Method for analyzing stable isotopes of particulate organic carbon (POC) and nitrogen in seawater using elemental analysis-isotope ratio mass spectrometry (EA-IRMS)

A method for analyzing stable isotopes of particulate organic carbon (POC) and nitrogen in seawater using elemental analysis-isotope ratio mass spectrometry (EA-IRMS), comprising the following steps:

• a. material preparation:
• burning a φ25 mm GF/F glass fiber filter membrane at 450° C. for 5 h in advance before use;
• b. sample collection:
• collecting a water sample at a desired station and in a desired layer;
• using the φ25 mm GF/F glass fiber filter membrane to filter 200 mL of the water sample and then filter 100 ml of distilled water;
• and wrapping a resulting membrane sample with tin foil, and storing the resulting membrane sample at −20° C. for later use;
• c. sample pretreatment:
• subjecting the resulting membrane sample to acidification for 30 min to obtain an acidified membrane sample, drying the acidified membrane sample in a drying oven at 60° C. for 24 h immediately after the acidification to obtain a dried membrane sample, and storing the dried membrane sample in a desiccator;
• d. sample coating:
• coating the dried membrane sample obtained in step c to obtain a coated membrane sample, and letting the coated membrane sample to be ready for test on a machine;
• e. sample determination:
• cleaning and debugging instruments of an elemental analyzer and an isotope ratio mass spectrometer according to operational provisions of the instruments to make the instruments in a sample test status;
• adjusting general settings of the instruments as follows:
• setting an oxygen addition time to 80 s and a combustion tube temperature to 700° C. for the elemental analyzer, and modifying ion parameters and setting a trap current to 300 μA to 400 μA for the isotope ratio mass spectrometer;
• and starting a sample test;
• f. sample loading order:
• adding three or more isotopic standards certified by the International Atomic Energy Agency (IAEA) or working standards with stable isotope values, wherein 5 to 6 replicates are set for each standard of the isotopic standards and the working standards;
• adding the coated membrane sample, wherein a standard is added every 10 samples;
• and after all samples are added, starting the instruments for test;
• and
• g. sample data processing:
• observing whether 5 to 6 results of the each standard are stable, wherein if a standard deviation (SD) is lower than 0.2, data are available;
• if test results of the isotopic standards and the working standards are stable, averaging the 5 to 6 results of the each standard;
• using Excel to plot a standard curve with an average value measured by the instruments as an x-coordinate and a true value as a y-coordinate;
• and substituting a measured value of a sample into the standard curve for calculation to obtain a true value of the sample, substituting a measured value of a standard inserted among all samples into the standard curve for calculation, and comparing a result with the true value, wherein if a correction coefficient is in a range of 0.9 to 1.1, data are credible.