Rapid determination of serum melatonin by ESI-MS-MS with direct sample injection

This paper describes a rapid, simple and sensitive analytical method for the quantitative determination of melatonin in human serum by ESI-MS-MS with direct serum sample injection and on-line extraction. The method uses N-acetyltryptamine as the internal standard. It has high specificity and sensitivity for serum melatonin analysis. The internal calibration curve shows a wide linear range from 0.500 to 200 ng/ml with a correlation coefficient, R(2) > 0.999. The limit of quantitation is 0.500 ng/ml and the limit of detection is 0.100 ng/ml with 10-microl sample injection. The recoveries of serum melatonin at three levels are approximately 70%. The intra-assay precision (n = 5) is between 0.8 and 2.0% and the inter-assay precision (n = 3) is between 1.5 and 5.9% over the calibration range. This method has a total analysis time of less than 9 min. It can be used for the measurement of melatonin in human blood.