Hi Masumi,
here are references for method papers on d15N ammonium  measurements:
Sorensen et al., 1991 
Sigman et al. (i think ,1997)
Holmes et al., 1998
I have measured d15N of pore water ammonium using these methods, so feel free to write me if you have more questions.
Masha


----- Original Message -----
From: Masumi Yamamuro <[log in to unmask]>
Date: Monday, December 26, 2005 12:15 am
Subject: [ISOGEOCHEM] 15N of interstitial ammonium
To: [log in to unmask]

> Greetings!
> 
> I would like to determine the natural abundance of 15N of ammonium in
> interstitial water and overlying water in brackish lake.
> 
> I would greatly appreciate any information i.e. literatures, name 
> of the
> expert person, or key words to search papers. Searching with key 
> words of
> "ammonium" and "stable isotope" gave me too much papers of no 
> relations.
> Sincerely,
> Masumi
> 
> -------------------------------------------------------------------
> Masumi YAMAMURO (D Sc)
> Institute of Geology and Geoinformation
> Geological Survey of Japan
> AIST Tsukuba Central 7
> Tsukuba Ibaraki 305-8567 Japan
> Tel & Fax 81-298-61-3766
> E-mail [log in to unmask]
> http://staff.aist.go.jp/m-yamamuro/en_Folder/top_english.html
> +++++ Updated on 25 November 2005 +++++
> -------------------------------------------------------------------
>