## 5 Development of the First TF-SPME Device and Improvements up to 2019
Moreover, the carbon mesh support granted much better mechanical stability compared to previous iterations of TF-SPME, allowing the device to undergo more rigorous agitation compared to previously developed TF-SPME devices. After curing the extraction phase, the devices were then cut to different sizes well suited for TD, allowing more practical device introduction compared to previous methods. As a result of these recent developments, this version of TF-SPME is now the first commercially available TF-SPME device, currently distributed by GERSTEL Inc. (Gerstel GmbH, Mulheim, Germany). Since then, much of the development of TF-SPME devices has followed the same convenient format. Different extraction phases have been tested but not commercialized yet, including hydrophilic-lipophilic balance (HLB) phases that offer a wider range of extraction [30]. With the development of new mixed-mode extractive membranes that were both thermally and mechanically stable, new TF-SPME devices were able to overcome the limitations of previous variants of TF-SPME technology [12; 16; 32; 49]. Initial devices, being made of pure monophasic extraction phase, exhibited poor structural rigidity making their practical application cumbersome in immersive extraction [12; 49]. As a consequence of the large volumes of PDMS utilized in these devices, significant siloxane bleed was found, resulting in unacceptably high backgrounds. This was at times circumvented by use of single ion monitoring (SIM), avoiding detector saturation but nonetheless a technique not well suited for untargeted analysis. With the development of glass fiber supported TF-SPME devices [29; 32], better structural rigidity was met, with the first device [32] still lacking suitable mechanical stability and thermal stability for repeated analysis. The second glass fiber supported device [29], however, not only achieved better mechanical and thermal stability compared to its earlier counterparts but also demonstrated a wider range of extraction than found previously by the use of mixed-mode extraction phases. While there is certainly a benefit of using glass fiber as a structural base for TF-SPME devices, the large amount of extraction phase and binder used is still cause of concern in terms of high siloxane backgrounds. Further developments of TF-SPME devices have been focused on self-supported membranes with the incorporation of extractive particles, allowing them to achieve efficient extraction of a wide range of analytes. The incorporation of these extractive particles provided newfound mechanical stability [29], permitting the devices to be well suited for direct immersion extraction. Given that the qualities indispensable to the sampling method and characteristics important for the ruggedness of any GC-amenable extraction device--thermal stability, mechanical stability and extraction efficiency--were finally met, a refinement of TF-SPME as a whole was essential and further development was needed. Although it is true that the developed phases are thermally stable, a large volume of extraction phase will still cause more background and bleed compared to a smaller volume. In light of this, the newest iteration of TF-SPME devices [19], applying the extraction phase to a carbon mesh, solves this issue by retaining comparable sensitivity but with drastically reduced siloxane bleed, all the while being more mechanically stable than previous TF-SPME devices. As much as the geometry is far more convenient than previous iterations, the planar surface still poses a practical obstacle in the engineering of automated sampling and desorption. The major caveat in the new design is that currently only two phases currently are offered (DVB/PDMS and Car/PDMS).