Evaluating the stability of synthetic cathinones in liquid urine and dried urine spots: impact of pH and storage conditions.

Synthetic cathinones comprise a class of new psychoactive substances associated with significant morbidity and mortality. Their detection in biological samples is complicated by their instability, which can compromise quantification or interpretation. This study evaluates the stability of five synthetic cathinones - 4-chloroethcathinone (4-CEC), 4-ethylmethcathinone (4-EMC), N-ethylhexedrone (NEH), methylenedioxypyrovalerone (MDPV) and 4-chloro-α-pyrrolidinopropiophenone (4-Cl-α-PPP) - in dried urine spots (DUS) and liquid urine at acidic (pH 5.98) and mildly basic (pH 7.63) conditions. Fortified samples were stored at room temperature (25 °C) and refrigerated (4 °C) for up to 14 days and analysed using the Thermo Fisher VeriSpray™ Paper Spray Ion Source coupled to a Thermo Fisher Altis™ Plus Triple Quadrupole Mass Spectrometer. Urinary pH was the critical determinant of stability. Under acidic conditions, 4-EMC, 4-Cl-α-PPP, and MDPV retained > 90% of their initial concentration by day 14 in both matrices. Conversely, under basic conditions, 4-CEC and NEH degraded rapidly at room temperature (< 20% remaining by day three). Refrigeration slowed but did not prevent degradation. DUS offered a minor stabilising effect for most analytes, although NEH exhibited greater degradation. Statistically significant differences between acidic and basic conditions were observed (p < 0.0083), while matrix format had limited influence at acidic pH. Overall, pH rather than matrix type was the main driver of synthetic cathinone stability in urine. Nevertheless, DUS offer practical benefits for sample transport and storage in forensic settings. Future research should further characterise the physicochemical mechanisms underlying synthetic cathinone degradation in urine to inform the interpretation of operational toxicology workflows.