茶游离氨基酸总量测定方法的改进

对国家标准GB/T8314—2002中茶游离氨基酸总量测定方法存在的问题进行探讨,并针对存在问题从方法的原理、标准工作液配制、反应体系缓冲液的选择、加样量、样品分析中参比的选择等方面进行研究并提出改进措施。改进的游离氨基酸总量测定程序为:取茶汤或标准工作液1.0mL于25mL容量瓶中,分别加入pH6.81的1/15mol/L磷酸氢二钠-磷酸二氢钾缓冲液0.5mL和20g/L茚三酮溶液0.5mL,在沸水浴中加热15min,冷却后加水定容至25mL,放置10min后,在570nm处测定吸光度。标准曲线测定以水代替标准工作液按同样操作作为参比,样品分析以水代替茚三酮溶液按同样操作作为参比。改进的方法在谷氨酸浓度为0.1～0.3mg/mL范围内呈现良好线性,精密度高、重复性好。

Improvement on determination of free amino acids content in tea

The internal defects of the method of Chinese National Standard GB/T8314—2002 for the determination of free amino acids content in tea were investigated,and the improving measure to these defects were also put forward from the theory of method,standard working solution preparation,buffer selection,sampling volume and blank test selection.The procedure for the free amino acids determination of the improved method was as follow:1.0 mL of the tea soup or standard working solution was mixed with 0.5 mL of disodium hydrogen phosphate-potassium dihydrogen phosphate buffer(pH 6.81,1/15 mol/L) and 0.5 mL of ninhydrin solution(20 g/L) in 25 mL volumetlric flask,then the mixture was heated by boiling water bath for 15 min.The reaction solution was diluted with water to 25 mL after cooled in air,and kept at room temperature for 10 min.The optical density of the diluted solution was measured by the absorbance at 570 nm.The blank test for the calibration curve determination was prepared according to the same procedure as mentioned above after substituted water for standard working solution.However,the appropriate blank test for the sample determination of tea soup was prepared according to the same procedure as mentioned above after substituted water for ninhydrin solution.Calibration curve of the improved method showed the linearity was in the range of 0.1 mg/mL to 0.3 mg/mL with L-glutamic acid.The improved method was precise,repeatable.