Buprenorphine d4

Analytical grade buprenorphine was purchased from LGC Standards (Teddington, UK) and buprenorphine-D4 was purchased from Sigma-Aldrich (Darmstadt, Germany). Ethyl acetate (POCH, Gliwice, Poland), n-hexane (Merk, Darmstadt, Germany) and ammonia solution 25% (Stanlab, Lublin, Poland) were of HPLC or analytical grade. Formic acid and methanol were from Sigma-Aldrich (Darmstadt, Germany) and were of analytical grade and LC-MS grade, respectively.
The plasma samples used in the method development and validation were obtained from rabbits involved in the pharmacokinetic/pharmacodynamic study. All blank plasma samples were obtained from animals which did not receive any drugs except isoflurane used in inhalatory anesthesia.

Morphine, Morphine-d₃, tramadol, ¹³C-tramadol-d₃, buprenorphine-d₄, chlorpromazine-d₃, and phenylbutazone were obtained as solutions from Sigma-Aldrich Chemie GmbH (Buchs, Switzerland). Butorphanol and buprenorphine were obtained as solutions from ReseaChem GmbH (Burgdorf, Switzerland) and Lipomed AG (Arlesheim, Switzerland), respectively. Ketoprofen-d₃ and phenylbutazone-(diphenyl¹³C₁₂) were purchased as powders Sigma-Aldrich Chemie GmbH (Buchs, Switzerland). Detomidine, acepromazine, meloxicam, ketoprofen, chlorpromazine, flunixin and fluphenazine were analytical standards available as powders in the authors’ laboratory. All solvents and other chemicals were purchased from Sigma-Aldrich Chemie GmbH (Buchs, Switzerland). All standards were dissolved in methanol, except for meloxicam that was dissolved in dimethylformamide, to yield stocks of 1 mg/ml. Firocoxib was obtained as an injectable for veterinary use. Chromatography-grade water used for LC-MS/MS analysis was processed by a PURELAB Option-Q system by ELGA LabWater (Labtec Services AG, Villmergen, Switzerland).