## 1 Introduction  
The development and accessibility of next-generation sequencing (NGS) technologies has made the generation of large amounts of genomic and transcriptomic data relatively straightforward. On the other hand, fast and reliable screening methods to characterize enzymes identified from these data are generally lacking, resulting in an ever-increasing number of enzymes awaiting biochemical characterization. Several methods have been developed, but the use of hyphenated analytical chemistry technologies especially has been shown to have the potential for fast biochemical screenings. This includes techniques such as gas chromatography-mass spectroscopy (GC-MS) and liquid chromatography-mass spectroscopy (LC-MS) [1]. Sesquiterpenoids are a subgroup of terpenoids that contain a core structure of 15 carbon atoms. Sesquiterpene hydrocarbons and sesquiterpenoids with a single hydroxyl group are often very volatile, and these are best analyzed using gas chromatography (GC) combined with mass spectroscopy (MS) [2,3]. The first step of sesquiterpen biosynthesis is performed by sesquiterpene synthases (sqTPS) [4]. SqTPSs utilize farnesyl pyrophosphate as their substrate and characterization of sqTPSs from plants can be achieved by expression of the genes in heterologous hosts like _Escherichia coli_, _Saccharomyces cerevisiae_ (yeast), or _Nicotiana benthamiana_, followed by product analysis using GC-MS [5, 6, 7]. Expression of sqTPSs has previously been successful in all these systems. However, in yeast it has been found that the pH of the media decreases during growth, which can cause acid-induced rearrangement of unstable compounds [7]. _N. benthamiana_ are fast-growing plants and expression in this host system offers the advantage of a native plant cell environment along with availability of codons, which are optimal for expression of plant TPSs in general [5]. Some biosynthesized sesquiterpenes are thermo labile and unstable in acidic conditions [7,8]. For example, it has been shown that germacrenes are prone to heat-induced rearrangements [9]. Germacrenes contain an open 10-membered ring structure (Figure 1). Due to the instability of germacrenes, it was initially suggested that they were an enzymatic intermediate in the biosynthesis of other sesquiterpenes [10,11]. It has since been shown that germacrenes are enzymatic end-products themselves, but that they are unstable and rearrange when exposed to high temperatures, acids, or reactive materials [7,9]. One such describe rearrangement is the heat-induced Cope rearrangement where the germacrene ring is transformed into the elemene structure (Figure 1b), but acid-assisted rearrangement has also been described [9,12,13]. The inherent instability of many sesquiterpenes under commonly used GC-MS conditions means their analysis must be carried out with caution [7,8]. Head Space-Solid Phase Micro Extraction (HS-SPME) fibers are used in many areas of volatility studies and are an obvious choice for fast screening of volatile sesquiterpenoids on GC-MS [5,14, 15, 16, 17]. Continued studies have led to a variety of different fiber coating materials, and combinations thereof, which are optimal for different kinds of volatiles [18,19]. SPME fibers often consist of a fused silica core coated with different materials that make them suitable for different analytes. Commercially available SPME fibers can be divided in two groups with regards to the mechanism involved in the interaction between the analytes and the SPME phase, _ab_sorption and _ad_sorption [20]. With _ab_sorption, the analytes are extracted by partition in the bulk of the SPME stationary phase; with _ad_sorption, the analytes physically interact or chemically bind to the surface of the stationary phase. Stationary phases that have _ab_sorption properties include polydimethylsiloxane (PDMS) and polyacrylate. The diffusion of small molecules in PDMS is similar to that in organic solvents; this provides a fast diffusion of the analytes into PDMS and an _ab_sorption type of extraction.