A decarbonation and preservation method for the analysis of organic C and N contents and stable isotope ratios of low-carbonated suspended particulate material

The aim of this study was to determine a simple routine procedure to preserve, decarbonate and analyse low-carbonated filters of suspended particulate organic matter (POM) for particulate organic carbon and nitrogen content, δC and δN. Our goal was to analyse these four parameters from a single and entire filter of POM without altering the organic material. First, freezing (-20°C) versus oven-drying (60°C) were compared as the initial preservation step. Afterwards, non-acidified samples were compared to acid-treated samples using 0.12N HCl (diluted HCl rinsing at the end of the filtration) or 12N HCl (filters exposed to HCl fumes for four hours in a dessicator). Regarding the preservation methods, our results indicate that freezing increases the uncertainty of δN measurements and, in combination with concentrated HCl treatment, leads to a loss of particulate nitrogen and an alteration of the δN signature. Consequently, we recommend drying to preserve filter samples. Regarding acid treatments, we found that (i) diluted HCl would not be sufficient to fully remove the carbonate from our samples, (ii) in contrast, a four-hour exposure of the filters to the HCl fumes was enough to remove all the inorganic carbon, and (iii) the concentrated HCl treatment did not alter the nitrogen measurements (only when drying without freezing is used to preserve the filters). Consequently, we propose that low-carbonated POM filters are preserved by drying and carbonates are removed by exposing the filters to HCl fumes (four hours) for the analysis of particulate organic C and N content and isotope ratios. 2 ha l-0 04 52 45 0, v er si on 1 2 Fe b 20 10