Discrimination and Simultaneous Determination of Analogs by Derivative Spectrum Method

由于对羟基苯甲酸甲酯、对羟基苯甲酸乙酯、对羟基苯甲酸丙酯和对羟基苯甲酸丁酯等防腐剂的吸收光谱相似，在253-254 nm处有最大吸收峰，仅凭吸收光谱不能区分它们。在其水溶液中加入表面活性剂后，其最大吸收波长发生移动。这种增溶现象使测定防腐剂在水相和胶束相之间的分配参数成为可能。在所有实验温度下，烷基对羟基苯甲酸酯在十二烷基硫酸钠溶液中的参数值顺序为：丁基-GT；丙基-GT；乙基-GT；甲基对羟基苯甲酸酯。这意味着烷基对羟基苯甲酸酯的酰基链越长，越能溶解到胶束相中。这最终导致更长的移位长度和更大的分区参数值。同样的结果不仅在十二烷基硫酸钠溶液中观察到，而且在十六烷基三甲基溴化铵溶液、吐温20溶液和八乙二醇正十二烷基醚溶液中也能观察到。通过对配分参数的测定，实现了对羟基苯甲酸酯的鉴别，证实了混合对羟基苯甲酸酯的位移长度与其组成的摩尔比有关。这使得只知道给定(最大)波长下的位移长度和总吸光度的大小就可以确定对羟基苯甲酸酯的浓度。只要表面活性剂能引起吸收位移，这种方法就可以应用于任何发生吸收位移的体系。这是一个很大的优点，一旦设定了校准曲线，我们只需要移动长度和吸光度就可以计算出各组分的浓度。在产品分析等需要在较短时间内处理大量样品的情况下，这种方法可能更有用。

As the preservatives, such as methylparaben, ethylparaben, propylparaben, and butylparaben, have similar absorption spectrum which show maximum at 253-254 nm, it is not possible to discriminate them by absorption spectrum only. The addition of surfactant into their aqueous solution caused the shift of the wavelength of maximum absortion. This solubilization phenomena made it possible to measure the partition parameter of the preservative between aqueous and micellar phase. The parameter values of alkylparabens in sodium lauryl sulfate solution were in the order of butyl->propyl->ethyl->methylparaben at all temperatures where experiment were done. This means that the longer the acylchain of alkylparaben, the more can be solubilized into micellar phase. This ended up greater shift length and bigger partition parameter values. The same results were observed not in sodium lauryl sulfate solution, but also in cetyltrimethylammonium bromide solution, Tween 20 solution, and octaethyleneglycol-n-dodecylether solution. Thus the discrimination of alkylparabens were attained through the measurement of partition parameter.It was confirmed that the shift length of the mixed parabens depended on the molar ratio of its constituents. This made it possible to determine the concentration of parabens only knowing the magnitude of shift length and total absorptivity at given (maximum) wavelength. This method can be applied to any systems where absorption shift occurs as long as surfactant can cause the shift. It is a great advantage, once the calibration curve are set, then we need only the shift length and absorptivity to calculate the concentration of each components. This method can be more useful where so many samples should be handled in shorter time such as products analysis.