## 2 From Fiber to Thin Film Format: Pros and Cons  
In common with all non-exhaustive extraction techniques, the mechanism of extraction for SPME is based on the equilibrium-driven diffusion of analytes between the sample matrix and the extraction phase [6]. The amount extracted at equilibrium between these phases is described in Equation (1) and explained in Section 4, \"Fundamentals of TF-SPME\". This equation implies that for most SPME-based extractions, the only parameters that are consequential when optimizing extraction efficiency for a non-exhaustive extraction are the distribution coefficient between the sample and the extraction phase (\(\mathrm{K_{es}}\)), and the volume of the extraction phase (\(\mathrm{V_{e}}\)). During method development and optimization, \(\mathrm{K_{es}}\) is maximized through changes in different physical parameters of the extraction, such as temperature, agitation, ionic strength, the amount of organic solvent in the sample (if any) and most notably extraction phase chemistry [6]. As a result, during the development of an SPME device the physiochemical properties of the coating must be carefully selected as they affect both the extraction efficiency and specificity for the targeted analyte. Additionally, the extraction phase must also be able to be efficiently desorbed, by either thermal desorption (TD) (the phase then needing to be thermally stable) or by desorption in an organic solvent (the phase not swelling in organics). Beyond the previously mentioned parameters, an increase in extraction phase volume also contributes to the enhanced efficiency of the sampling device [12]. This phase volume allows improved capacity for the analyte, which in turn enables a more sensitive extraction, applicable to ultra-trace level analysis, doing so, however, poses practical challenges in both engineering of the device and mass transfer phenomena. For example, when optimizing the volume of the extraction phase (\(\mathrm{V_{e}}\)) for fiber SPME, a simple increase in the diameter of the coating, as seen in Equation (2), drastically prolongs the equilibration time and negatively affects the desorption efficiency. An increase in phase volume for a fiber SPME also requires a redesign of the whole device assembly, as in the case of the recently introduced Arrow-SPME [13; 14]. This device, while important to the overall development of SPME due to its enhanced capacity, will not be further discussed as it does not seek to maximize the device's surface-to-volume ratio as other TF-SPME devices do. There have been multiple developments throughout the years to increase the extraction phase volume of microextraction devices to enhance their sensitivity with varying success. Still commonly used today, stir bar sorptive extraction (SBSE) seeks to increase the volume of extraction phase by coating a magnetic stir bar with polydimethylsiloxane (PDMS) for the immersive extraction of aqueous analytes, and has proven suitable for both direct immersion and headspace extraction [15]. This geometry allows higher capacities than conventional SPME fiber due to the larger extraction volume, however, it still has yet to overcome the difficulties discussed previously in timely extraction [16] (especially in regards to large sample volumes) [17] and efficient desorption. While the large capacity can grant greater sensitivity, the extraction time can take 24 h [18] and desorption can be long as well depending on the molecular weight and volatility of the analytes [17]. Thin film solid phase microextraction (TF-SPME), a novel SPME device first developed in 2003 [12], overcomes these limitations, extraction efficiency and capacity, by the use of an alternative geometry. TF-SPME consists of a large-volume thin layer of extraction phase (originally pure PDMS) for the pre-concentration of analytes.