## 2 Basis of the QuEChERS Method
### Modifications in the Clean-Up Sorbents  
Concerning the second dispersive clean-up step in the QuEChERS procedure, the original procedure proposes using 25 mg of PSA and 150 mg of MgSO\({}_{4}\)[17]. PSA removes the acid matrix interferets co-extracted during the first step of the process (such as organic polar acids, polar pigments, sugar, fatty acids) due to its weak anion exchange properties, while MgSO\({}_{4}\) eliminates the water of the sample extract and reduces its polarity, thus leading to precipitation of polar interferets and improving their retention in the PSA sorbent [15; 16; 17; 18]. However, PSA is sometimes not able to remove excessive interferences in complex matrices [43]. For this reason, over the years, the QuEChERS procedure has been modified by the introduction of many different types of clean-up sorbent materials to improve the extraction efficiency [15; 16]. The main ones have been silica-bonded octadecylsilane (C18) and graphitized carbon black (GCB), which have been used in combination with PSA or separately [15; 16]. C18 is suitable for the effective removal of non-polar interfering substances such as lipids, and its combination with PSA is indicative of a more efficient clean-up procedure of fatty and complex matrices. Conversely, GCB is particularly effective in removing pigments, namely carotenoids and chlorophyll; thus, it is suitable for vegetables (carrots, red sweet pepper, spinach, rucola, lettuce) and plant-based products (e.g., teas and aromatic herbs) [15; 16; 39]. However, one of the drawbacks of GCB is that it may have affinity for compounds with planar structures; thus, there is risk to loose analytes with this type of structure when this sorbent is used in the dSPE clean-up step [15; 16; 39]. Nonetheless, alternative sorbents are continuously being proposed to improve and broad the range of application of the QuEChERS approach. In this context, many researchers have searched for and evaluated other novel clean-up sorbents for QuEChERS, such as: multiwalled carbon nanotubes (MWCNTs), magnetic nanoparticles, zirconia-based sorbents, sol-gel organic-inorganic hybrid sorbents, organic polymers, florisil, alumina, chitosan, molecularly imprinted polymers, ordered mesostructured silicas, graphene, etc. [15; 28; 43; 44; 45; 46; 47; 48; 49; 50]. Likewise, over time, the amount used of these clean-up sorbents has also been modified from the initial amounts proposed. In the case of PAs, many different types of clean-up sorbents, as well as combinations of them, have been used in the QuEChERS procedure (Table 1). For instance, three works performed the clean-up step with the original combination of PSA and MgSO\({}_{4}\), but modifying in some cases the original amounts (Table 1). However, no other sorbents or combinations of them were tested in these works. Conversely, other authors used the combination C18 and MgSO\({}_{4}\) for the clean-up extraction of PAs from herbal dietary supplements, leak, wheat and tea (Table 1) [29; 31]. In both works, it was observed that the C18 sorbent has great potential for the removal of non-polar co-extractants, slightly reducing matrix effects without affecting the recovery of the analytes [29; 31]. These sorbents have also been used individually and combined in the QuEChERS clean-up step for the determination of PAs. In this sense, [PERSON] et al. (2020) evaluated the individual use of PSA, C18 and Z-Sep as clean-up sorbents for sorghum, oregano and mixed herbal tea matrices [33]. It was concluded that 100 mg of any of these sorbents is feasible to achieve good recovery of the targeted analytes, as no recovery losses were detected [33]. Likewise, PSA has been individually used for the clean-up of honey, herbal beverages and forage grass with satisfactory results (Table 1). In these works, the authors evaluated and compared the PSA with different types of sorbents. For instance, [PERSON] et al.