Approximately 50 mg of nicotinic acid; 50, 150 and 250 mg
of topical formulations analyte-free; and 5, 15 and 25 mg of
standard ascorbic acid were weighed precisely and dissolved,
separately, in 50mL of the mobile phase, with subsequently
mixing. Samples were centrifuged at 3000 rpm for 5 min at room
temperature (24.0±2.0 ◦C). Supernatants were discarded.