Hi Wendy,

Peak at m/z=28 may result not only from N2, but also from CO background 
(provided that CO2 background is OK). Such increase from 5E-12 to 1.2E-11 
seems to be normal after venting the ion source.
Stan Halas

At 15:58 06-03-13 -0500, you wrote:
>Hi, Everyone,
>
>    We are using Isoprime IRMS to run CO2 samples in DI mode. After the 
> ion source was cleaned, we run background scan (Nupro Valve Closed), the 
> N2 signal is ~12 e-12, it is much higher than before (0.8 e-12) and above 
> the acceptable value (5 e-12). However, the other signals, such as H2O, 
> O2, Ar, are not excess of their maximum acceptable values.
>     And to our experience, if there is a leak, Ar and H2O would show a 
> higher than normal background. Does anyone know what else may be causing 
> the high N2 background if it is not the leak? Any suggestion would be 
> greatly appreciated.
>
>
>Wendy Zhang
>Air Quality Research Division
>Environment Canada
>4905 Dufferin Street
>Toronto, Ontario M3H 5T4
>Tel: (416) 739-5792 (office)
>Tel: (416) 739-5822 (lab)
>Fax: (416) 739-4664
>Email:[log in to unmask]
>
>

Dr. Stanislaw Halas
Mass Spectrometry Laboratory
Institute of Physics, UMCS
20-031 Lublin, Poland
phone: +48 81 537-62-75
fax: +48 81 533-36-69