Testing the validity of reference standard materials and stock solutions of veterinary drugs using LC-MS/MS.

This paper reports a practical approach for the stability testing of 37 veterinary drugs in stock standard solutions stored at -20°C for 1-3 years and the study of expiry date extension of 7 expired reference standard materials stored at 4°C. Stored stock solutions were compared versus freshly prepared stock solutions and concentrations determined using LC-MS/MS. The validity of expired reference materials was tested by new valid reference materials. LC-MS/MS method and parameters were optimised to get the maximum signal stability of the analytes. Statistical analysis was developed and performed to evaluate the stability results according to the acceptability criteria of 10% set by the European Commission guidance document SANTE/11813/2017. The stability of most of the stock solutions of the following veterinary drug families: β-agonists, illegal dyes, inhibitors, macrolides, penicillins, quinolones, sulfonamides and tetracyclines ranged from 12 to 36 months. β-Agonist compounds have the maximum stability period of 36 months while penicillin's stock solution in methanol showed the least stability. The results of testing the expiry date extension of reference standard materials demonstrated that there was no any deterioration of all tested compounds after manufacturer expiry date by 4-7 years.