## 3 Results and Discussion
### Antioxidant Activity and Total Phenolic Content  
The AOX of chemical compounds and their mixtures can be explained by two mechanisms: hydrogen atom transfer (HAT) or single electron transfer (SET). The combination of both is also possible. In HAT, the antioxidant donates a hydrogen atom to free radical species to stabilize it. In SET, the antioxidant donates an electron to the target molecule. The chemical structure of the antioxidant compound determines the mechanism of AOX. It should be highlighted that plant extracts containing phenolic compounds synergistic effect can be observed. It influences AOX, and it depends on the type of compounds and their concentration [37]. Moreover, a single compound AOX can be changed in its mixture with other compounds by synergistic, additive, or antagonistic effect [38]. Different methods for evaluating AOX depend on the AOX mechanism, as described above. Table 3 presents the AOX obtained by ORAC and DPPH methods. The half-maximal inhibitory concentration indicates the AOX of samples. In this case, the sample collected in Loarre in September has the strongest AOX. DPPH methods show the mixture of compounds that scavenge free radicals or act as hydrogen donors. Figure 4: Results of Pearson correlation for the most important compounds presented as heat map with hierarchical clustering. On the other hand, in the case of the ORAC method, the high values of the obtained results indicate strong AOX of the plant extracts. In this case, the sample collected in Pina de Montalgrao has the strongest AOX. The ORAC method shows the extracts contain compounds with good electron donor capacities. Analyzing the results, an interesting tendency can be seen, as both methods correspond to different antioxidant mechanisms. Thus, the sample with the best AOX measured by the DPPH method has the lowest AOX measured by the ORAC method and vice-versa. Therefore, the obtained results indicate that the sample LO_S contains a high number of compounds acting as hydrogen donors and very few compounds acting as hydrogen donors, while an opposite tendency is seen in the case of the PI sample. It should be highlighted that the methods applied for evaluating AOX are not specific to volatile compounds. Obtained results of AOX can be related to both volatile and non-volatile compounds. Qualitative analysis of non-volatile compounds in samples of _Arctostaphylos uwa-ursi_ was presented in our previous work [21]. Methanolic extracts prepared from samples collected in September and November 2015 were employed for total phenol content determinations. The total phenolic contents results show that it ranges from 133.5 \(\pm\) 18.8 mg GAE/g DW (sample PI) to 177.1 \(\pm\) 11.0 mg GAE/g DW (LO_N). The plant with the highest total phenolic content was the Loarre sample collected in September (171.9 \(\pm\) 19.4 mg GAE/g DW) and November (177.1 \(\pm\) 11.0 mg GAE/g DW) and the region with the lowest total phenolic content was Pina de Montalgrao (133.5 \(\pm\) 18.8 mg GAE/g DW). These results agree with the results obtained in the AOX study, showing similar behaviour in the same two populations. Antioxidant Markers  
All data presented in this investigation based on the untargeted metabolomics performed by HS-SPME-GC-MS completed with data analysis by chemometric tools let us identify antioxidants as quality markers shown in Table 4.  
The determination of antioxidants as quality markers allows the differentiation of samples from different locations. In this case, 20 quality markers were proposed for samples of _Arctostaphylos uwa-ursi_. Moreover, it facilitates samples characterization, quality control, AOX control and even authentication in case of fraud. It could also be applied to deal with the problem of product counterfeiting and inflated product prices.