In Vitro Dermal Absorption of m-Phenylenediamine Sulfate in Mini-Pig Skin Using LC–MS/MS Quantification

m-phenylenediamine sulfate (mPDS) is used as an oxidative hair dye intermediate and is permitted in cosmetic products in Korea at concentrations up to 3%. However, quantitative data on its dermal absorption remain limited. This study evaluated the in vitro dermal absorption of mPDS using a Franz diffusion cell system. An LC–MS/MS method was developed and validated for quantifying mPDS in receptor fluid, skin, wash solution, and tape-stripped stratum corneum samples. The method showed excellent linearity (r2 ≥ 0.9951), with overall accuracy of 85.9–110.9% and precision of 0.7–10.9%, meeting regulatory validation criteria. Dermal absorption studies were performed using mini-pig dorsal skin that satisfied integrity requirements. Oxidative hair dye formulations containing mPDS at 0.7% or 2.1% were applied at 10 mg/cm2, and receptor fluid samples were collected for 24 h. Most of the applied dose was recovered in the wash fraction after removal at 30 min. Dermal absorption was 2.90 ± 1.27 and 4.75 ± 2.06 µg/cm2 for 0.7% and 2.1% mPDS, respectively. These findings provide quantitative dermal absorption data to support regulatory exposure characterization of mPDS.