(a) topical formulations analyte-free without glutathione and
sodium metabisulfite:
Approximately 250 mg of topical formulations (o/w
emulsion and aqueous gel) 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. Aliquots of samples were diluted to final
concentrations of 10.0 gmL−1 of o/w emulsion and aqueous
gel on the mobile phase.
(b) individual analysis of ascorbic and nicotinic acids, antioxidants
(glutathione and sodium metabisulfite) and oxalic acid
(degradation product of Vitamin C):
Approximately 25 mg of standard ascorbic acid, nicotinic
acid, glutathione, sodium metabisulfite and oxalic acid
were weighed precisely and transferred singly to volumetric
flasks and dissolved in 50mL of the mobile phase. Aliquots
of each sample were diluted to required concentration of
10.0gmL−1.