Determination of total nitrogen in atmospheric wet and dry deposition samples

A microwave-assisted persulfate oxidation method followed by ion chromatographic determination of nitrate was developed for total nitrogen determination in atmospheric wet and dry deposition samples. Various operating parameters such as oxidation reagent concentrations, microwave power, and extraction time were optimized to maximize the conversion of total nitrogen to nitrate for subsequent chemical analysis. Under optimized conditions, 0.012 M K₂S₂O₈ and 0.024 M NaOH were found to be necessary for complete digestion of wet and dry deposition samples at 400 W for 7 min using microwave. The optimized extraction method was then validated by testing different forms of organic nitrogen loaded to pre-baked filter substrates and NIST SRM 1648 (urban particulate matter), and satisfactory results were obtained. In the case of wet deposition samples, standard addition experiments were performed. The suitability of the method for real-world application was assessed by analyzing a number of wet and dry deposition samples collected in Singapore during the period of March-April 2007. The organic nitrogen content was 15% (wet) and 30% (dry) of the total nitrogen. During the study period, the estimated wet fluxes for nitrate (NO₃ ^-), ammonium (NH₄ ⁺), organic nitrogen (ON), and total nitrogen (TN) were 16.1 ± 6.5 kg ha^-1 year^-1, 11.5 ± 5.7 kg ha^-1 year^-1, 3.8 ± 1.5 kg ha^-1 year^-1and 31.5 ± 13.2 kg ha^-1 year^-1, respectively, while the dry fluxes were 2.5 ± 0.8 kg ha^-1 year^-1, 1.4 ± 0.9 kg ha^-1 year^-1, 2.3 ± 1.4 kg ha^-1 year^-1 and 7.5 ± 2.6 kg ha^-1 year^-1, respectively.