Thank you all for your suggestions, both on the list as directly to me. It is a good idea to calibrate our organic working standard (labstandard) against organic reference standards like USGS 40 and USGS 41, instead of the inorganic ones from the IAEA (N1 and N2).
So the easiest thing to try is to buy phenylalanine in a large quantity and to order the USGS 40 and 41.
I couldn't find the natural range in nitrogen 15 of all the amino acids. If someone knows if there is a better choice then phenylalanine, please let me know.
Thanks again, it was very helpful,

Anita Aerts-Bijma
Centre for Isotope Research, University of Groningen.

Centrum voor IsotopenOnderzoek (CIO), Rijksuniversiteit Groningen
Nijenborgh 4, 9747 AG Groningen, Netherlands
tel +31-50-3634716 fax +31-50-3634738