Direct Determination of Readily Accessible Quercetin-Reactive Aluminum and Ascorbate-Reducible, Ferrozine-Reactive Iron in Cosmetic Emulsions

Aluminum in antiperspirants is relevant to formulation control and compliance, whereas reactive iron can promote oxidative instability in emulsions. We developed smartphone-based colorimetric assays relying on metal–reagent complexation directly in oil-in-water (O/W) emulsions, without digestion or extraction. The aluminum assay measures quercetin-reactive aluminum from aluminum chlorohydrate (ACH). Calibration data were fitted by linear regression (R2 ≥ 0.971). Acceptable accuracy (relative bias within ± 8.5%) and intermediate precision (CV ≤ 9.4%) were obtained upon quality control (QC) analysis (above the LLOQ). Relative bias in commercial antiperspirants and corresponding in-house preparations was within ± 15.2%. The iron assay measures ascorbate-reducible, ferrozine-reactive iron. Its calibration data were also fitted by linear regression (R2 ≥ 0.981), while QC analysis showed acceptable accuracy (relative bias within ± 13.7%) and intermediate precision (CV ≤ 12.6%). Comparison with portable energy-dispersive X-ray fluorescence (EDXRF) yielded a regression slope of 1.14 and an intercept of 0.40 μg/g, suggesting a proportional difference and a possible constant component. Common chelators and certain metals interfered with both assays. The effect of interferents in commercially relevant concentration was, however, minimal (relative bias within ± 15.2% for Al and at 4.5% for a Co/Cu/Fe-fortified model emulsion) after matrix matching or dilution. Although mostly inferior in terms of technical characteristics compared to established instrumental methods, this methodology is sufficiently accurate, precise and selective under the defined conditions.