Determination of melatonin in sleep supplements by high-performance liquid chromatography

Abstract

The aim of this study was to quantify the actual melatonin content in commonly available over-the-counter melatonin sleep supplements marketed in Lithuania. For this purpose, a high-performance liquid chromatography (HPLC) method was optimised and validated. The calibration curve was linear (R2 = 0.9996) over a concentration range of 0.5–50.0 mg/L, and the limit of detection was 0.17 mg/L. Recoveries of melatonin from spiked supplement samples ranged from 96.8 to 104.1%, with relative standard deviation values below 3.2%. Melatonin was quantified in 20 commercial sleep supplements representing 10 brands and three dosage forms (capsules, tablets and liquids), with two different lots analysed for each brand. Overall, the measured melatonin content ranged from –7 to +26% relative to the labelled amount, indicating a low variability among melatonin products sold in Lithuania. Only one product contained melatonin amount outside the ±20% margin of the labelled claim. The least variable products were liquid supplements, which generally contained the simplest ingredient compositions.