Spectrometric methods use the chlorophyll density optically to infer the nitrogen content. The TKN Kjeldahl method is the standard chemical analysis. Atomic spectrometric techniques are also employed. There are many public and private laboratories which perform these analyses.
Dear Khitam,
if you want to measure protein content of your leaves you can opt for TKN as mentioned by Paul Reed Hepperly. For total N I recommend ultimate analysis which measures CHNS/O (http://www.astm.org/Standards/E870.htm). TEM coupled with X-ray microanalysis can also be used if carefully calibrated and samples suitably prepared (http://linkinghub.elsevier.com/retrieve/pii/S0025326X97000076). Good luck Rob.
Please refer to the following article:
Ferreira, E. V. D. O., Novais, R. F., Médice, B. M., Barros, N. F. D., & Silva, I. R. (2015). Leaf total nitrogen concentration as an indicator of nitrogen status for plantlets and young plants of eucalyptus clones. Revista Brasileira de Ciência do Solo, 39(4), 1127-1140. http://www.scielo.br/scielo.php?pid=S0100-06832015000401127&script=sci_arttext
Are you considering a destructive method? If so, then wash representative sample of leaves to get rid of other possible external N source, oven dry at low temperature (e.g. 70 oC) and grind. If your lab has a LECO CN analyzer, simply weigh out required amount (based on your expected N but usually about 0.15 - 0.5g) of your ground leave sample into a crucible and analyse for total N. You can also do an acid digestion following the Kjeldahl method.
There are SPAD meters basically measures spectrometrically the greenness or chlorophyll content correlates well with Nitrogen contents. These devices can be used directly over field canopies. Pretty neat and not overly expensive. Check them out.
1. By Kjeldahl method in the laboratory
2. Estimating by SPAD meters which is correlate with N content
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