VITAMIN A AND LIPOIC ACID STABILITY 411 It can be deduced from the results that the absence of antioxidants favors the degradation of vitamin A. The presence of ascorbyl palmitate, magnesium ascorbyl phosphate, and vitamin C seem to protect vitamin A as well as BHT at pH 5 and 7. Ascorbyl palmitate (system D) seems less to protect vitamin A palmitate and more to protect lipoic acid in these conditions. Although we have determined an interaction between lipoic acid and BHT (15), the addition of vitamin A in the proportion indicated improves the stability of these preparations at pH 5 and 7. Content was observed in these formulations: the higher the pH, the greater the stability. Based on these results, we propose systems C and D at pH 7 as the most stable for formulations containing vitamin A and lipoic acid. Table II pH Variations in Systems A–D Time (days) 0 43 83 111 195 321 438 System A (pH 3) 3.00 2.98 2.83 2.79 2.79 2.84 2.67 System A (pH 5) 5.00 5.04 4.57 3.84 3.76 3.54 3.34 System A (pH 7) 7.00 7.01 6.83 6.76 6.74 6.63 6.58 System B (pH 3) 3.00 3.08 3.00 2.81 2.89 2.94 2.77 System B (pH 5) 5.00 5.27 5.35 5.30 5.21 5.02 4.91 System B (pH 7) 7.00 6.96 6.89 6.88 6.75 6.60 6.70 System C (pH 3) 3.00 2.96 3.01 2.94 2.99 2.97 2.80 System C (pH 5) 5.00 4.97 5.32 5.25 5.30 5.14 5.18 System C (pH 7) 7.00 6.98 7.02 7.15 7.15 7.08 6.87 System D (pH 3) 3.00 2.98 3.04 2.96 3.03 3.04 2.90 System D (pH 5) 5.00 5.01 5.05 4.92 4.92 4.86 4.76 System D (pH 7) 7.00 6.93 6.84 6.84 6.85 6.83 6.70 Figure 4. Percentage of vitamin A palmitate vs time in system A.

JOURNAL OF COSMETIC SCIENCE 412 CONCLUSIONS The photoirradiation of vitamin A palmitate with UVA light results in the formation of photodecomposition products and reactive oxygen species through three distinct mecha- nisms: a UVA-initiated free-radical mechanism, an ionic photodissociation mechanism, and vitamin A palmitate photosensitization. The photoirradiation of vitamin A palmitate generates singlet oxygen. The presence of antioxidants, such as BHT, ascorbyl palmitate, magnesium ascorbyl phosphate, and vitamin C increases the photostability and stability Figure 5. Percentage of vitamin A palmitate vs time in system B. Figure 6. Percentage of vitamin A palmitate vs time in system C.