## 3 Results and Discussion
### Chemical Characterization of C. australasia Peel Using HPLC-QTof-MS  
The elucidation of the compounds in each chromatographic peak was achieved through the analysis of the candidate formula with a mass accuracy limit of 5 ppm. The characterization strategy was based on several factors: on the accurate MS, their CID fragments and the use of authentic standards for those that were commercially available, as well as with information available in the existing literature. The SciFinder Scholar ([[http://scifinder.cas.org](http://scifinder.cas.org)]([http://scifinder.cas.org](http://scifinder.cas.org)), accessed on 15 October 2021), MassBank ([[http://massbank.jp](http://massbank.jp)]([http://massbank.jp](http://massbank.jp)), accessed on 15 October 2021), METLIN Metabolite ([[http://metlin.scripps.edu](http://metlin.scripps.edu)]([http://metlin.scripps.edu](http://metlin.scripps.edu)), accessed on 15 October 2021) and the Food ([[https://foodb.ca/](https://foodb.ca/)]([https://foodb.ca/](https://foodb.ca/)), accessed on 15 October 2021) databases were employed for the acquisition of chemical structural information. As many as 22 different compounds were detected in the ethyl acetate fraction, of which 15 were tentatively identified in the _C. australasia_ L. peel (Figure 4 and Table 1). Table 1 shows the retention time (RT) of each peak, the experimental \(m/z\) of deprotonated molecular ions ([M \(-\) H]\({}^{-}\)), the molecular formula, the error (ppm), the main MS/MS fragments and the tentative identification for each compound/peak shown in the chromatogram. These compounds were numbered according to their elution time. Additionally, the compounds identified for the first time in _C. australasia_ L. peel are in bold. The identified compounds belonged to different chemical classes from the phenolic compounds' families, i.e., seven flavonoidsand derivatives, three phenolic compounds derivatives classified as other polyphenols, two phenolic acids and derivatives, two limonoids and one organic acid. Figure 4: Base–peak chromatogram (BPC) of ethyl acetate fraction of _C. australasia_ L. peel using HPLC-QTof-MS.  
Figure 3: Total phenolic content (TPC) and antioxidant activity of _C. australasia_ L. peel and pulp fractions were obtained using different solvents. Data are expressed as mean \(\pm\) standard deviation and mean values are provided on top of each bar. Mean values with different superscript letters are significantly different (\(p\) < 0.05). Data shown in Table S2. Thirteen phenolic compounds and derivatives were tentatively identified in the _C. australsica_ L. peel. Among them, the identification of two phenolic acids, i.e., caffeic acid (peak 3) and 2-coumaric acid (peak 9), were confirmed using commercial standards. Although coumarin has been widely detected in other citrus fruits [28], and other coumarins in _C. australsica_ L. [29], this is the first time that this compound has been reported in _C. australsica_ L. In addition, seven flavonoids were identified in the studied matrix. Flavonoids are one of the most common phenolic compounds found in cirtuses [30; 31; 32]. This has been reflected also in _C. australsica_ L. samples where ruin (peak 5, \(m/z\) 609.1456), hyperoside (peak 6, \(m/z\) 463.0877), naringin (peak 7, \(m/z\) 579.1714), isorhamnetin (peak 8, \(m/z\) 315.0505), didymin (peak 11, \(m/z\) 593.1870) and naringenin (peak 14, \(m/z\) 271.0606) were identified using the commercial standards (Table 1). Finally, peak 10, with a [M-H] ions at \(m/z\) 471.1291, was tentatively identified as ononin-O-acetate on the basis of its fragmentation pattern, which included main fragments at \(m/z\) 307, 266 (formononetin), 205, 163 and 59 (acetate functional group), and this is depicted in Figure S1 (Supplementary Materials). Onorina has been reported in four _Citrus_ species (namely, _C. aurantium_, _C. grandis_, _C. limonia_ and _C. sinensis_) via HPLC-MS-SIM, while ononin-O-acetate has only been identified in _Citrus aurantium_ so far [33].