# Systematic Evaluation of Different Coating Chemistries Used in Thin-Film Microextraction
## 4 Conclusions  
In this study, eight different adsorbents used as TFME coatings were systematically evaluated, using as models various compounds characterized by a wide range of log P values. In summary, carbon-based coatings i.e., graphene, graphene oxide, MWCNTs and MWCNTs-COOH, afforded better extraction capabilities towards compounds with high log P values, with hydrophobic and \(\pi\)-\(\pi\) types of interactions as the main driving forces for adsorption. Specifically, the graphene oxide coating showed better extraction performance for polar compounds owing to its hydrophilic functional groups on the surface. Nevertheless, the overall extraction efficiencies of the carbon-based coatings for all model analytes were significantly low. Two different factors are postulated to contribute to such low recoveries: (i) poor desorption efficiencies for non-polar compounds; (ii) low dispersibility in PAN slurry, which makes carbon-based nanomaterials easily stack together, thus decreasing their surface area for adsorption. In this regard, several useful strategies can be employed to improve the extraction performance of carbon-based nanomaterials. One such strategy would be to exploit surface functionalization with appropriate organic molecules or polymers through covalent or non-covalent reactions. Functionalized graphene or MWCNTs would be more suitable for analysis of polar compounds, non-polar compounds, or both. Another way to improve extraction would be through  
Figure 3: (**a**) Structural scheme of eight different types of adsorbents and (**b**) photograph of thin-film coatings prepared from eight different adsorbents. immobilization of carbon nanomaterials onto appropriate supporting substrates, e.g., silica particles, to avoid their aggregation and increase the effective adsorption area. In comparison to carbon-based coatings, SPE-based coatings i.e., PS-DVB, C18, HLB and PBA, showed much higher extraction efficiencies toward the tested probes. In particular, PS-DVB and HLB coatings exhibited favorable ability to extract basic, neutral, and acidic compounds with a wide range of log P values at physiological pH conditions. The C18 coating was demonstrated to be useful for extraction of non-polar compounds with very high sensitivity due to its long-chain hydrophobic alkyl groups. In the present case, the PBA coating showed lower extraction efficiencies in comparison to the other SPE-based coatings, but previous work demonstrated that the PBA sorbent can be successfully used for targeted metabolomics analysis of urinary nucleosides due to selective binding of _cis_-diol groups available on the ribose sugar group under basic conditions [33]. In sum, coating selection should take into consideration both the properties of the analytes of interest as well as those of the coating. In this regard, the current research work can provide analysts with useful information regarding coating selection for TFME applications. The following are available online. The file of supplementary information is available online, and contains the following information: Section S1.1: Chemicals and Materials. Section S1.2: Stock solutions and working solutions. Section S1.3: Liquid chromatography and tandem mass spectrometry conditions. Table S1: Absolute recovery and carryover data of each analyte from PS-DVB coating by use of four desorption solvents. Solvent 1: methanol/acetonitrile/water (25/25/50, _v/v/v_), solvent 2: methanol/acetonitrile/water (40/40/25, _v/v/v_), solvent 3: methanol/acetonitrile (80/20, _v/v_), solvent 4: acetonitrile/water (80/20, _v/v_). Formic acid (0.1%, _v/v_) was added into all the desorption solvents. Table S2: Determination of correction factors for coating comparison. Table S3: Normalized extraction efficiency of each analyte from eight thin-film coatings at various pH values. Extractions were performed in triplicate at pH 3.0, 7.4 and 10.0, using an extraction time of 2 h. Description was performed for 1 h using desorption solvent 2.