Experimental In this analysis we used the standard reagents of melatonin, 5-methoxytryptamine, and N-acetylserotonin (Fig. 1), preparing each as a 1 mg/mL methanol solution. For a matrix, we used 2,5-hydroxybenzoic acid (DHB) dissolved in methanol. The sample and matrix solutions were mixed at 1: v/v. We dropped 1 μL of mixed solution onto the target plate and allowed it to air dry. Product ion spectra were acquired using TOF-TOF positive ion mode.