference value.(4)The high temperature oxidation method for the determination of four compounds(caffeine,glucose,phthalic acid,and potassium acetate)in mixed DOC solutions showed that the tested ?13C value was-25.59‰,which was negative 0.34‰compared with the reference value,and the standard deviation 0.12‰,the carbon conversion rate is between 111.31%?125.12%.(5)Select samples of different properties(river water,karst water,sea water)to test the DOC content and ?13C value using the high temperature oxidation TOC-IRMS combined instrument.The standard deviation of the DOC content measurement is between 0.01mg/L?0.41mg/L,the standard deviation measured by the ?13C value is between 0.00‰?0.17‰.(6)Select five DOC standard samples of the same concentration,and compare the test results of high temperature oxidation method and wet chemical oxidation method.Except for humic acid,the carbon conversion rate of high temperature oxidation method is between 98.60%?106.20%,while wet oxidation method is only 72.81%?91.13%.The deviation of the ?13C value between the high-temperature oxidation method and the reference value is within-0.82‰?0.11‰,and the standard deviation of the measurement is between 0.01‰?0.03‰;the deviation between the?13C value and the reference value of the wet oxidation method is within 3.48‰?1.78‰,the standard deviation of the measurement is between 0.08‰?0.69‰.