>Dear All:
>
>I found the isotope values of the air nitrogen near our lab fluctuating.
>Sometimes they can be off by 0.5 per mil. Does anybody have the similar
>experiences? Are there any tricks in collecting air nitrogen for standard
>calibration?
>
>Thanks.
>
>Dachun Zhang
>Global Geochemistry Corp.
>6919 Eton Ave.
>Canoga Park, CA 91303

You should try the following reference :

Mariotti A., 1983, Atmospheric nitrogen is a reliable standard for 15N
abundance measurements, Nature, 303, 685-687.

Maybe your problem is coming from Ar correction.

David Widory
Laboratoire de Geochimie des Isotopes Stables
I.P.G.P.
Universite Paris 7-Denis Diderot
Tour 54-64 1er etage
4, place Jussieu
75252 Paris Cedex 05
France
Tel. (33) 01 44 27 60 88
Fax. (33) 01 44 27 28 30