## 3 Research Progress in Analytical Techniques of Nitrogen and Oxygen Isotopes  
It has been proven that nitrogen and oxygen isotopes have been used as effective approaches for identifying the source and fate of nitrate pollution in surface water. Thus, it is crucial to ensure that the testing technique of isotopic compositions is accurate, rapid, and inexpensive. Ion-exchange method, denitfier method, and Cd-azide reduction method are the most common approaches currently used for analytical techniques for \(\delta^{15}\)N and \(\delta^{18}\)O determination. Ion exchange method was developed by [PERSON] et al. [35]. This method has three steps, i.e., transformation, purify, and mass spectroscopy analysis. Briefly, nitrate is purified and concentrated by passing samples through ion exchange resin columns, and then can be simultaneously analyzed \(\delta^{15}\)N and \(\delta^{18}\)O values via isotope ratio mass spectrometer (IRMS). The adsorption rate and elution rate during the transformation are two key indicators for experimental operation. Incorrect operation will cause nitrogen isotopic fractionation, which results in smaller measured values. [PERSON] [36] found that the absorption rate of anion exchange resin (Rowex 1-X8) was more than 97%, and the recovery of saturated K\({}_{2}\)SO\({}_{4}\) solution was close to 100%. No obvious isotopic fractionation occurred during the adsorption and desorption of zwitterion, and the deviation was below 0.3%. In traditional mass spectroscopy, prepared AgNO\({}_{3}\) samples are put into sealed tube for high temperature combustion so that the AgNO\({}_{3}\) are converted into N\({}_{2}\) and CO\({}_{2}\) and then values of \(\delta^{15}\)N and \(\delta^{18}\)O can be measured through IRMS. Mass spectroscopy is not widely used because it is cumbersome in preprocessing steps and needs lots of AgNO\({}_{3}\) samples. Recently, a new testing technique of higher accuracy has gotten extensive applications. In this method, prepared AgNO\({}_{3}\) samples are placed in aluminum or silver tube at 1400 \({}^{\circ}\)C in an element analyzer, samples can be conducted by conversion to N\({}_{2}\) and CO. Subsequently, the isotope values are measured through IRMS. This method has the following demerits: (1) the amount of require water sample is huge, and a relatively large amount of N in samples (>1 umol N) is essential to avoid contamination from atmospheric N\({}_{2}\)[37]; (2) chemical reagents (e.g., silver oxide) used for sample preparation are very expensive; (3) influence of other ions (e.g., SO\({}_{4}\)\({}^{2-}\) and Cl\({}^{-}\)) on the quantification of nitrate concentration [38]. The merits of this method can be summarized as follows: (1) it achieves a high level of sensitivity and is easy to operate [12]; (2) no harmful preservatives are added into sample water and thus cross contamination can be effectively avoided; (3) the preservation time is long (two years when refrigerated in humid environment); (4) isotopic fractionation only occurs to a small extent. This method is a well-developed approach for the measurement of \(\delta^{15}\)N and \(\delta^{18}\)O, and has been widely used in field experiments. Denitrifier method was put forward by [PERSON] et al. [39] and [PERSON] et al. [40]. In this method, the simultaneous determination of \(\delta^{15}\)N and \(\delta^{18}\)O of N\({}_{2}\)O produced from the conversion of nitrate by denitifying bacteria which naturally lack N\({}_{2}\)O-reductase activity. This method has four experimental steps, namely, bacterial selection, bacterial cultivation, conversion, and extraction of N\({}_{2}\)O. This method has many advantages. First of all, it only requires a small amount of sample water and is able to be used for the analysis of low-concentration samples. Apart from that, it only converts nitrate and is completely uninfluenced by organic nitrogen. Besides, it shortens the analyzing time because the preparation procedure is simple.