1：Experimental Design and Method Selection:

The study employed electrochemical impedance spectroscopy (EIS) and differential pulse voltammetry (DPV) to evaluate and select electrodes for detecting Quaternium-

2：The rationale was to find a sensitive and cost-effective alternative to HPLC methods. Sample Selection and Data Sources:

Quaternium-73 was purchased from Hayashibara Company. Cosmetic samples (anti-acne cleansing gel and spray) were commercially obtained and prepared by dilution and centrifugation.

3：List of Experimental Equipment and Materials:

Equipment included a potentiostat/galvanostat (SP-150; Bio-Logic SAS), HPLC system (L-7100 pump, L-7455 detector; Hitachi), and various electrodes (carbon fiber, carbon paste, metal oxide-modified carbon paste). Materials included metal oxide nanoparticles (tin, cerium, cobalt, iron, nickel oxide; Sigma-Aldrich), graphite powder, paraffin oil, and buffers.

4：Experimental Procedures and Operational Workflow:

Electrodes were prepared by mixing graphite powder, paraffin oil, and metal oxides. EIS and DPV measurements were conducted in buffer solutions at specific pH and potentials. Cosmetic samples were extracted, filtered, and analyzed.

5：Data Analysis Methods:

Data were analyzed using EC-Lab software for EIS and DPV, with impedance spectra interpreted for electron transfer resistance. HPLC data were integrated for comparison.