## 6 TF-SPME Coating Methods  
The various extraction phases and coating methods used in the development of TF-SPME devices (Figure 4) are chosen based on the chemical properties of the analyte, the surrounding matrix and the adopted desorption method. There are many techniques for the coating of TF-SPME devices, including dip coating [39; 50], spin coating [51] bar coating [34; 36; 52], electrospinning [53] and spray coating [54]. Among these different coating methods, bar and spin coating are the most common coating methods for the production of TF-SPME devices amendable for TD. The most common coating method discussed in this review is bar coating in which the liquid extraction phase is set on a substrate and then this extraction phase is spread by a bar to develop the device [24]. The first TF-SPME device made by bar coating was prepared using DVB particles impregnated on PDMS and this device was later used for air sampling with good results [36]. Moving forward, [PERSON] et al. prepared bar coated DVB/PDMS onto a carbon-based mesh support coupled with a portable GC-MS for the quantitation of volatile and semi-volatile organic compounds [19]. The bar coating procedure is needed to be repeated on both sides of the carbon mesh support to ensure an even coating for the final device [9]. In 2017, [PERSON] et al. used bar coating for the preparation of DVB/PDMS TF-SPME devices for the analysis of common pesticides in surface water samples. They found that the use of TF-SPME, in comparison to LLE, provided enhanced selectivity, reproducibility and faster rates of extraction [9]. In another study by [PERSON] and colleagues in 2018, bar coating was utilized in the development of novel TF-SPME devices in an effort to analyze various pesticides found in river water, demonstrating a greener and more sensitive alternative than LLE [55]. The use of these developed DVB/PDMS TF-SPME devices for the quantification of pesticides demonstrated enhanced sensitivity and extraction efficiency for both onsite and bench-top analysis. In the spin coating method, in similar fashion to bar coating, a layer of extraction phase is placed on the substrate, after which by spinning the substrate, a thin layer of extraction phase is homogenously produced [24]. In both bar coating and spin coating, the thickness of the extraction phase can easily be controlled by the pressure of the bar onto the substrate and the intensity of spinning, respectively. Spray coating, one of the simplest methods for the preparation of TF-SPME devices, utilizes a dissolved mixture of extractive phase in a suitable solvent to be sprayed onto a stage until the formation of a uniform film [24]. An example of this technique, [PERSON] et al. prepared polyacrylamideonitrile-polystyrene (PAN-PS)-DVB and polyacrylamideonitrile-phenylboronic acid (PAN-PBA) TF-SPME devices for the analysis of a variety of pharmaceuticals from human plasma [56]. Through the development of these two new TF-SPME devices for consequent analysis by LC-MS/MS, the analysis of a wide spectrum of polar compounds in human plasma with high efficiency and rapid throughput was achieved. Finally, another coating method which is often used for TD is electrospinning or electrospray coating. In this method, a mixture containing a polymer is sprayed by electrical energy on the surface of substrate [24]. In 2015, TF-SPME devices were prepared by the electrospinning method using polyimide nanofibers for the investigation and quantification of phenol compounds in environmental water using GC-MS [53]. Extraction devices were first activated by acetone, increasing hydrophilicity, resulting in greatly enhanced extraction efficiency with results demonstrating limits of quantification (LOQs) in the ppt level.