High throughput screening of sub-ppb levels of basic drugs in equine plasma by liquid chromatography-tandem mass spectrometry.

This paper describes a high throughput LC-MS-MS method for the screening of 75 basic drugs in equine plasma at sub-ppb levels. The test scope covers diversified classes of drugs including some alpha- and beta-blockers, alpha- and beta-agonists, antihypotensives, antihypertensives, analgesics, antiarrhythmics, antidepressants, antidiabetics, antipsychotics, antiulcers, anxiolytics, bronchodilators, CNS stimulants, decongestants, sedatives, tranquilizers and vasodilators. A plasma sample was first deproteinated by addition of trichloroacetic acid. Basic drugs were then extracted by solid-phase extraction (SPE) using a Bond Elut Certify cartridge, and analysed by LC-MS-MS in positive electrospray ionization (+ESI) and multiple reaction monitoring (MRM) mode. Liquid chromatography was performed using a short C(8) column (3.3 cm L x 2.1mm ID with 3 microm particles) to provide fast analysis time. The overall instrument turnaround time was 8 min, inclusive of post-run and equilibration time. No interference from the matrices at the expected retention times of the targeted masses was observed. Over 60% of the drugs studied gave limits of detection (LoD) at or below 25 pg/mL, with some LoDs reaching down to 0.5 pg/mL. The inter-day precision for the relative retention times ranged from 0.01 to 0.54%, and that for the relative peak area ratios (relative to the internal standard) ranged from 4 to 37%. The results indicated that the method has acceptable precision to be used on a day-to-day basis for qualitative identification.