Dissolved organic nitrogen measurement using dialysis pretreatment

Dissolved organic nitrogen (DDN) is important for ecological and engineering researches. Quantification of low DDN concentrations in waters with elevated dissolved inorganic nitrogen (DIN) using existing methods is inaccurate. In this study, a dialysis-based pretreatment technique was optimized and adopted to reduce the interference from DIN to the quantification of DDN in natural water. A cellulose ester dialysis tube (nominal molecular weight cutoff = 100 Da) was used in batch and continuous-flow dialysis steps with model compounds, natural organic matter isolates, and bulk waters to develop a dialysis pretreatment approach that selectively reduces DIN from solutions containing DDN. By reducing DIN concentrations, propagation of analytical variance in total dissolved nitrogen (TDN) and DIN species concentrations allows more accurate determination of DDN (DDN = TDN - ND₃^- - ND₂^- - NH₃/NH₄⁺). Dialysis for 24 h against continuously flowing distilled water reduced DIN species by 70%. With dialysis pretreatment, DDN recoveries of more than 95% were obtained for surface water and finished drinking water, but wastewater experienced a slight loss (∼10%) of DDN possibly due to the adsorption of organics onto the dialysis membrane, permeation of low molecular weight fractions, or biodegradation. Dialysis experiments using surface water spiked with different DIN/TDN ratios concluded that dialysis pretreatment leads to more accurate DDN determination than no dialysis when DIN/TDN ratios exceed 0.6 mg of N/mg of N.