*3.3. LC-ESI-QTOF-MS*/*MS Characterization*

LC-MS/MS has been widely used for the identification and characterization of bioactive compounds, including phenolics from different fruits, vegetable, and medicinal plants. An untargeted qualitative analysis of phenolics from twenty different fruit peel samples was achieved via LC-ESI-QTOF-MS/MS analysis in both negative and positive modes of ionization (Table S1, Figures S1 and S2—Supplementary Data). Phenolics present in different fruit peel samples were tentatively identified and characterized from their *m*/*z* value and MS spectra in both negative and positive modes of ionization ([M − H]−/[M + H]+) using Agilent LC-MS Qualitative Software and Personal Compound Database and Library (PCDL). Compounds with mass error < ± 5 ppm and PCDL library score more than 80 were selected for further MS/MS identification and *m*/*z* characterization and verification purposes. In our study, LC-MS/MS enabled the tentative identification and characterization of 176 phenolics in twenty different fruit peel samples, including phenolic acids (49), flavonoids (86), lignans (11), stilbene (5) and other polyphenols (25) listed in Table S1 (Supplementary data).

#### 3.3.1. Phenolic Acids

Phenolic acids are the most abundant bioactive compounds present in different fruits [5]. In our study, a total of 49 phenolic acids were tentatively characterized, including hydroxybenzoic acids (12), hydroxycinnamic acids (31), hydroxyphenylacetic acids (2), and hydroxyphenylpropanoic acids (4).

Hydroxybenzoic acids are widely present in different fruits such as mango, apple, custard apple, citrus, strawberries, and raspberries with significant antioxidant potential. Compound **1** presenting in mango, pear and kiwifruit was proposed as vanillic acid 4-sulfate based on the observed *m*/*z* at 246.9911 in negative ionization mode and further confirmed by the MS/MS experiment which displayed a characteristic loss of SO<sup>3</sup> (80 Da) at *m*/*z* 167 [49]. Most of the phenolic acids showed the loss of CO<sup>2</sup> (44 Da) and hexosyl moiety (162 Da) [50]. Compound **3** (*m*/*z* 169.0146), compound **6** (*m*/*z* 137.0244) and compound **8** (*m*/*z* 153.0193) were identified as gallic acid, 2-hydroxybenzoic acid and 2,3-dihydroxybenzoic acid, showing product ions at *m*/*z* 125, at *m*/*z* 93 and at *m*/*z* 109, represented the loss of CO<sup>2</sup> from the precursor ions [50,51]. Previously, Kim, et al. [52] had also tentatively identified gallic acid from white and red dragon fruit peel and pulp samples.

Hydroxycinnamic acids contained collectively a larger number of detected compounds than in any other subclass in this study. In our study, a total of 31 hydroxycinnamic acids were identified with remarkable antioxidant potential. Six caffeic acid derivatives were successfully identified in our work. Compound **15** (*m*/*z* 341.0861) and compound **26** (*m*/*z* 355.0686) exhibited a product ion at *m*/*z* 179 (caffeic acid ion) by losing glucoside (162 Da) and glucuronide (176 Da) in negative mode and identified as caffeoyl glucose and caffeic acid 3*-O-*glucuronide [53].

Ferulic acid (Compound **23**) was also observed in eight different peel samples. In an MS<sup>2</sup> experiment, ferulic acid displayed the product ions at *m*/*z* 178, *m*/*z* 149, and *m*/*z* 134, indicating the loss of CH3, CO2, and CH<sup>3</sup> with CO<sup>2</sup> from the precursor, respectively [54]. Compound **25** (RT = 19.319 min) was tentatively identified as *m*-coumaric acid with the precursor [M − H]<sup>−</sup> *m*/*z* at 163.0406 and confirmed by the MS/MS spectra (Figure 2), which exhibited the fragments at *m*/*z* 119 due to the loss of CO<sup>2</sup> [54]. Compound **47** (dihydroferulic acid 4*-O-*glucuronide, *m*/*z* at 371.0986) and compound **49** (dihydrocaffeic acid 3*-O-*glucuronide, *m*/*z* at 357.0811) were both detected only in the negative ionization mode, and the characteristic loss of the glucuronide (176 Da) moiety was observed in both compounds, which produced the fragment ions at *m*/*z* 195 and at *m*/*z* 181, respectively [55].

− − − − **Figure 2.** The LC-ESI-QTOF-MS/MS characterization of *m*-coumaric acid. (**A**) A chromatograph of *m*-coumaric acid (Compound 25, Table S1—Supplementary Data) in the negative mode [M − H]<sup>−</sup> which was tentatively identified and characterized in fifteen different fruit peel samples; (**B**) a mass spectrum of *m*-coumaric acid with a precursor of *m*/*z* 163.0406 in the apple peel; (**C**) MS/MS spectrum of *m*-coumaric acid with the product ion of *m*/*z* 119 (confirmed from online LC-MS library and database); (**D**) a fragmentation pattern of the *m*-coumaric acid in negative mode [M − H]−, with precursor of *m*/*z* 163 and a product ion of *m*/*z* 119 due to the loss of CO<sup>2</sup> .
