An ultra-high sensitive bioanalytical method for plasma melatonin by liquid chromatography-tandem mass spectrometry using water as calibration matrix.

For the endogenous substances with an ultra-low level in biological fluids, such as melatonin, the blank biological matrix is obviously not "blank". This problem leads to a serious issue of the bioanalytical methods development and validation by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). This work developed and validated an ultra-high sensitive bioanalytical method for plasma melatonin by LC-MS/MS using water as calibration matrix. The lower limit of quantitation of the method was verified to be 1.0 pg/mL and the method exhibited a linear range of 1-5000 pg/mL. Potential matrix effects, accuracy and precision were fully monitored and validated by two complementary quality control approaches respectively using water and the pooled plasma as matrix. The intra-run and inter-run precisions were less than 11.5% and 12.2%, respectively, and the relative error was below ± 13.8% for all of 5 quality control levels. The method was successfully applied to investigate the daytime (8:00 AM-8:00 PM) baseline level of endogenous plasma melatonin, as well as the pharmacokinetic profiles of exogenous melatonin after oral administration in beagle dogs.