3.5 DIC carbon isotope test
For one sample, 8 drops of anhydrous phosphoric acid was added to the 12 mL sample bottle (labco) and the bottle was put on the constant temperature sample disk in sequence. The air blowing needle was fixed, the working procedure of GC PAL automatic sampler was set, and the sample bottle was successively emptied with helium for 5min to remove the influence of the air in the bottle on the determination of C isotope ratio. 0.2mL water sample was added into the sample bottle, reacted at 45 °C on the dry heater for 45 minutes, and centrifugated before sample measurement.
The mixture of high purity helium and CO2 was separated from other impurities by gas chromatography at 75 ℃. The separated CO2 is carried into Delta V detector by helium, and ionized by high-energy electron beam. After accelerated electric field, gaseous ions with different mass charge ratios (m/z44, m/z45, m/z46) enter the magnetic field to separate into different ion beams, which enter the receiver and convert them into electrical signals to determine the carbon isotope ratio. The accuracy of δ13CDIC value is ± 0.08 ‰.
3.6 Methane isotope test
The instrument used in methane and carbon dioxide isotope analysis was the on-line analysis and test system (GC/C/IRMS) produced by ThermrFinnigan company, it mainly composed of Trace Ultra Chromatograph, combustion furnace (GC Combustion Ⅲ) and stable isotope mass spectrometer Delta V Advantage. The mass spectrometry test conditions were set as follows: ion source high pressure was 3.0 kv, ion source emission current was 1.5 mA. The chromatography conditions were set as follows: Poraplot Q capillary column (27 m×0.32 mm×20.00 μm) was chosen as chromatographic column, high purity He (99.999%) was used as carrier gas, column flow rate was 1.5 mL/min, sample inlet temperature was 120 ℃.
For δ13C-CH4 analysis, the temperature of the oxidation furnace (NiO/ CuO/ PI) was 960 ℃ and the temperature of the reduction furnace was 640 ℃. Then methane was oxidized into carbon dioxide gas in combustion furnace, and then introduced into isotope mass spectrometer to detect carbon isotope value.
For δD-CH4 analysis, the carrier gas was helium (1.2 mL/min), the split ratio was between 1:8 and 1:40 depending on methane concentration, and the temperatures of the GC oven and injector were 40°C and 200°C, respectively. Analysis of δD-CH4involved on-line transfer of samples from a high temperature conversion reactor (containing an empty ceramic tube covered with graphite layer that was kept at a temperature of 1440°C) in which compounds were pyrolyzed to molecular hydrogen, carbon, and carbon monoxide, prior to their transfer into the mass spectrometer via a ConfloIV interface. Then molecular hydrogen was introduced into isotope mass spectrometer to detect hydrogen isotope value.