> I recall seeing a paper on the effect that Ar had
>on N2 measurement on different dual inlet mass spectometers from the early
>80's, but unfortunately I cannot find the original reference.
>
>Paul Brooks.

Try : Mariotti A., 1984. Natural 15N abundance measurements and atmospheric
nitrogen standard calibration, Nature, 311 : 251-252.

        Hope it will help.
                                Boris