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Nitrogenous species, such as ammonia, nitrite, nitrate, hydroxylamine (NH2OH) and nitrous oxide (N2O), are recycling through microbial processes in the hydrosphere.1-3 NH2OH has been identified as an intermediate in the nitrogen cycles, such as the oxidation of ammonia to nitrite and the reduction of nitrite to ammonia by several microorganisms in aquatic and sedimentary systems. 4,5 Furthermore, NH2OH is often described as an intermediate for producing N2O. N2O is one of the greenhouse gases, and is also associated with the reduction of ozone in the stratosphere. Hence, NH2OH is an important nitrogen species not only for understanding the nitrogen cycle, but also for clarifying the mechanism of N2O production.It has been reported that millimolar solutions of NH2OH are stable for several hours at pH 4.0, but only for 60 min at pH 7.8 in the presence of air. 6,7 Thus, the direct determination of NH2OH is difficult in environmental water because of its low concentration and instability. Several methods, such as spectrophotometric methods 6,9-11 and a titrimetric method 12 for the determination of NH2OH, have been reported. The former method is based on the combination of the oxidation of NH2OH to nitrite with iodine and the usual Griess-Romijin method. This method, however, is subject to interference from nitrite. 13The latter method is based on the oxidation of NH2OH using ferric ammonium sulfate with a copper sulfate catalyst. In this method, Fe(III) was reduced by NH2OH and the resultant Fe(II) was titrated with potassium dichromate. This method is applicable in the millimolar range. In general, these methods were employed only in media containing a relatively high concentration of NH2OH. On the other hand, Marta et al. developed a gas chromatographic method for the determination of nanomolar concentrations of NH2OH by its oxidation to N2O using Fe(III) ions.14 However, the yield for the conversion of NH2OH to N2O by this method was 50%.In a previous paper, 13 we reported the spectrophotometric determination for NH2OH in environmental water samples. The interference from nitrite in the water sample described above was solved by removing the resultant azo dye from nitrite using a Sep-Pak C18 cartridge prior to the analysis of NH2OH. Although this method was available for environmental water samples from fresh-water to seawater, the procedure was very tedious. In this paper, we propose a new and simple method for the determination of nanomolar NH2OH in fresh-water samples by its oxidation to nitrous oxide using hypochlorite as an oxidizing agent. The N2O produced in this manner was subsequently measured by using a gas chromatograph with an electron-capture detector (ECD). Experimental Reagent and apparatusReagent solutions were prepared with high-purity water from a Millipore Milli-Q purification system. A standard NH2OH solution (500 mgN L -1 ) was prepared by dissolving 0.2481 g of hydroxylammonium chloride in 100 mL of water deoxidated with nitrogen gas. It was fleshly prepared for each analysis. A hypochlorit...

Lake Puma Yumco is a typical alpine lake (altitude 5030 m) located in the pre-Himalayas of Tibet, China, and this study was the first limnological investigation ever conducted on it. Lake Puma Yumco (28°34ЈN, 90°24ЈE) has the following morphometric properties: maximum length 31 km, maximum width 14 km, mean width 9 km, shoreline 90 km, surface area 280 km 2 , and shoreline development 1.5. Transparency was approximately 10 m, even in the thawing season. The extinction coefficient of the lake water was calculated as 0.15 m Ϫ1 . Annual maximum transparency was estimated from the depth of the Chara zone to be 30 m. Dissolved oxygen was 7 mg O 2 l Ϫ1 and showed saturated values, and salinity was 360 mg l Ϫ1 . The chemical type of the lake water was Mg-Ca-HCO 3 -SO 4 , and it was slightly alkaline in character. Total nitrogenous nutrients (sum of ammonia, nitrite, nitrate, and urea nitrogen), phosphate, and silicate were extremely low at 1, 0.02, and 9 µM, respectively. Dissolved organic carbon, nitrogen, and phosphorus concentrations were 160, 11, and 0.08 µM and the molar ratio was calculated as 2100 : 140 : 1. Chlorophyll a concentration was 0.2 mg m Ϫ3 . Phytoplankton and zooplankton were dominated by Aphanocapsa sp. and Diaptomidae. Both nitrogen and phosphorus appear to be the limiting parameters for phytoplankton growth. Organic carbon and nitrogen contents in lake sediments were low and the sediments contained a large amount of CaCO 3 . The grain size of sediment was that of silt-sand in most cases. The present results indicate that the pre-Himalayan alpine freshwater Lake Puma Yumco is an ultraoligotrophic lake.