# Systematic Evaluation of Different Coating Chemistries Used in Thin-Film Microextraction
## 2 Results and Discussion
### Comparison of Extraction Efficiency of Various Coatings at pH 7.4
The main noticeable difference between HLB and PS-DVB was observed for the extraction of phenylalanine, tryptophan, and adenine, where PS-DVB showed superior performance compared to HLB. These three compounds are characterized as small polar molecules (molecular weight below 205 Dalton) bearing in their structure aromatic moieties and amine groups. Although both PS-DVB and HLB sorbents contain divinylbenzene functionalities able to display the \(\pi\)-\(\pi\) type of interactions, the smaller pores of PS-DVB (50 A for PS-DVB and 80 A for HLB particles) may have probably allowed for improved extraction of the aforementioned compounds. Similar results corresponding to PS-DVB performance have already been reported by [PERSON] et al. [21]. The C18 extraction phase, in turn, showed poor extraction capabilities toward the most polar probes (log P values under 1) and, as expected, superior normalized recoveries for compounds of medium to high hydrophobicity, namely dexamethasone, carbamazepine, diazepam, testosterone, and propranolol. As shown in Figure 2b (dot bar), hydrophobic interactions provided by C18 particles were sufficient to allow for an almost two-fold increase in normalized recoveries as compared to HLB and PS-DVB coatings. Another factor that should also be considered regards the differences in particle size. The small size of C18 particles (5 \(\upmu\)m) permits for more efficient arrangement of the sorbent in a given volume, and this, in turn, might result in higher normalized recoveries. The only one exception to this phenomenon was phenanthrene; although characterized by a high log P value (4.46) it yielded a lower recovery by the C18 coating in comparison to the results obtained using PS-DVB and HLB phases. The aromatic-ring-rich structure of phenanthrene provides stronger \(\pi\)-\(\pi\) interactions with PS-DVB or HLB phases rather than the C18 phase. In addition, it is worth mentioning that riboflavin showed a normalized recovery different from the expected trend. As presented in Figure 2, among all the tested extraction phases, the highest normalized recovery for riboflavin was obtained with C18. Taking into account the high polarity of this compound (log P -1.46), it is reasonable to consider that the ion-exchange capacity provided by the low concentration of silanol groups present in the silica may play an important role during the extraction process of basic solutes (e.g., riboflavin) [22]. For the PBA coating, the multiple functional groups provide different types of interactions including hydrogen bonding interactions, ionic interactions, as well as Van der Waals and \(\pi\)-\(\pi\) interactions for various kinds of substances [12]. Therefore, the PBA coating offers universal extraction capacities, toward both polar and non-polar compounds. However, the much lower normalizedextraction efficiencies of the PBA coating are attributed to its having the largest coating volume among these four SPE-based sorbents, resulting in the lowest correction factor for calculation. Moreover, in comparison to PS-DVB (owning same particle size with PBA), the density of phenyl-boronic acid groups grafted on PBA-modified silica particles would provide less abundant adsorptive sites, facilitating much lower extraction efficiency.