## 5 Development of the First TF-SPME Device and Improvements up to 2019  
As discussed previously, the simultaneous increase of extraction phase volume and surface area for TF-SPME devices allows enhanced sensitivity with as good or better extraction rates compared to traditional fiber SPME. The development of the underlying theory of this phenomena took root in 2000 when [PERSON] and colleagues described the kinetics of a thin layer of extraction phase, predicting what would be the driving force for both TF-SPME and passive sampler development [43]. While not a traditional TF-SPME device, the first technique to exploit a higher surface area-to-volume ratio to increase both extraction capacity and efficiency was developed in 2001 by [PERSON] and colleagues [31]. The procedure utilized a thin film (0.05 and 0.33 \(\upmu\)m) of ethylene-vinyl acetate as the extraction phase which was coated onto 22 mm diameter glass disks serving as the support. This passive sampling method, exhibiting a surface area to volume ratio over 1000 times higher than comparable 100 \(\upmu\)m SPME fibers, demonstrated faster equilibration times but with the caveat of having less phase volume. Due to the decreased phase volume, sensitivity did not exceed traditional SPME fibers, however, the method has still been adopted throughout the years with success as environmental passive samplers [44; 45; 46; 47; 48]. While important to the development of TF-SPME, this technique does not share the same geometry or sensitivity as traditional TF-SPME devices and thus is only mentioned due to its importance in the development of parallel sampling technologies based on thin adsorbent layers. First developed in 2003 by [PERSON] and colleagues [12], the first TF-SPME device consisted of a pre-manufactured 25.4 \(\upmu\)m sheet of polydimethylsiloxane (PDMS) as the extraction phase. This thin sheet of PDMS was attached to a stainless steel rod as support, being affixed in a \"flag-like\" manner during extraction and wrapped around the rod prior to manual TD in a PTV GC inlet (Figure 3). Using polycyclic aromatic hydrocarbons (PAHs) as model analytes, the study demonstrated the practical use of TF-SPME as an alternative geometry to fiber SPME in both direct immersion extraction and headspace extraction. As the extraction efficiency was up to 20 times higher when using a 1 cm \(\times\) 1 cm sheet of PDMS compared to a 100 \(\upmu\)m PDMS fiber, with the extraction rate exceeding the already-developed SBSE [15], TF-SPME devices were further developed and optimized for better sensitivity and a more convenient sampling approach. Being limited to PDMS as an extraction phase, due to the availability of pre-made sorbents, priority was given in the geometric optimization of TF-SPME devices. In 2006, [PERSON] and colleagues modified a PDMS sheet into a 127 um thick house-like shape (Figure 3) supported by a stainless steel wire [49]. This TF-SPME device, dimensions of 2 cm \(\times\) 2 cm with the triangular portion of the device being 1 cm in height, boasted an increased phase volume of 0.0635 cm\({}^{3}\) compared to the 0.00255 cm\({}^{3}\) phase volume achieved by the previous developed TF-SPME device [12]. An increase of over 20 times the volume of the extraction phase, the achievable amount of extracted analyte was greatly increased, and thus greater sensitivity was attained. Moreover, the house-like geometry of the TF-SPME device permitted an increase in surface area while still allowing the device to be easily wrapped around the support, ensuring ease of injection into the GC inlet. Additionally, the same study demonstrated the efficacy of using TF-SPME for field analysis of aqueous media, establishing TF-SPME as a convenient onsite extraction tool [12]. A distinct departure from previous developments in both chemistry and design, [PERSON] and colleagues [32] demonstrated the use of glass wool fabric as solid support for TF-SPME.