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HPLC-MS chromatograms of (a) fraction A, (b) fraction C, and (c) fraction D obtained for solid phase extraction (SPE)-preconcentrated red wine sample spiked with TRP (1 mg/L), TEE (50 μg/L), and MEL (50 μg/L). Peak identity: TRP, tryptophan; TEE, tryptophan ethyl ester; MEL, melatonin.
