Mass spectrometric characterization of efaproxiral (RSR13) and its implementation into doping controls using liquid chromatography–atmospheric pressure ionization‐tandem mass spectrometry

Efaproxiral (2‐[4‐[[(3,5‐dimethylanilino)carbonyl]methyl]phenoxyl]‐2‐methylpropionic acid, formerly referred to as RSR13) is prohibited in sports according to the World Anti‐Doping Agency (WADA). The drug as well as structurally related compounds and a stable isotope‐labeled derivative have been synthesized to elucidate the fragmentation pathway of efaproxiral, using electrospray ionization (ESI) and tandem mass spectrometry by employing a novel linear ion trap—orbitrap hybrid mass spectrometer—in positive and negative ionization modes. The elimination of 2‐methyl acrylic acid (−86 u) has been identified as a major fragmentation process in both charge states. Negative ionization and collision‐induced dissociation (CID) caused an additional release of carbon dioxide (−44 u), and positive ionization the loss of formic acid (−46 u). Efaproxiral was incorporated into an existing screening procedure for doping controls using solid‐phase extraction (SPE) followed by liquid chromatography‐tandem mass spectrometry, enabling a limit of detection of 2.5 ng/ml and interday precisions ranging from 7.9 to 13.0%. Copyright © 2006 John Wiley & Sons, Ltd.