I have problem with measurements of GF/F filters. My samples are freshwater lagoon sediments. Nitrogen isotope ratio +20‰. We have Delta V Advantage and FlashEA1112. Salt concentration in the samples is very law. I removed carbonates keeping filters in desiccators with concentrate HCl. After 4 hours I kept my filters in 50oC for about 24 hours. I always was using this method and didn’t have any problems. Can it be chlorine? Maybe I had to keep filters in fume hood for Cl evaporation? Or maybe may GC column is “tired”. I had cleaned GC column by increasing its temperature. I also replaced the oxidation column filling it with double dose of silvered cobalt oxide. It didn’t help. I also replaced magnesium perchlorate column. The measurements of the other samples (not filters) are good. If it is chlorine, what should I do with these filters?