利用紫外光谱法分析水体COD时, 当样品达到一定浓度之后(仍处于朗伯-比尔定律通常被适用的浓度范围内), 其溶液紫外吸光度与溶液COD数值会明显偏离线性关系, 此现象在许多学者发表的紫外光谱论文中都曾有所提及。 对此, 选用海洋光学公司的S2000微型光纤光谱仪作为光谱测量仪器, 选用PX-2脉冲氙灯作为激发光源, 在温度为20 ℃(±0.5 ℃)及湿度为35%(±5%)的暗室中进行光谱实验, 测量了COD值为40~680 mg·L-1的34组邻苯二甲酸氢钾溶液样品的紫外吸收光谱, 并基于样品紫外吸收特性进行了分析建模。 采用相关系数法选取优势波段, 通过对比样品紫外吸收光谱的第二特征峰和水质COD分析中常用波长处的COD-吸光度关系曲线动态特性, 选定优势波长为275 nm。 采用逐点延伸的方式, 在较低浓度段利用鲁棒线性回归、 较高浓度段利用非线性最小二乘回归, 反复拟合线性或指数方程, 滑动预测下一个数据点, 根据均方根误差和相对误差判断预设低浓度临界点和较高浓度临界点, 确定了低浓度段和较高浓度段COD-吸光度关系模型的分段点分别为300和560 mg·L-1, 得到低浓度段模型和较高浓度段模型。 通过在优势波长处进行低浓度段、 较高浓度段和全浓度范围的鲁棒线性回归和非线性最小二乘法回归等不同模型的拟合精度比较, 表明40~300和300~560 mg·L-1范围内COD-吸光度关系的线性-指数分段数学模型, 不仅拟合精度最高, 而且预测效果好, 低浓度段的预测均方根误差为4.944 9, 较高浓度段的预测均方根误差为6.768 9, 整体预测均方根误差为5.664 7。 研究结果对紫外光谱应用于较高COD的水质测量和分析具有一定的参考价值。

The relationship between the UV absorbance and the COD significantly deviate from the linear relationship when the samples reach a certain concentration(still within the concentration range where Lambert-Beer’s law is often applied)when using UV spectroscopy to analyze COD in water, this phenomenon has been mentioned in many scholars’ literature on UV spectroscopy. In thisregard, the S2000 micro-fiber spectrometer and the PX-2 pulsed xenon lamp of Ocean Optics were selected for experiments in a dark room with a temperature of 20 ℃(±0.5 ℃) and humidity of 35%(±5%),andUV absorption spectra of 34 groups of potassium hydrogen phthalate solutions with COD values of 40～680 mg·L-1 were measured for analytical modeling. The dominant wavelength band was selected by correlation coefficient method combine withthe UV absorption characteristics of the sample, and the dominant wavelength was determined to be 275 nm after compared the dynamic characteristics of the COD-absorbance curves at the second characteristic peak of the ultraviolet absorption spectrum of the sample with the wavelength commonly used in water quality COD analysis. A point-by-point extension method is used, which select robust linear regression and the unary nonlinear least squares regression in low concentration segment and higher concentration segment, respectively, repeated fitting of linear or exponential equations and sliding prediction of the next data point,and the segmentation points of the COD-absorbance relationship model of the low concentration segment and the higher concentration segment are determined to be 300 and 560 mg·L-1, respectively, according to the root mean square error and the relative error,and a low concentration segment model and a higher concentration segment model were obtained. The comparison of the fitting accuracy of the linear or nonlinear model in a low concentration range, higher concentration range and the full concentration range at the dominant wavelength indicates that the linear-exponential segmentation model for the relationship between absorbance and COD of water quality in the range of 40 to 300 mg·L-1 and 300 to 560 mg·L-1 has the highest precision, and the prediction effect on the prediction set samples is better: the prediction root mean square error of the model at low concentration is 4.944 9, 6.768 9 at higher concentration and 5.664 7 overall, the prediction effect is ideal for the prediction set. The research result provides a reference value for the measurement and analysis of water with higher COD by UV spectroscopy.