1. US Geological Survey standard USGS I-2700-85, 〈http://nwql.usgs.gov/tech_memos/nwql.1993-05.pdf〉 (accessed 18.09.14).
2. US Environmental Protection standard EPA 370.1, 〈http://www.epa.gov/region1/info/testmethods/pdfs/testmeth.pdf〉 (accessed 18.09.14).
3. US Environmental Sciences Section ESS 360.2, 〈http://www.epa.gov/greatlakes/lmmb/methods/methd360.2.pdf〉 (accessed 05.12.14).
4. Inorganic interferences in the 2,4-xylenol spectro-photometric method for nitrate and their elimination