Are you looking for inorganic (lab) standards to preserve IAEA-N1 and IAEA-N2 (both ammonium sulphates) or are you looking for organic (lab) standards?
If it is the former you after you could purchase USGS25 and USGS26 (both ammonium sulphates too) with d15N values of -30.41 and +53.75 per mil, respectively.
If you require (lab) standards for scale calibration and quality control of organic samples I would recommend getting started by purchasing USGS40 and USGS41 (both glutamate) with d15N values of -4.52 and +45.57 per mil, respectively. Both these materials are recognised reference materials so you could use these to determine scale calibrated d15N values of lab standards, for which amino acids (as suggested by Stan) would be a good choice since their chemical make-up (in particular their C/N ratio) is a good match for most organic materials.
As also mentioned by Stan, some AAs are a pain to weigh out (either lumpy or electrostatically charged like the dickens). In these instances, we prepare aqueous stock solution which we dilute to end up with solutions where for example 10 microL contain 0.4 mg. In line with the desired sample weight, we dispense small volumes of the dilution into tin capsules sitting in a well-plate made of solid metal. This metal well-plate is eventually placed in a tray filled with liquid N2 to flash-freeze the samples and the frozen well-plate is subsequently placed in a freeze dryer. This procedure yields standards of highly reproducible sample weight (and, hence, N2 and CO2 peaks).
in order to preserve the IAEA standards N1 and N2 we like to have labstandards with delta 15 values of 0 ‰ and around 20 ‰. The zero value doesn't appear to be a problem, but a material with the higher value we can't find until now.
Does anyone of you find a readily available material with a d15N of approx 20‰?
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