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N2 incorrectly identifying on GCMS after H2 carrier gas swap
Posted: Mon Sep 26, 2022 1:25 pm
by morgan2
I have thoroughly searched the internet for someone else also having this same issue but have come up short. We have recently switched the carrier gas of our mass spec from helium to hydrogen. Other than the shift in retention times and flow this was completed with little to no problems. However, I have noticed since the swap that what has always been our Nitrogen peak is now integrating as "Diethyl azodicarboxylate" which has a large base peak of 29. The similarity index rating for it being diethyl azodicarboxylate is 88. The Nitrogen base peak of 28 barely shows up and nitrogen isn't even listed in the possible match list. The second closest match is Formaldehyde. I have tried several different ways of integrating the peak, i.e. only scanning the top of the peak, scanning the whole peak, subtracting baseline, etc. No matter what I do it will not show as nitrogen. This is true even when injecting a >99% Nitrogen sample. What do you think is going on here? We are using an Agilent Poraplot Q column. It is a Shimadzu GCMS 2010-plus.
Re: N2 incorrectly identifying on GCMS after H2 carrier gas swap
Posted: Mon Sep 26, 2022 5:14 pm
by James_Ball
You are probably forming N2H or some similar species through a chemical ionization in the source.
We tried hydrogen carrier doing EPA work and had difficulty passing a tune check which requires injection of 4-Bromofluorobenzene and evaluation of the resulting mass spectra. This check requires that m/z 96 be at 5-9% of the intensity of m/z 95, but the first check we did with hydrogen carrier it was nearly 100%. We were able to get it down to about 12% but with a carrier flow of only 0.3 ml/min. The higher the carrier flow, the higher the mass ratio.
Classic EI spectra found in the NIST and similar libraries were acquired using Helium carrier so they may not match when using Hydrogen. With the need to switch for many labs it would be nice if there were a database generated using hydrogen carrier to help with this problem.
Re: N2 incorrectly identifying on GCMS after H2 carrier gas swap
Posted: Tue Sep 27, 2022 7:22 pm
by Steve Reimer
The higher % of 96 is normal for hydrogen. Thus in method 8260D we upped the acceptance to 15% for hydrogen. A couple of the target analytes have a tendency to react in the inlet and may give erratic results.
Re: N2 incorrectly identifying on GCMS after H2 carrier gas swap
Posted: Thu Sep 29, 2022 2:21 pm
by James_Ball
The higher % of 96 is normal for hydrogen. Thus in method 8260D we upped the acceptance to 15% for hydrogen. A couple of the target analytes have a tendency to react in the inlet and may give erratic results.
I wish they would rewrite 524 with this exception since we do more of those than anything.
8270 works well though.