This article determines the content of total nitrogen in water by TOC/TN instrument method ,and analyzes the sources which affect the uncertainty in the process of determination :repeatability ,standard solu-tion ,regressive curve ,injection volume ,etc .Based on this analysis ,it evaluates the uncertainty of measure-ment and calculates the synthetic uncertainty and expanded uncertainty .Finally ,it puts forward the solving methods to improve the accuracy of TN analysis .% 采用T OC/T N分析仪测定了污水处理厂出水中的总氮含量，分析了测量过程中不确定度的来源：重复性、标准溶液配置、标准曲线拟合、进样体积等，在此基础上对各不确定度分量进行了评定，并计算得到合成不确定度和扩展不确定度，提出了在测定过程中减小不确定度的有效途径。
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