以碳──環氧組成電極伏特安培法同時測定化妝品產品中之維生素C及其衍生物   全文下載

Simultaneous Voltammetric Determination of Ascorbic Acid and Its Derivatives in Cosmetics Using Epoxy-carbon Composite Electrodes

維生素C（ascorbic acid，簡稱AA），維生素C磷酸鎂鹽（magnesium ascorbyl phosphate，簡稱MAP）及維生素C十六酯（ascorbyl palimitate，簡稱AP），這三種成分在乳化製品中對皮膚的抗老化有顯著的有效性，我們以伏特安培法進行定量。所使用的是自製的環氧樹脂與碳粉的組成電極，並改變不同的電解液及酸鹼值，探討其對定量測定的影響。從伏特安培法的分析圖譜發現，AA、MAP與AP成分的陰極電位隨著條件的差異分別位在0.35±0.07、0.81±0.11及0.23±0.03 V，每一分析物在10-300 μg/mL的濃度範圍都有好的線性關係（相關係數r2 > 0.997），且他們的偵測極限分別少於0.17、0.46及0.09 μg/mL。在化妝品產品中，這三種成份的相對標準偏差在6.3%以下，回收率範圍則從92.2至104.5%。在產品中共存的有機干擾物對定量的影響也被探討。而容易發生氧化的維生素C，其不穩定性是分析過程中重要的誤差來源。由實驗結果發現，通氮去氧是穩定維生素C最有效且方便的方法。

We used a voltammetric method to determine the levels of ascorbic acid (AA) and two AA derivatives (magnesium ascorbyl phosphate (MAP) and ascorbyl palimitate (AP)) in cosmetic products. These compounds are believed to affect the process of skin aging and are widely used in skin care products. An electrode composed of an epoxy resin and graphite powder can be easily fabricated. We studied the effects of solutions’ pH and different supporting electrolytes on the performance of a carbon-epoxy electrode which we used as a working electrode to take voltammetric measurements that would quantify the levels of AA, MAP and AP in various cosmetic products effectively. In the voltammogram, anodic peak potentials of AA, MAP and AP were obtained at about 0.35 ± 0.07, 0.81 ± 0.11, and 0.23 ± 0.03 volts in various pH and different supporting electrolytes respectively. Linearity of peaks from plots of current vs. concentration (correlation coefficient r2 > 0.997) was over the range of 10-300 μg/mL for all analytes. The limits of detection (LOD) for AA, AMP and AP were lower than 0.17, 0.46 and 0.09 μg/mL respectively. For the three analytes in cosmetic formulations, the relative standard deviations were below 6.3%, and the range of recovery ratio ranged from 92.2 to 104.5%. Various compounds coexisting in the formulations that were potential sources of organic interferences in voltammetric analysis were explored. Ascorbic acid oxidizes easily to dehydroascorbic acid, but its derivatives are relatively stable. Material instability could result in a significant error in the analytical method. Different approaches were used to inhibit ascorbic acid degradation, and purge of oxygen from the analyzed solution with nitrogen was found to be the most effective method.