## 3 Types of Desorption Modes for TF-SPME
### Desorption by Thermal Desorption Unit (TDU)
The first stage, the TDU, is most often run at a constant temperature to desorb all compounds enriched on the TF-SPME device, however, a temperature ramp program is possible if desired. This temperature must then be optimized depending on the volatility of the analyte (typically recommended to be 50 \({}^{\circ}\)C below a compound boiling point) and the thermal stability of the extraction phase, the latter usually taking priority as different extraction phases require different thermal thresholds to desorb efficiently. In most instances, the lowest operating temperature that can be used for the desorption of enriched analytes with minimal carryover is optimal, as this approach prolongs the health of the TF-SPME device and minimizes the amount of bleed from the extraction phase. As an example, the operating temperatures for the two commercially available TF-SPME devices, Carboxen(r)PDMS (Car/PDMS) and divinylbenzene/PDMS (DVB/PDMS), go up to 250 \({}^{\circ}\)C. This parameter, like many others, needs to be optimized by trial and error at the beginning of the analytical procedure. Additionally, the split ratio must be optimized like any other injector, however, as the TDU does not directly transfer analyte into the column but the  
Figure 1: Two-stage thermal desorption unit for thin film solid phase microextraction (TF-SPME) (photo courtesy of GERSTEL Inc.). For the purpose of clarity, the CIS (cooled injection system) is referred to as a cryo-trap in the text. TDU, thermal desorption unit; GC, gas chromatography. cryo-trap, the split-mode of the TDU does not necessarily reflect the overall behavior of the injector. During desorption by way of the TDU, the analyte is transferred through a heated capillary to a cryo-trap. After desorption, the TDU is cooled and the TF-SPME device removed to ensure that no residual analyte is erroneously transferred from the TDU to the cryo-trap during the second stage. Furthermore, the heated transfer capillary connecting the TDU and cryo-trap should always be at a greater temperature than the highest temperature of the TDU to ensure complete transfer of all analyte into the cryo-trap, preferably 20-30 \({}^{\circ}\)C higher. Contrary to one-stage desorption, in two-stage TD, after the first stage desorbs all volatile and semi-volatile compounds from the extraction phase, the second stage is used to pre-focus analytes before their introduction into the analytical column. This pre-focusing step, while a boon to any TD method, is especially crucial when desorbing large-volume phases due to the amount of analyte desorbed and the longer desorption times typically required. During the desorption of such large-volumes, as is the case of TF-SPME, the loss of highly-volatile compounds and ultra-volatiles during a single-stage desorption method would be unavoidable. The pre-focusing step is usually achieved by the inclusion of either a sorbent tube or temperature-controlled unit (here referred to as a cryo-trap). While a sorbent-based second-stage can exhibit suitable trapping capacities of analyte, the addition of a second sorbent can complicate method optimization and thus cryo-trap stages are generally preferred. Depending on the cryo-trap device used, the trap can be cooled by liquid gas (such as nitrogen and carbon dioxide) or a solid-state device. In the use of a cryo-trapping device, while the analytes are desorbed at high temperature in the TDU, the cryo-trap is held at a low temperature, \(-40\) to \(0\)\({}^{\circ}\)C being appropriate for most volatiles [16; 32; 33] with highly volatile substances requiring lower temperatures trending toward \(-150\)\({}^{\circ}\)C to properly pre-focus [8; 19; 29; 34; 35; 36]. Only after all compounds are thermally desorbed from the TF-SPME device and condensed in the cryo-trap, the temperature of the cryo-trap is increased at a high rate (usually \(12\)\({}^{\circ}\)C/s), achieving discrimination-free transfer of analytes into the analytical column with minimal sample loss.