## 3 Types of Desorption Modes for TF-SPME
### Desorption by Thermal Desorption Unit (TDU)
Proper method development and TF-SPME workflow (Figure 2), with the use of a cryo-trap for the focusing of volatiles, results in sharper chromatographic peaks and ensures that all compounds extracted are introduced into the analytical column, increasing the reproducibility and sensitivity of the analytical method. In case of direct immersion extraction, it is advisable to gently wipe the TF-SPME device before introduction into the desorption liner, as to avoid potential matrix contaminants such as water and oil entering the TDU. Figure 2: Optimized TF-SPME workflow (photos courtesy of GERSTEL Inc.) for extraction and thermal desorption. Care must be taken when deciding the type of split for the cryo-trap as it is synergistic with the TDU (Table 1). It should be mentioned that the gas flow of a two-stage unit is restricted by the cryo-trap during both stages. As a result, if splitless injection is desired a solvent-venting cryo-trap is still necessary to maximize gas flow, thus allowing efficient desorption. This solvent-vent must then be closed when the cryo-trap begins to increase in temperature, ensuring a splitless injection with maximum desorption efficiency. In short, for maximum sensitivity both stages should be run in splitless mode, the cryo-trap being operated under solvent-vent conditions before desorption into the column. In the presence of solvent, the cryo-trap can typically run in a mode that vents the solvent if the boiling point differs greatly from the analyte (approximately a 150 \({}^{\circ}\)C difference). This allows better chromatography with no loss of analyte, provided the boiling point temperature difference is sufficient and the cryo-trap is heated after the solvent vent closes, effectively making this method a splitless injection onto the column. For unknown analysis, it is typical to run both stages in split mode, and with concentrations in the ppm level, the TDU must have an appropriate split ratio with the cryo-trap being performed in either a splitless or solvent venting fashion. It should be noted that alternative forms of TD for TF-SPME devices have been recently reported. These methods use \"transmission mode\" devices consisting of a coated mesh-like surface to enrich analytes and subsequently both desorb and ionize in one step using ambient ionization methods [37; 38; 39]. Direct analysis in real time (DART), an ambient ion source that has been shown to be well suited for onsite analysis [40], can be simply coupled to TF-SPME (coated mesh-like geometry) by positioning the device in-between the ion source and the mass spectrometer (MS). In this way, a stream of heated plasma (most often helium) desorbs the TF-S4 PME transmission device while also ionizing analytes at the same time [37; 38]. Another ambient ionization source, dielectric barrier discharge ionization (DBDI), has also been proven to be well suited for the analysis of illicit drugs using TF-SPME devices [39]. However, as the DBDI source does not thermally desorb compounds as the DART source can, a separate TD chamber was constructed using an aluminum body and temperature controller. This chamber was then filled with pre-humidified nitrogen to facilitate desorption, the flow of this now-analyte enriched gas entering the DBDI source [39].