Quantification of melatonin in human saliva by liquid chromatography–tandem mass spectrometry using stable isotope dilution

A method for the determination of melatonin in human saliva has been developed using high-performance liquid chromatography–tandem mass spectrometry (HPLC–MS–MS). Saliva was collected in plastic tubes. 7-D-Melatonin was added as internal standard and the samples were cleaned and concentrated by solid-phase extraction. The limit of detection was 1.05 pg ml−1 and the limit of quantification was 3.0 pg ml−1. The accuracy of the method was ±14% at 5.60 pg ml−1 and ±9% at 19.6 pg ml−1. The precision was ±13% at 6.18 pg ml−1 and ±11% at 31.2 pg ml−1, respectively. Our HPLC–MS–MS method shows a high sensitivity and specificity for melatonin and more reliable results compared with a radioimmunoassay. The chromatographic method has been used to determine the circadian rhythm of melatonin among three nurses working the night shift and a patient suffering from an inability to fall asleep at night.