# Solid-Phase Microextraction Techniques and Application in Food and Horticultural Crops
## 1 Introduction
This consists of a stainless steel rod onto which the sorbent is coated, and is capped with a conical tip, which facilitates penetration through a septum. The diameter and length of the rod is much larger than that of a a normal SPME fiber, enabling a much larger amount of sorbent to be used, for example, a 1 cm \(\times\) 100 \(\upmu\)m (0.6 \(\upmu\)L) classical SMPE fiber compared to a 3 cm \(\times\) 250 \(\upmu\)m (15.3 \(\upmu\)L) SPME Arrow (Figure 1) [15]. However, in order to extract a larger amount of the sample, adsorption and desorption times will be longer. A SPME fiber normally has a length of 1 cm and consists of a solid fused silica substrate coated with a suitable polymeric stationary extraction phase, such as divinylbenzene (DVB), polyacrylate (PA), polydimethylsiloxane (PDMS), Carboxen (CAR), Carbowax (CW), and templated resin (TPR). These are used either individually or as a mixture of two to three components [16; 17; 18]. Despite the advantages in using SPME for sample preparation, analyses of complex matrices such as food create significant challenges, especially when developing methods for analysis for analytes present at very low concentrations. In the past, its use in food  
Figure 1: Schematic diagram of (**a**) a classical SMPE coated fiber, and (**b**) a SPME Arrow coated rod. analysis has been limited by the number of commercially available sorbent coatings, their short lifetime, sensitivity and matrix effect. Commercially available SPME fibers are fragile with a relatively low operating temperature due to the thermal instability of the sorbent coating [19]. Furthermore, the direct immersion of the extraction phase in a complex sample matrix can cause damage to the extraction phase, due to the irreversible adsorption of macromolecules from the sample matrix that will shorten the coating lifetime and change the method's sensitivity or the extraction properties of the coating (matrix effect). Potential swelling in common organic liquid chromatography solvents may also cause matrix effects [19]. Stir bar sorptive extraction (SBSE) can also be used when larger samples need to be extracted from a sample matrix. This involves the use of a magnetic stirrer bar with a sorbent coating to extract analytes from a sample matrix [20]. This also has the advantage of not requiring an extraction solvent but results in more labor-intensive analysis when compared to that in SPME, which can automated. The extent of the adsorption of analytes onto a SPME fiber is dependent on temperature, and on the physicochemical properties of the analyte and the stationary phase on the fiber. This is quantified via the distribution coefficient (\(K_{\mathrm{fs}}\)) between the two phases where,  
\[K_{\mathrm{fs}}=[\mathrm{A}]_{\mathrm{f}}/[\mathrm{A}]_{\mathrm{s}} \tag{1}\]  
\(K_{\mathrm{fs}}\) gives a measure of the extent of the adsorption of analytes onto the fiber, where \([\mathrm{A}]_{\mathrm{f}}\) and \([\mathrm{A}]_{\mathrm{s}}\) are the equilibrium concentrations of the analyte in the fiber and sample phases, respectively. Note that the actual amount of analyte extracted is dependent on both \(K_{\mathrm{fs}}\) and the amounts of stationary and sample phases present [14]. Generally, a polar fiber coating will more effectively absorb polar analytes while a non-polar fiber coating will more effectively absorb non-polar analytes. Additionally, increasing the thickness/amount of the coating present on the fiber will result in more of the analyte being adsorbed by the coating [21]. The kinetics of the adsorption process can vary considerably. The rate of analyte adsorption is dependent on the adsorbent used, nature of analyte(s), thickness of the adsorbent film, amount of sample agitation, and temperature. Typical adsorption times vary from around 5 min to around 60 min [22; 23]. The appropriate selection of a fiber coating is one of the most critical steps of SPME method development.