If you have access to LC, you can run a standard for the retention time and then collect a fraction of the eluant to catch the melatonin. I used to do that for LSD before the capillary columns could separate LSD from lampa, but the method is quick and easy. Run enough samples thru the LC to get enough to run ir with.

Please look at the following below links which may help you in your analysis:

https://www.tandfonline.com/doi/pdf/10.1080/00365510600548777?needAccess=true

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5263157/pdf/fpls-08-00064.pdf

Thanks

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