Determination of plasma free carnitine by electrospray tandem mass spectrometry

We have evaluated a tandem mass spectrometry method for determining free ca rnitine concentrations in plasma and have compared its performance with tha t of an existing radioenzymatic assay. In this method, plasma was mixed wit h an internal standard, carnitine-d(3) (600 nmol/L), and butylated before a nalysis on a Quattro II tandem mass spectrometer. The detection limit of the tandem mass spectrometric (MS) method was 4 mu m ol/L and carryover between samples was 2.2%. Precision of the method was 1. 6 5.4% between injections and 4.0-5.6% between samples. Between batch preci sion was 10 13%. The method was linear up to a free carnitine concentration of 300 mu mol/L and good agreement was found with the existing radioenzymatic assay. We conclude that the tandem MS method is a precise and robust method for th e determination of free carnitine concentrations in plasma that overcomes t he disadvantages of the radiochemical method.