## 3 SPME Process
### Derivatization  
Under some conditions target analytes cannot be efficiently extracted or determined by the applied method due to the physicochemical properties of the analytes or the composition of the matrix. In these cases derivatization may modify the analytes and improve their extraction, desorption, or chromatography relative to matrix interferences. Derivatization might also improve the separation performance of analytes with poor chromatographic behavior [78]. Desired requirements for derivatization reactions are a fast and quantitatively formation at room temperature as well as a high stability and solubility in the applied solvent or phase [74].  
There are different derivatization methods applicable for SPME (see Figure 7). The main difference of the various derivatisation approaches is the point of time when the derivatization reactions take place. Firstly, the derivatization reaction can be conducted by directly adding the derivatizing agent to the sample, named direct or pre-extraction derivatization. Furthermore, the reaction can take place during the extraction process (simultaneous derivatization and extraction) or immediately after that (derivatization following extraction or post-extraction derivatization). Under these conditions, the derivatization is performed by exposing the enriched SPME fiber to the derivatization agent solution after the extraction [74]. Another option is the post-extraction derivatization, which take place in the injection port of a gas chromatograph through thermal conversion (derivatization in GC Injector Port (see Figure 7).  
In _direct derivatization_ the chemical structure of the analyte is modified. Afterwards the common extraction and desorption procedures are performed. The obtained derivatives show better performance characteristics than the original analytes by respective extraction phases [79]. Direct derivatization is a simple derivatization technique and is frequently applied and easy to implement [74].  
Another option for derivatization takes place _simultaneously with the extraction process_ or _after the extraction process_. Due to the combination of extraction and derivatization this procedure is quite convenient to apply and the time needed for the analysis can be significantly reduced [74]. Barriers of this procedure are the compatibility of the derivatizing agent with the extraction phase and with the applied extraction method [74]. However, optimization of the composition of the derivatizing agent is  
Figure 7: Classification of derivatization techniques in SPME, adapted from [PERSON] and [PERSON] (1997) [79].  
frequently required [74]. Therefore, post-extraction derivatization is preferred when extraction and derivatization cannot be conducted simultaneously or the partition coefficients of the derivative agents are lower than those of the analytes [74].  
_Injection port derivatization_ is a post-extraction derivatization procedure, for which analytes containingpolar functional groups such as -OH, -SH, -NH, and -COOH are thermally converted into derivatives.  
Derivatization in food analysis for example has been applied for analysis of chlorophenols and chloroanisoles in wine [80] or formaldehyde in fish products [81]. Interesting fields of application of derivatization in environmental samples are, for example, the analysis of formaldehyde and other carbonyl compounds in indoor air [82], organometallic compounds (mercury, lead, tin) in river and seawater [83] or organotin compounds in sediment [84]. Further applications of derivatization in the fields of food [80, 81, 85, 86] and environment [87, 88, 89, 90, 91, 92, 93, 94, 95, 96] are provided in Section 3 and 4, respectively. Recently, commonly used derivatization agents were reviewed by [PERSON] and [PERSON] [97] and [PERSON] [78].  
The main drawback of derivatization, however is the formation of artefacts or increased matrix interferences [74]. Furthermore, derivatization reactions favors the generation of waste and therefore, it is recommended to avoid them where possible [98].