My Shimadzu TOC/TN analyzer is yielding inconsistent total dissolved nitrogen results for samples, but not standards and is likely due to sparging. The samples are all filtered and the instrument has an internal sparging unit. I typically analyze samples for NPOC and TDN with more injections for the TDN portion, i.e. 3/5 injections for NPOC and 4/6 for TDN. When sample is analyzed only for TDN the area for injections is higher than when TDN is analyzer for both NPOC/TDN when internally sparging is on. For example, the first three NPOC/TDN injections are very precise and similar and the second three injections are very precise, similar, and higher. I have compared samples with internal addition of acid and sparging, internal addition of acid and no sparging, and no acid addition and sparging. Results with no acid and no sparging had the largest area counts, followed by no acid with sparging (about 10% lower), and acid with sparging yielded about 20% lower area counts for TDN. I have also manually externally sparged samples with and without acid, without any internal sparging and all results were the same. Using multiple injections from one autosampler draw or drawing each injections one by one and sparging yields the same results and issues.

I ran these samples on a different Shimadzu in a different lab and these issues did not occur. Results were a little different, but this may be due to whatever is happening with my Shimadzu. I'm unsure of what to try next. I do not see any signs of leaking. It is very strange that the issue occurs on the samples, but not the standards.