二阶导数光谱法快速测定硝酸盐氮和亚硝酸盐氮

紫外吸收方法中,硝酸盐氮(NO-3-N)的紫外吸收峰在202.0nm左右,而亚硝酸盐氮(NO-2-N)的紫外吸收峰在210.0nm左右,两者吸收峰位置距离很近,因此,在分析过程中两者的紫外吸收曲线严重重叠,相互之间严重干扰,不经过分离很难用单波长对二者的含量进行测定而常用的国标方法过程又过于繁琐,耗时较长。为了准确、快速、环保的实现环境水体和饮用水中的硝酸盐氮和亚硝酸盐氮快速监测,避免国标方法中对二者测定的诸多不足,结合紫外吸收和二阶导数光谱法,在不经过任何预先分离处理的情况下,建立了水体中这两种物质的快速分析方法,实现水样中二者的快速准确测定。研究采用优级纯试剂配制硝酸盐氮和亚硝酸盐氮系列标准溶液。以去离子水做参比,采用紫外-可见光分光光度计扫描其在195～250nm范围内的紫外吸收光谱,之后采用Origin软件对所获得的光谱图做二阶导数处理,并采用Origin软件中的Savitzky-Golay方法对处理后的二阶导数光谱进行平滑处理以去除其他无关的干扰和噪声。通过观察上述所得两组二阶导数光谱图,得出以下结论,不同浓度的亚硝酸盐氮样品在223.5nm处吸光度的二阶导数均为0,不同浓度的硝酸盐氮样品在216.5nm处的吸光度的二阶导数也均为0。通过实验可见硝酸盐氮和亚硝酸盐氮混合样品的紫外吸收光谱的二阶导数在这两个特定波长处符合朗伯比尔定律。实验通过配制硝酸盐氮和亚硝酸盐氮混合样品,并扫描混合样品的紫外吸收光谱,采用上述方法对所得光谱做二阶导数及平滑去噪处理。研究混合样品二阶导数光谱图可以看出在硝酸盐氮浓度相同而亚硝酸盐氮浓度不同时,亚硝酸盐氮的浓度变化会对硝酸盐氮的吸光度的二阶导数有影响,但是各种混合样品的二阶导数光谱在223.5nm处几乎交叉于一点,说明此处亚硝酸盐氮的浓度不同不会对硝酸盐氮的二阶导数吸光度有影响。且在223.5nm处硝酸盐氮二阶导数吸光度随浓度增加而线性增加。因此,223.5nm可作为混合组分中硝酸盐氮的测定波长。参照以上方法,可得亚硝酸盐氮的测定波长为216.5nm。在223.5nm处对单组分的硝酸盐氮的浓度值及其相应的吸光度的二阶导数进行线性回归,其线性关系良好,得到标准曲线的回归方程为C=438.69A+0.015,R2=0.995 9。同理,得到亚硝酸盐氮在216.5nm处回归方程为C=-657.29A+0.068 8,R2=0.998。为了验证这种方法在实际水样测量中能否成立,取秦皇岛市新河、汤河以及戴河三种河水水样进行实验验证,结果表明,回收率在96.7%～103.0%之间,相对标准偏差在1.46～3.68之间。该方法结果较准确,且操作更加简便,成本较低,可同时实现硝酸盐氮和亚硝酸盐氮快速在线监测。

Rapid Determination of Nitrate Nitrogen and Nitrite Nitrogen by Second Derivative Spectrophotometry

In the ultraviolet spectrophotometry, we know that the absorption peak of nitrate nitrogen is around 202.0 nm, and the absorption peak of nitrite nitrogen is around 210.0 nm. It can be seen that the absorption peaks of the two are very close, and the absorption curves overlap seriously and interfere with each other badly. It is difficult to use single wavelength to determine the content of the two without the separation, and the national standard method is too complicated and time-consuming. In order to monitor the nitrate nitrogen and nitrite nitrogen in environmental water and drinking water more accurately, rapidly and eco-friendly, and to avoid many shortcomings in the national standard method, in this paper we studied the rapid analysis method of the two in water. This method combines with ultraviolet spectrophotometry and the second derivative spectrometry which are more rapid and accurate. And it does not need any pre-separation treatment. Nitrate nitrogen and nitrite nitrogen series solution were prepared by guarantee reagent. Using deionized water as a reference, UV-Vis spectrophotometer was used to scan the UV absorption spectrum in the range of 195～250 nm. After that, we used the Origin software to do the second derivative spectra of the obtained spectrogram, and used the Savitzky-Golay method in the Origin software to smoothen the second derivative spectra. By observing the above two groups of second derivative spectrogram, we found that the second derivative absorbance at different concentrations of nitrite nitrogen samples at 223.5 nm was 0 and nitrate nitrogen samples at 216.5 nm was also 0. The second derivative absorbance of the UV spectra of the mixed samples were observed by experiments. We found that they conformed to Lambert-Beer law. Then the nitrate nitrogen and nitrite nitrogen mixed samples were prepared. The UV absorption spectra of the mixed samples were scanned. We did the second derivative spectra of the obtained spectrogram and smoothened them. After that, we observed the second derivative spectra of mixed samples. It could be seen that when the concentration of nitrate nitrogen is the same and nitrite nitrogen concentration is different, the concentration of nitrite nitrogen will affect the second derivative absorbance of nitrate nitrogen. However, the second derivative spectra of various samples at 223.5 nm are almost overlapping, indicating that the concentration of nitrite nitrogen at this wavelength will not have any effect on the second derivative absorbance of nitrate nitrogen. At this wavelength, the value of the second derivative absorbance of the mixed samples increase linearly with the increase of nitrate nitrogen concentration. Therefore, 223.5 nm was chosen as the determination wavelength of nitrate nitrogen in the mixed samples. In the same way, the determination wavelength of nitrite nitrogen is 216.5 nm. Linear regression analysis of the nitrate nitrogen single component samples was performed between the second derivative absorbance and the concentration at 223.5 nm. The linear relationship was good, and the regression equation of the obtained standard curve was C=438.69A+0.015, R2=0.995 9. In the same way, we obtained that the regression equation of the nitrite nitrogen at 216.5 nm was C=-657.29A+0.068 8, R2=0.998. In order to test the application of this method in actual water sample measurement, we took three kinds of water samples from New River, Tang River and Dai River in Qinhuangdao to carry out experiments. The results showed that the recovery rate was between 96.7% and 103%, the relative standard deviation was 1.46~3.68. The method is relatively accurate and easy to operate and costs less. The rapid on-line monitoring of nitrate nitrogen and nitrite nitrogen can be realized at the same time.