## 3 SPME Process
### Extraction and Desorption Techniques  
The extraction process of fiber SPME can be conducted in three common ways: _direct or immersion extraction (DI), headspace (HS) and membrane protection extraction_ (see Figures 5 and 6). In _Direct SPME_ the fiber is directly immersed in a gaseous or liquid sample. In case of a liquid sample certain agitation is required to reduce the extraction time. For volatile compounds in gaseous samples, the natural occurring air flow is often sufficient to reach the equilibrium [65]. In _HS-SPME_ the fiber is exposed to the vapor phase above the liquid or solid sample [9]. The advantages are the protection of the fiber from damaging substances of the sample matrix and a possible adjustment of pH conditions. Only for very volatile compounds a significant difference between DI-SPME and HS-SPME techniques was observed [66]. The extraction kinetics are governed by [PERSON]'s law [67]. If the Henry's constant of a given substance is high, then the concentration of the compounds in the headspace is high, too. Under these conditions a rapid extraction from the headspace takes place. Figure 4: Extraction and desorption techniques for SPME applications. In case of aqueous sample matrices, volatile and non-polar compounds are extracted faster than semi-volatiles and polar volatiles [65]. Moreover, increasing sample temperature and agitation efficacy may decrease the extraction time [65]. In static headspace sampling, diffusion occurs between the fiber and the sample without any interference, whereas dynamic headspace sampling involves air movement devices like air sampling pumps that serve to move the headspace air [69]. To collect and pre-concentrate the headspace gas, the headspace air is transferred into another chamber where the SPME filament or another extraction trap is present [70]. Figure 5: SPME procedure for total-immersion and headspace sampling according to [PERSON] _et al._ (2009) [2], reprinted with permission of Springer-Verlag. Figure 6: SPME procedure for membrane protection sampling according to [PERSON] and [PERSON] (2004) [68], reprinted with permission of Elsevier B.V. (HFM: hollow fiber membrane). An advantage of static headspace sampling in comparison with dynamic headspace sampling is that static headspace sampling does not require careful calibration processes and expensive air sampling pumps [71]. However, differences in temperature and pressure affect the efficiency of the static sampling process as well as wall effects from sampling containers that also play a role [69]. In summary static headspace sampling shows advantages of simplicity, sensitivity, selectivity and ease of automation over dynamic headspace sampling techniques [72]. The third extraction technique is the _membrane protection extraction_. Extraction which is typically applied to the analysis of samples containing interfering compounds such as proteins, humic acids and fatty material [68]. It is important to prevent the extraction of these high molecular weight compounds which potentially complicate, or even inhibit, sample analysis [68]. The extraction is conducted using a membrane which is selectively permeable for analytes of interest [68]. Membran protection extraction is slow in comparison with other extraction techniques, but effectively extracts compounds with low volatility [65]. All extraction procedures require an optimization of performing parameters in order to achieve good reproducibility, resulting from a compromise between sensitivity and extraction time. Method optimization parameters include agitation conditions, extraction time, extraction technique, temperature, headspace and sample volume vial shape, condition of fiber coating, depth of the SPME fiber inside the vial, geometry of the fiber, and pH conditions [33, 73]. After extraction, target analytes are introduced into a suitable instrument for detection [74].