Most likely CO2 is breaking through your LN2 trap and is being ionized to 12C16O+ giving you (a lot of) m/Z 28. I have seen this happen when LN2 evaporates or more often during the first run of the day when I have forgotten to fill the LN2 trap.
----- Original Message -----
From: Matheus Carvalho <[log in to unmask]>
Date: Tuesday, February 22, 2011 7:05 pm
Subject: Problems in aminocids measurements
To: [log in to unmask]
> Dear Isogeochem members:
> I have been trying to measure the d15N of aminoacids using a GCcIRMS setting, and I have been obtaining strange values around 400ppt for all samples. I have been trying many things, and nothing has worked so far. Interestingly, I could get reasonable values for d13C, between -20 and -40ppt for the same samples. Some more details:
> - injecting air through the column I obtain typical values between -1 and 1ppt for d15N.
- injecting N2O I get similar values as for the air, which indicates that the reduction furnace is ok (I did balak tests and values were negligible, so the N2 peak as really N2O, and not air).
- I am injecting 1uL of sample, splitless, constant pressure @ 25psi, temperatures from 45 to 190oC; column has 50m in length, 0.22mm ID, 0.25um of film thikness
- the mass 30 is very high for some samples, but not for all
> Any ideas or suggestions are appreciated.
> Matheus C. Carvalho
> Senior Research Associate
> Centre for Coastal Biogeochemistry
> Southern Cross University
> Lismore - Australia