# Systematic Evaluation of Different Coating Chemistries Used in Thin-Film Microextraction
## 2 Results and Discussion
### pH Effect on the Extraction Performance of Different Coatings
It should be considered that the adsorption/extraction of these drugs on hydrophobic surfaces is based on two factors, i.e., the hydrophobicity of the drug (conditioned by the value of log P) and the ionization of the molecule (determined by the values of pH and pKa). Therefore, log D is a pH-dependent modified log P value and is relevant for compounds that are partly dissociated or protonated [29]. It can be calculated as Equations (1) and (2) [30]:  
For acidic molecules, log D is determined as:  
\[\log\text{D}=\log\text{P}-\log(1+10^{\text{(pH-pKa)}}), \tag{1}\]  
Whereas for basic molecules,  
\[\log\text{D}=\log\text{P}-\log(1+10^{\text{(pKa-pH)}}), \tag{2}\]  
For example, the modified log D is:  
The D values of propranolol at pH 3.0, 7.4, and 10.0 were found to be -2.94, 1.47, and 3.38, respectively. As pH value increases, the hydrophobicity of propranolol gradually increases, thus facilitating higher extraction efficiency through hydrophobic interactions with these coatings. Moreover, atenol and pindolol showed the same variation tendencies of log D values with increasing pH values, and improvements in extraction were also obtained. Similar trends for these three drugs adsorbed on the C18 coating at different pHs were also observed due to the hydrophobicity of the C18 phase. For the graphene oxide coating, the best extraction performances for most analytes took place under acidic and basic conditions, demonstrating that alternative binding forces i.e., hydrogen bonds under acidic conditions [31] and electrostatic interactions under basic conditions [32], respectively, contributed to extraction. For SPE-based coatings, PS-DVB and HLB coatings showed a similar tendency toward most of the analytes with respect to variations in pH values. As the C18 coating is strongly hydrophobic, no significant changes were observed under various pH conditions in relation to trends of analytes with high log P values (dexamethasone, carbamazepine, thiabendazole, diazepam and testosterone) adsorbing on the C18 coating. For the PBA coating, the most favorable recoveries occurred at physiological conditions due to the multiple interactions between analytes and PBA particles. It should be mentioned that the dissociation of boronate moieties occurs under basic conditions (pKa 8.8); thus, the negatively charged surface of PBA at pH 10 was favorable for extraction of compounds containing N-H groups such as atenol and pindolol. However, compounds with hydroxyl groups or negative charges were less extracted by the PBA coating (phenylalanine, tryptophan, morphine and dexamethasone). The extraction efficiency of 3-phenylpropionic acid was observed to significantly increase at pH 3.0 for all SPE-based coatings. It is reasonable that 3-phenylpropionic acid is protonated under strong acidic conditions, making it easier to be extracted onto SPE-based coatings in its protonated form. In contrast, for arachidonic acid (log P 6.98), recoveries were lower under acidic conditions in comparison to recoveries under neutral and basic conditions when SPE-based coatings were used. These results would indicate that deprotonated arachidonic acid shows higher affinity toward both PS-DVB and HLB coatings due to the weak ion-exchange properties of PS-DVB and the hydrophilic moieties of HLB. Besides, the silanol groups present in the silica support of the C18 and PBA adsorbents were effectively activated under strong basic conditions, thus providing alternative hydrophilic interactions with deprotonated arachidonic acid molecules, and consequently resulting in higher extraction efficiencies.