Determination of L-carnitine by flow injection analysis with NADH fluorescence detection

A flow injection analysis method for determining L-carnitine is reported. The system uses the enzyme L-carnitine dehydrogenase covalently immobilized to Eupergit C. The NADH produced by the action of the enzyme, which is proportional to the L-carnitine concentration, is quantified using fluorescence detection. The system response was rapid and had a wide range of linearity. At a flow rate of 0.2 ml/min, a detection limit of 1 microM (20 pmol) was obtained for L-carnitine, peak areas were linear up to 100 microM, and samples could be injected every 4 min. The method performed well as a routine assay, showing high sensitivity (54,000 AU/microM), a precision of 0.96%, and the ability to carry out 144 consecutive assays with an RSD of 1.47% (good stability). Comparisons were made with other known methods for L-carnitine determination. Presence of D-carnitine had no effect on L-carnitine assay. The analysis was valid for determining L-carnitine concentrations in commercial pharmaceutical preparations.