Water soluble nitrate sometimes can be measured quickly as absorbance at 206 and/or 250 nm wavelengths. However, other organic molecules, especially those with double bonds also absorb radiation at same wavelengths. How can I minimize such interferences?
It is quite impossible to measure nitrate concentration by direct UV ansorbance at 250nm. First because nitrate absorptivity is very low at 250 nm, second because DOM also absorbs UV light, with a max between 240 and 290 nm.
It is possible to quantify nitrate by UV measurements ONLY if you use multiwavelength absorptiometry (see an old paper of mine : Fresenius Journal of Analytical Chemistry 12/1989; 338(3):234-237. DOI: 10.1007/BF00323014, or these two others : Deep-Sea Research I 49 (2002) 1291–1305) and Thomas O.; Gallot S.; Mazas N., 1990: Uv multiwavelength absorptiometry uvma for the examination of natural waters and wastewaters part ii determination of nitrate. Fresenius' Journal Of Analytical Chemistry. 338(3): 238-240
You also could find some infos on this website :http://link.springer.com/chapter/10.1007%2F978-3-642-01129-0_17#page-1
I agree with the above answer. Therefore, I suggest you use the second-derivative method developed by Ferree and Shannon 2001. Water Research 35: 327-332.
Hi - I have used the dual wavelength UV method for nitrate quantification in water/soil extracts. Please see my paper entitled "Assessing the potential for ion selective electrodes and dual wavelength UV spectroscopy as a rapid on-farm measurement of soil nitrate concentration" on my profile. I found it to be an excellent method.
Article Assessing the Potential for Ion Selective Electrodes and Dua...
It 's hard to believe that a simple equation like DOM205 = (2.841 × DOM300) - 0.0126 can be used to predict the DOM absorbance at 205 nm for all type of DOM in all kind of water...But try it !
You don't say if you are working freshwater or sea water. In seawater bromide absorbs at pretty much the same wavelength as nitrate, and of course organic matter gives you a broad spectrum absorbance below about 300 nm. The colorimetric method is much better although these days in situ nitrate analyzers do work to a greater or lesser extent, depending on the dissolved organic matter concentration.
From my experience with soil extracts and water samples from soils taken via suction cups, both the second-derivative method developed by Ferree and Shannon 2001, and the scanning method didn't work. by far the best way is conversion to Nitrite and then analyzing it colorimetric. the advantage of using the scanning method (220-2x275) over the second derivative is that it gives you a notion of the organic matter content.
good luck
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