## 3 Results and Discussion
### Quantification of Polar Compounds from C. australasia L. Peel Using UPLC-MS/MS  
The citrus industry generates a large amount of waste, and the use of its by-products as a source of nutraceuticals and natural food additives are of high priority [39]. Indeed, the by-products in citrus may have the ability to revolutionize the functional food industry [40]. Therefore, the accurate identification and quantification of the novel matrices of compounds of interest is a must. Fifteen standard calibration curves for quantifying the main potential bioactive compounds found in _C. australasia_ L. peel extract were prepared using the following available commercial standards: citric acid, pyrogallol, caffeic acid, rutin, coumarin, hyperoside, rutin, naringin, 2-coumaric acid, didymin, naringenin and isonhametin. The calibration curves had good linearity between different concentrations ranges, with regression coefficients near 0.999 in all cases. The quantitative results are presented in Table 2.  
Citric acid was the most abundant compound quantified (8106.7 ug/g sample d.w.), and was also previously detected and quantified in the same matrix, but at higher concentrations, in fresh weight (f.w.), ranging between 46,800 and 58,800 ug/g f.w. [41], which could be explained due to the study of different cultivars [25], where it was reported to be between 8260 and 20,750 ug/g f.w. Citric acid is abundant in C. _australasia_ L., marking this fruit as a potential source of organic acids [23]. After citric acid, the second most abundant compound was the flavonoid naringin (916.3 ug/g sample d.w.), which is commonly present in all citrus fruits, such as pomelo (3910 ug/g f.w.) or grapefruit, at higher concentrations, though this is dependent on the method of extraction (1800 to 2800 ug/g d.w. for conventional extraction and 2400 to 3600 ug/g d.w. for ultrasound-assisted extraction) [32; 42]. They are of interest because they have been found to exhibit strong antioxidant and anti-inflammatory activities, and several lines of investigation suggest that the use of naringin as a food supplement may be of benefit to treat obesity, diabetes, hypertension and metabolic syndrome [43]. Didymin was the second most abundant flavonoid, which was found at 111.8 ug/g sample d.w. It has been previously detected in citrus peel (rinds) at values around 400 ug/g d.w [44]. Currently, it is being studied to treat lung cancer in vitro and in vivo [45] and gastric cancer [46]. Isorhammetin and rutin (41.9 and 8.7 ug/g sam  
\begin{table}
\begin{tabular}{c c} \hline
**Compound** & **Average \(\pm\) SD** \\ \hline Citric acid & 8106.7 \(\pm\) 180.8 \\ Pyrogallol & 22.5 \(\pm\) 2.2 \\ Caffeic acid & 11.2 \(\pm\) 7.4 \\ Coumarin & 5.4 \(\pm\) 2.8 \\ Hyperoside & n.g. \\ Rutin & 8.7 \(\pm\) 1 \\ Naringin & 916.3 \(\pm\) 75.4 \\
2-Coumaric acid & 97.4 \(\pm\) 10.1 \\ Didymin & 111.8 \(\pm\) 19.1 \\ Naringenin & 2.7 \(\pm\) 0.2 \\ Isorhammetin & 41.9 \(\pm\) 3.4 \\ \hline \end{tabular} n.q. = not quantifiable. Data are expressed as average \(\pm\) standard deviation (SD).  
\end{table}
Table 2: Quantification of phenolic compounds in _C. australasia_ L., expressed as ug compound/g sample d.w. (n = 3).  
ple d.w., respectively) are compounds extended in cirtruses and which present anticancer properties [47], among other health benefits. Pyrogallol was present at 22.5 ug/g sample d.w. and it has also been quantified in other cirtruses, such as C. _aurantium_, but in bloom and at a higher concentration (541.27 ug/g d.w) [34]. Only trace levels of caffe acid were detected in the studied matrix, which is in agreement with previous work conducted on several citrus matrices (but finger limes were not studied) [48]. Coumarin compounds are generally distributed throughout the citrus species [49]. In this species, coumarin and 2-coumaric acid were quantified at 5.4 and 97.4 ug/g d.w, respectively.