I would recommend you to use a kit like this one: http://www.funakoshi.co.jp/data/datasheet/BAS/ENOS.pdf
However, if you are having problems of sensitivity (the limit of the kit is around 1pmol/ul) or similar I would give a try to the radiochemical assays that are usually more sensitive. The article that Combet et al. published in nature is a really good example of this method. You can find it here: