Solid phase extraction/cyclodextrin-modified micellar electrokinetic chromatography for the analysis of melatonin and related indole compounds in plants

• A simple and robust methodology for melatonin and related indole compounds is proposed.
• A dual pseudostationary phase (SDS plus β-cyclodextrin) enabled unique selectivity.
• The SPE followed by CD-MEKC allowed the analysis of target compounds in less than 10 min.
• Limits of detection (LODs) down to low ppb levels were obtained for complex matrices.

The identification of melatonin in plants has inspired new investigations to understand its biological function. In this work, a robust, reliable, low-cost, quick and simple method based on solid phase extraction followed by micellar electrokinetic chromatography for the extraction, preconcentration and simultaneous determination of melatonin, tryptophan, serotonin and indole-3-acetic acid in plant material is proposed. Extraction of indole compounds from plant tissues was enhanced by ultrasound and solid phase extraction, which was carried out with C8 SPE cartridges. The use of a dual pseudostationary phase system, involving a mixture of SDS anionic micelles and β-cyclodextrin, enabled to reach adequate selectivity. A BGE of 10 mM sodium tetraborate (pH 9.2), containing 20 mM β-CD, 20 mM SDS, and 10% (v/v) of acetonitrile, allowed baseline separation in less than 10 min. The proposed methodology provided limits of detection (LODs) down to low ppb levels. Under the optimal conditions, a successful application on Arabidopsis tissue, green, and linden tea leaves confirmed the validity of this method for food analysis and as a tool to contribute to the elucidation of the biological role of melatonin in plants.