6. Specificity
In order to assess the separation of ascorbic, nicotinic and oxalic acids and possible interferents from the excipients of the pharmaceutical/cosmetic formulations containing glutathione and sodium metabisulfite, as antioxidants, specificity was considered in two levels:
(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 10.0_gmL−1.