Separation of carnitine and acylcarnitines in biological samples: a review

Carnitine and its acylesters are a family of compounds that can be used in the early diagnosis of many diseases. Carnitine and acylcarnitines have a crucial role in fatty acid transportation. The increased level of free carnitine, total carnitine, or the acylesters can act as biomarkers for many metabolic disorders, including diabetes, encephalopathy and cardiomyopathy. The determination of these compounds is difficult owing to the simple aliphatic structure, the chiral center and the permanent positive charge. Although MS detection can be enough to differentiate between some carnitine derivatives, closely related structural isomers of the acylcarnitines must be separated before detection because they form the same base peak and second most abundant ion peak. Different separation methods are discussed in this review, including reversed-phase, hydrophilic interaction, ion exchange, ion pairing, mixed mode liquid chromatography, gas chromatography and electrophoresis. Representative example chromatograms are shown. The sample preparation and the different derivatization reactions are also covered. A table that summarizes the most important analytical methods by detailing the analyte mixture, the sample matrix, the separation mode and the detection method is provided.