# Systematic Evaluation of Different Coating Chemistries Used in Thin-Film Microextraction
## 1 Introduction  
Thin-film microextraction (TFME), an alternative configuration of solid-phase microextraction (SPME), was first introduced in 2003 [1] and has since been employed for various LC and GC applications [1; 2; 3; 4]. The merit of the TFME format is the increase in the volume of extracting phase, which in some cases may improve the analyte recoveries and enhance the sensitivity for trace level analysis [5; 6]. In LC applications, the extracting phase is immobilized on a blade-shaped substrate instead of the traditional rod-shaped geometry, providing a higher surface area-to-volume ratio which enables an increase in the extracting phase volume, and consequently allows for improved extraction efficiency [7]. Moreover, multiple thin films can be easily assembled in a 96-well plate compatible format to achieve high-throughput sample preparation [8; 9; 10].  
Careful selection and evaluation of extracting phases play an important role in TFME method development. Unfortunately, the limited information about the relations of various extracting phases with different compounds is still challenging in the TFME technique. For instance, commercially available adsorbents typically used in solid phase extraction (SPE) can be employed as coating materials, such as C18 particles [8], polystyrene-divinylbenzene (PS-DVB) [11], and phenylboronic acid (PBA) [12], among others. [PERSON] et al. systematically evaluated these SPE sorbents for high-throughput analysis of Escherichia coli metabolomics in the 96-blade format. However, the effect of extraction conditions, e.g., pH, on the TFME extraction was not performed in this work [13]. In addition to the above SPE phases, carbon nanomaterials including graphene and carbon nanotubes (CNTs) have been employed as potential adsorbents for SPE and SPME applications due to their large surface area and tunable surface properties [14; 15; 16; 17]. In this regard, CNTs have been employed in TFME coatings for high-throughput analysis of phenolic compounds in water [18]. However, evaluations of the extraction abilities of carbon nanomaterials as thin-film coatings for LC-MS analysis of various types of compounds are still limited. In addition, the effect of extraction conditions, such as pH, on the extraction performance of TFME coatings must be thoroughly evaluated to provide analysts with a better understanding of the extraction behaviors of various kinds of compounds on these adsorbents. Such information would help better inform analysts with respect to coating selection during TFME method development of real applications.  
In this study, eight different types of adsorbents were evaluated as TFME coatings for LC-MS analysis. These adsorbents can be divided into two classes, namely the carbon-based coatings graphene, graphene oxide, multi-walled carbon nanotubes (MWCNTs) and carboxylated MWCNTs (MWCNTs-COOH), and the SPE-based coatings PS-DVB, C18, PBA and hydrophilic-lipophilic balance (HLB). Generally, various kinds of interactions between adsorbents and compounds, such as hydrophobic, hydrophilic, electrostatic, \(\pi\)-\(\pi\) stacking, and covalent bonding, are all involved in these materials, indicating their representativeness among plenty of materials. These adsorbents were further coated onto 96-blades through a developed spraying method with polyacrylonitrile (PAN) as glue [8]. A total of 24 compounds from different chemical classes and of widely varying polarities (log P of \(-\)2.99 to 6.98) were selected as probes for the coating evaluation. Extractions were performed at physiological pH conditions and the desorption solvent was optimized to achieve favorable extraction efficiency. The influence of pH (3.0, 7.4 and 10.0) on the extraction capacities of the selected coatings was further investigated in order to better elucidate the extraction mechanisms of these analytes on the eight different coatings.