Book - Standard Methods for Analysis of Water and Wastewater is an appropriate reference for research work. Most libraries or water quality labs should have a recent edition.

Depending on the nitrogen concentration in your samples you can use Kjeldahl method. You need 3 aliquots of your filtrate/sample:

1. aliquot: direct steam distillation for NH4-N

2. aliquot: steam distillation after addition of Devarda's alloy for NH4-N + NO3-N

3. aliquot: steam distillation after digestion for analysis of NH4-N and organic N

As an alternative you can use a spectral photometric approach combine with an UV assisted oxidation of organic nitrogen to NO3-N. This approach is e.g. described by Kroon (1993):

https://ac.els-cdn.com/0003267093803963/1-s2.0-0003267093803963-main.pdf?_tid=079362de-2e25-4398-a67a-adadab0f8d05&acdnat=1534521010_3d65552f012a8d215808d3ceba37376f

The author compares these approach with Kjeldahl method for analysis of water samples. We use it for the analysis e.g. of NH4-N, NO3-N and Nt in CaCl2 extracts of soils and organic fertilizers.

Thank you so much for your answers @ Dieter Lohr, @Willium F. Hansen . It will be very helpful for my research.

Hello

There is a photometric method as well as test kits manufactured by Hach or Mettler Toledo for example. This systems are easy to use and cost effective. Maybe you check the web for Hach.com. there you should find the documents for the existing different methods. Kjeldahl is quite complicated and you need a full equiped Lab. The test kits are much cheaper

BR

Delia

Flow Injection Analyser (FIA) can give you Nitrate-N, Ammonium-N, Nitrite-N, total-N. The model we are using FOSS FIAstar 5000 twin channel analyser.