Thanks for replying me about my issues. I followed the suggestions trying to figure out the problem:
Simon - Our system has carrier flow and the purge flow the carrier around 65 ml/min and 48ml/min, respectively. The reaction tubes were changed recently and no leak was detected. I baked the GC column at 140C, I haven't for a while, and improved the linearity line. I can see the samples dropping and flashing.
Harry - I am using ANCA and reactor columns are new. The flow rates are normal and no leak was detected.
Chris - I am analyzing GF/F filters and I have to split the filter in half and analyze two capsules at the time. There is a possibility to get stuck due to their sizes but if that happens I should see a signal increase on the next sample that is not the case, the flowing sample has normal signal. I blanks are empty capsules, because I use two capsules per sample, I also use two empty capsules for the blanks. The variation d15N is normal due to low nitrogen on the standard (1.36%), I start to see a precise d15N measurement with weight >2mg.
I did a series of empty capsules (20 capsules) after baking the column and it ran fine. I do not believe baking the column would fix the problem. Like I mentioned, this is a random problem, but started increase lately.
Thank you for all suggestion, hopefully will not happens again.
All the best,
P.S: I have sent this reply long time ago and just noticed that was not posted. My apologies.....