Codeine

Carbinoxamine, chlorpheniramine, and chlorpheniramine-d₆ were purchased from TRC (North York, Canada). Individual solutions of 6-AM, 7-aminonitrazepam, 7-aminonimetazepam, amphetamine, benzoylecgonine, C6G, codeine, codeine-d₆, ketamine, methamphetamine, mephedrone, 4-methylephedrine, 3,4-methylenedioxymethamphetamine hydrochloride, M3G, M6G, morphine, morphine-d₆, and norketamine (each with a concentration of 1 mg mL⁻¹) were purchased from Cerilliant (Round Rock, TX, USA). Three brands of cold syrups (Consrine, ssBuron, and Pabron) manufactured in Taiwan were purchased from local pharmacies. Consrine contains codeine phosphate (0.48 mg mL⁻¹), chlorpheniramine (0.25 mg mL⁻¹), dl-methyl ephedrine HCl (1.6 mg mL⁻¹), and glycyrrhizic acid (0.25 mg mL⁻¹), ssBuron contains codeine phosphate (0.5 mg mL⁻¹), guaifenesin (3.33 mg mL⁻¹), chlorpheniramine maleate (0.14 mg mL⁻¹), and caffeine (1.33 mg mL⁻¹), Pabron contains codeine phosphate (0.48 mg mL⁻¹), dl-methyl ephedrine HCl (2.5 mg mL⁻¹), and carbinoxamine maleate (0.4 mg mL⁻¹). Formic acid and methanol (high-performance LC grade) (≥ 98%) were purchased from J.T. Baker (Pittsburgh, PA, USA). Ultrapure water (18.2 mΩ·cm) obtained from a Milli-Q ultrapure water system (Millipore, Billerica, MA, USA) was used in this study.