## 2 SPME Techniques
### Non-fiber SPME Techniques  
Non-fiber SPME techniques developed over the last two decades can be divided into static methods using sample stirring and dynamic techniques relying on the flow-through of the sample. The former include stir-bar sorptive extraction (SBSE) and thin-film microextraction (TFME). The latter can be classified as in-needle and in-tip SPME [21, 47]. Figure 3 provides an overview of these new techniques. Figure 2: Internally cooled SPME device (**A**) and its automation (**B**) according to [PERSON] and [PERSON] (2006) [53], reprinted with permission of the American Chemical Society. In _stir-bar sorptive extraction_ (SBSE), a magnetic stir bar coated with polydimethylsiloxane (PDMS) is stirred in or positioned above an aqueous sample [21,50]. After extraction, the stir bar is removed, which is usually done manually with tweezers. In case of a SBSE-GC coupling, desorption of the analytes is induced by inserting the bar into the heated GC injection port or by placing it in a small vial and back-extracting with a few microliters of an appropriate liquid solvent. In case of LC, the mobile phase can be added directly to the stir bar. Compared to fiber SPME, SBSE has a higher concentration capacity as the PDMS and the layer is 50 to 250 times thicker [21,50]. However, SBSE is not yet as widely accepted as fiber SPME due to the difficulty of achieving full automation and the limited number of commercially available coatings [21,50]. Dual-phase twisters using different carbon-based adsorbents in addition to PDMS are being developed at the moment as well as other phases covering a wide range of polarities and containing polymers like poly(methacrylic acid stearyl ester-ethylene dimethacrylate) and polypyrrole [21,50]. In _thin-film microextraction_ (TFME), a flat film with a high surface area-to-volume ratio is used as the extraction phase [57,58]. This higher surface-to-volume ratio as well as an increased volume of extraction phase result in an enhanced sensitivity compared to conventional SPME without sacrificing the sampling time.The first step in TFME is the conditioning of the blades. Subsequently, the blades are exposed to the sample to extract the analytes [57,58]. A 96-well plate is placed on an orbital shaker for agitation. After washing with an appropriate solution to remove interfering substances, the blades can be transferred to a second 96-well plate containing the desorption solvent which is eventually injected into the chromatograph. A robotic workstation providing full automation of all the TFME steps including conditioning, extraction, washing and desorption may be used [57,58]. Moreover, coupling to liquid and gas chromatographic systems is possible [57,58]. TFME can be applied to gaseous, liquid and solid samples and is used in different sampling formats. In combination with liquid chromatography, it can be easily performed in a 96-well plate thus allowing for simultaneous analyses. The first TFME application used 96 pieces of small cut SPE disk membranes mounted on a stainless steel wire for support [57,58]. Nowadays, the extraction phase is coated onto a blade-shaped substrate and PDMS is the most common sorbent [57,58]. Figure 3: Overview of SPME techniques–techniques marked in grey are more recent developments (SBSE: stir-bar sorptive extraction; TFME: thin-film microextraction; SPDE: solid-phase dynamic extraction; FNME: fiber-packed needle microextraction; MEPS: microextraction by packed syringe). In case of coupling TFME to a gas chromatograph, desorption may also be achieved by direct thin-film injection inside a thermal desorption unit adapted to the special requirements of the thin-film with its particular geometry. The thermal desorption unit contains a large volume inlet and a programmed temperature vaporizer. The latter cryotraps the desorbed compounds. Once the thermal desorption process is completed, the cryotrap is heated and the trapped substances are transferred to the GC column [57,58].