Simultaneous Voltammetric Determination of Ascorbic Acid and Its Derivatives in Cosmetics Using Epoxy-carbon Composite Electrodes
MIAW-LIN CHANG1 AND CHUR-MIN CHANG2*
1. Department of Cosmetic Science, Chia-Nan University of Pharmacy and Science, Tainan County, Taiwan, R.O.C.
2. Department of Applied Chemistry, Chia-Nan University of Pharmacy and Science, 60, Sec. 1, Erh-Jen Rd., Jen-Te Township, Tainan County 717, Taiwan, R.O.C.
(Received: March 7, 2005; Accepted: May 9, 2005)
ABSTRACT
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.
Key words: ascorbic acid (AA), magnesium ascorbyl phosphate (MAP), ascorbyl palimitate (AP), cyclic voltammetry (CV), differential pulse voltammetry (DPV), carbon-epoxy electrode, composite electrode, antioxidant, whitening agents