## 5 Development of the First TF-SPME Device and Improvements up to 2019
Opposed to previous attempts, which utilized pre-manufactured sheets of PDMS supported by steel wire, the SPME devices designed consisted of a polyacrylate (PA) extraction phase bound to glass wool. During the development of these devices, an amount of glass wool fabric was saturated with a solution of PA, being cured while sandwiched between two sheets of polyethylene foil to ensure homogeneity of the applied extraction phase. Being the first composite TF-SPME device developed, the sampling device demonstrated increased mechanical stability and the final device was 6 cm \(\times\) 0.3 cm after the sheets of foil were removed. These TF-SPME devices, named \"PA strips\", were compared to SBSE by performing their extraction with the same parameters and desorbing them into a TDU, very similar to the modern-day TD of TF-SPME devices. It was found, however, that these devices undergo thermal decomposition after multiple uses, resulting in high amounts of bleed into the GC and a lack of robustness of the device. Incorporating the house-like geometry [49] and the glass fiber support [32] of earlier TF-SPME devices, [PERSON] et al. [29] developed the first mixed-mode extraction phase for TF-SPME using Car/PDMS and PDMS/divinylbenzene (PDMS/DVB). Adsorptive particles (Carboxen(r) or DVB) were suspended in a solution of the binding agent (PDMS) before being applied to a thin sheet of glass wool fabric. Instead of allowing the polymer solution to absorb into the supporting material as previous methods did [32], the coating procedure was performed using the spin coating method due to its ease of use and greater control over the phase thickness. After curing and cutting the material to a 2 cm \(\times\) 2 cm  
Figure 3: A timeline of pivotal moments in the evolution of TF-SPME devices. PDMS, polydimethylsiloxane; DVB, divinylbenzene; Car, Carboxen®; HLB, hydrophilic-lipophilic balance. square with a 1 cm triangle (same geometry developed by [PERSON] et al. [49]), the TF-SPME device was held by a cotter pin during extraction of analytes. This newly developed TF-SPME device was then desorbed into a large volume inlet (TDU-2, Gerstel GmbH, Mulheim, Germany) with a CIS-4 (Gerstel GmbH, Mulheim, Germany) cryo-trap. Accordingly, results showed a marked increase in the mechanical stability of the device during extraction, as well as improved thermal stability during the desorption process. Building on the prior success of mixed-mode TF-SPME devices, the first self-supported particle loaded TF-SPME device was developed using DVB particles loaded onto PDMS [36]. This device was made with the intent of air-sampling using high amounts of DVB embedded into a PDMS base. It was found that when increasing the amount of DVB particles onto the PDMS membrane, up to 30% (w:w) DVB allows the membrane to achieve better mechanical stability compared to non-particle load membranes, 20% being found to be optimal for mechanical stability and extraction efficiency. As a result, the TF-SPME device boasted better sensitivity for the target analyte, benzene, than previous methods using fiber SPME or monophasic PDMS TF-SPME devices. This device, however, was still unable to appropriately be used in direct immersion extraction at high agitation rates, as the mechanical stability was not as great as previous glass-coated TF-SPME devices [29]. In 2016, [PERSON] and colleagues developed a sampler exhibiting far less siloxane bleed and greater robustness due to the carbon mesh support used therefore creating the first TF-SPME device well suited for untargeted analysis and onsite sampling by direct immersion extraction [19]. To achieve this, higher density PDMS was used to reduce bleed along with a slight reduction in phase volume. A mixture of two components, DVB particles embedded into PDMS, was then spread out onto a carbon mesh which primarily provided support along with some definitive properties. In spite of the reduced phase volume of this device, the new design still afforded a highly sensitive extraction with now far lower siloxane bleed.