# Solid-Phase Microextraction Techniques and Application in Food and Horticultural Crops
## 5 Applications of SPME and HPLC in Agricultural Products  
Desorption using SMPE-GC occurs under high temperatures, via the placement of the fiber directly into the gaseous mobile phase in the GC injection port, while HPLC desorption relies on the ability of the mobile phase to desorb the sample. If sample desorption does not occur quickly, undesirable peak broadening in the HPLC column will occur. This can be overcome by dipping the fiber in a small volume of the mobile phase or another suitable solvent until effective desorption takes place (static desorption). After desorption, the mobile phase/solvent sample is then injected into the HPLC column. However, the lifetime of SMPE fibers is often reduced when used with HPLC, due to swelling and break down of fiber coatings when exposed to mobile phase solvents [45]. The combination of SMPE with HPLC is particularly useful where analytes under investigation are non-volatile and/or break down under the elevated temperatures used in GC analysis [46]. This method has been widely used in the analysis of a wide range of pesticides [47]. Analysis of Pesticides, Herbicides and Environmental Pollutants in Agricultural Products Using SMPE and HPLC  
An early application of SMPE with HPLC was reported by [PERSON] et al. for the analysis of four pesticides (Figure 7) used on strawberries [48]. In developing their method, four SPME fibers were investigated. These consisted of a 100 \(\upmu\)m PDMS, 60 \(\upmu\)m PDMS/DVB, 75 \(\upmu\)m CAR-PDMS, and a 50 \(\upmu\)m CW/TPR fiber. Each  
Figure 7: Pesticide residues on strawberries extracted using a 60 \(\upmu\)m PDMS/DVB SPME fiber. fiber was subjected to an extraction time of 45 min with the solution stirred at 1000 rpm. A desorption time of 1 min in a small volume of the mobile phase (acetonitrile (55% \(v/v\))-water (45% \(v/v\))) was then used. The best SPME fiber for the extraction of these pesticides was the 60 \(\upmu\)m PDMS/DVB fiber. The use of a SPME fiber resulted in a sensitivity of around five times greater when compared to that under the direct injection of the pesticide mixture into the HPLC. Diethylustilbestrol (DES) is a synthetic non-steroidal estrogen that was widely used in the past to treat a number of diseases and also to promote growth in animals for improved food production [49]. However, DES and its metabolites do not break down easily in the environment and they have been found to be carcinogenic. Even though DES is no longer used as a medication and it is use in farming has been banned in many counties, it persists in the environment for a long period of time. Therefore, it is important to carefully monitor its presence both in the environment and in food products. [PERSON] et al. used sol-gel technology to fill wall pores of a carbon nanotube-reinforced hollow SPME fiber (CNTs-HF-SPME) with multi-walled carbon nanotubes (MWCNTs), in order to extract DES from milk products for HPLC determination [50]. The MWCNTs in the wall pores of the hollow fiber were found to selectively extract DES from milk products. Hollow fiber cartridges have the advantages of high surface area and good semi-permeability. They can limit large molecules such as proteins from entering small pores. Since MWCNTs are hydrophobic, 1-octanol was used as a wetting agent to promote the adsorption of DES from milk samples. Method optimization resulted in an extraction time of 30 min, with stirring (800 rpm) at 60 \({}^{\circ}\)C and a pH of 3.0. The desorption of DES from the fiber involved the use of methanol and ultrasonication over a period of 10 min. A wide range of linearity between 24 and 960 \(\upmu\)g/L, together with a low detection limit of 5.1 \(\upmu\)g/L, was obtained using this method. A major limitation of SPME Is Its inability to extract highly polar compounds from a sample. This was an issue for [PERSON] et al. when developing a SPME extraction method combined with HPLC and a diode array detector (DAD) to analyze three insecticides and seven fungicides commonly used in agriculture [51].