Fig. 3

The zero-order UV absorption spectrogram in methanol shows (A) nicotinamide (30 µg/mL), and its 262 nm peak is overlapped by (B1–B10) tretinoin standard series (B1 = 0.5 µg/mL, B2 = 1.0 µg/mL, B3 = 1.5 µg/mL. B10 = 5.0 µg/mL) with 348 nm peaks