## 3 Types of Desorption Modes for TF-SPME
### Desorption by Thermal Desorption Unit (TDU)  
Since the advent of thermally stable extractive phases and binders, the TD of sorbents has been an attractive method for sample introduction to analytical instrumentation, as it requires no additional organic solvent as many other methods do [7; 8; 9; 21]. As the science and engineering behind these thermally stable phases progress, the inherent background of newly developed phases (solid or liquid) is reduced, thus allowing the TD of appropriate sorbents to be applicable to ultra-trace level analysis [19; 29; 30]. Fundamentally, TD operates by heating a sorbent with hot gas to release all volatile analytes adsorbed onto the extraction phase, the increase in temperature driving the partition coefficient of the analytes to favor the gas phase thus releasing them from the sorbent. To its advantage, TD enables the introduction of all analytes to a gas chromatograph (GC), with the exception of any non-volatiles that either remain on the TF-SPME device or are desorbed and deposited in the liner/column (as can be the circumstance with liquid injection). This characteristic of TD prolongs the life of the analytical column, reducing the amount of maintenance needed due to the reduction of particulate matter introduced to the column. The ease-of-use and efficiency of TD, along with the inherent greenness of the method (when applicable to solid sorbents), allows TD to be the desorption method of choice for most volatile and semi-volatile compounds. The efficiency of TD coupled with the convenient geometry of fiber SPME has allowed it to be easily adapted for online analysis. Taking advantage of quick extraction times and a lack of organic solvent, the TD of fiber SPME has long been used for routine analysis. As in other SPME geometries, TD is the most efficient and green desorption method for the analysis of volatiles by TF-SPME, directly desorbing into the instrument and removing the need for an organic solvent. However, as TF-SPME devices boast a larger extraction phase volume compared to the conventional fiber SPME geometry, an adapter is needed for the GC inlet to accommodate its larger size. In the development of TD adapters for larger extraction phase volumes, [PERSON] and colleagues [31] first made a custom thermal desorption unit (TDU) that employed an external heating element to accommodate a 22 mm glass disk coated with extraction phase. Only a couple of years later, [PERSON] et al. [12] used a glass insert to introduce a sheet of monophasic PDMS to a commercial programmed temperature vaporizer (PTV) injector. Since then, as there is an ever-growing need for large-volume PTV injectors for various extraction techniques, the manufacturing of automated TDUs suitable for TF-SPME have become commonplace with units from companies such as GERSTEL, Inc. (Figure 1). The geometry of the TDU itself plays a large role in the development of TF-SPME technology, as the volume and overall shape of the TF-SPME device must be developed so the intended desorption unit can accommodate the device and effectively desorb it. The thermal desorption of TF-SPME devices is typically carried out by a two-stage device, as opposed to the simpler one-stage TD often used for small-volume phases, such as is the case of traditional SPME fiber devices. In two-stage TD, the first stage is responsible for the release of analytes from the extraction phase into the desorption unit. This first stage is in many ways equivalent to a one-stage TDU, however, there are more parameters to carefully optimize compared to traditional one-stage desorption (an example being a typical GC inlet). Opposed to only transferring the analyte directly into the column, with a two-stages desorption the analyte must be efficiently desorbed and transferred into the cryo-trap first, and then passed into the column. This requires optimization of the different split settings and temperature programming for each stage.