1610 / 2019-08-22 19:59:21

Developing direct method for simultaneous determination of dissolved organic nitrogen, nitrite, nitrate and ammonia using size exclusion chromatography coupled with nitrogen detector

18、地学仪器与系统 > 专题1：水体与湿地仪器

Quantification of dissolved organic nitrogen (DON) as well as inorganic nitrogen (DIN) species is important to understand the ecological N cycling issues in natural aquatic systems and the control of nitrogen-related water parameters in engineered water systems. The current standard method for DON determination in water and wastewater is an indirect method, which is common practice to subtract DIN (ammonia, nitrite, and nitrate) species from total dissolved nitrogen (TDN). However, the precision for DON analysis has been being a problem, as the subtraction of multiple independent measurements accumulates the errors of each single measurement into the finally determined DON concentration.
Huber et al. has developed the LC-OND/OCD system for directly measuring DON, which is based on three critical procedures: (i) isolating DON from DIN by size exclusion chromatography (SEC) according to their differences in molecular weight; (ii) oxidizing nitrogenous compounds into nitrate using a helical quartz capillary combined with UV lamp and (iii) quantifying nitrate concentration using a UV absorbance detector at λ=220 nm. However, the relatively low flow rate and high inner dimeter (I.D.=1.0 mm) of quartz capillary in oxidation unit lead to the broadening and overlapping of chromatogram peaks between low MW matter and nitrite/nitrate. To overcome the severe interferences from nitrite or nitrate on the analysis of DON fractions, Chon et al further developed electrodialysis as a pretreatment method prior to LC-OND/OCD analysis for high DIN/TDN wastewater samples.
Besides electrodialysis, there are possibly another three alternatives to enhance the separation of DON from DIN: (i) using longer SEC column packed with the same TSK HW50s material, (ii) using SEC column with low size exclusion limit and size permission limit and (iii) making the UV oxidation units with a small I.D. quartz capillary. In this work, we have developed an independent UV oxidation unit that can be inserted into the laboratory liquid chromatography system. The nitrogen recovery rate of this oxidation unit was evaluated as a function of retention time. With the SEC column and quartz capillary applied in this study, we demonstrated that the success improvement of the SEC-based method for simultaneous and direct determination of DON together with nitrite, nitrate and ammonia species.