Currently both sample are store in thermo scientific amber glass vial with septa. Each sample are preserve with 0.0008 M of HgCl2 and store in refrigerator
I would use a Leco CHNS Analyzer (carbon, nitrogen, hydrogen, sulfur) before and after ultrafiltering (0.1 um Teflon syringe filter). Check the 'American Association Methods of Water and Wastewater Analysis.