15N/14N isotope composition of nitrate formed during ozonation: Conversion to N2O and analy-sis by membrane inlet coupled with laser spectroscopy

During oxidative water treatment, nitrate can be formed as stable end-product of dissolved or-ganic nitrogen (DON) compounds. A better understanding of the reactivity between DON mod-el compounds and ozone can be achieved by analyzing the 15N/14N isotope composition of the formed nitrate. In this study, we developed an analytical approach based on existing redox con-version of nitrate (NO3-) to N2O by TiCl3, using a membrane inlet system followed by laser spectroscopy to analyze 14N/l5N in N2O at natural abundance levels (Figure 1). Two types of laser spectrometers were used, depending on the target NO3- concentrations. First, a quantum cascade laser absorption spectrometer (QCLAS) was applied for NO3- at 50 – 200 µM. Best performance was observed with 50 µM NO3-, with offset between actual and meas-ured δ15N values of 0.82 – 2.3 ‰ for four NO3- isotope standards ranging from -50.47 – 13.69 ‰ (Figure 2). Therefore, samples during ozonation of DON that contained NO3- concentration above 50 µM were diluted to 50 µM before conversion and analysis. Second, a cavity ring down spectrometer (CRDS) was tested for NO3- analysis at lower concentrations (3 - 6 µM). The performance in terms of accuracy and precision improved as the NO3- concentration in-creased (Figure 3). To stay within the operation range of the CRDS (1500 ppb of N2O), NO3- concentrations higher than 5 µM are not recommended, thus samples at higher concentration were diluted. At 5 µM NO3- concentration, offsets between actual and measured δ15N values were 1.7 – 2.6 ‰ for NO3- isotope standards ranging from -50.47 – 13.69 ‰ (Figure 3). The effect of natural organic matter was tested and showed minimal effects on the analysis of δ15N in NO3-. Overall, the developed chemical conversion and laser spectroscopic analysis technique is capa-ble of fast measurement of δ15N- NO3- in aqueous samples with acceptable accuracy at natural abundance levels, across a wide concentration range and even in complex water matrix.