# Solid-Phase Microextraction Techniques and Application in Food and Horticultural Crops
## 2 Optimizing SMPE Analyte Extraction  
### Effect of Ionic Strength on Analyte Extraction from Samples  
Extraction efficiency can also be significantly enhanced via the addition of a salt (usually NaCl) to the aqueous analyte sample. An increase in ionic strength makes hydrophobic compounds less soluble in the aqueous solution (salting-out effect) and also alters the nature of the phase boundary between the stationary phase and sample solution. This can result in more of the analyte being adsorbed by the fiber. For example, [PERSON] et al. used SPME to analyze compounds commonly found in food packaging (methyl-, ethyl-, propyl-, and butyl- paraben, and butylated hydroxyanisole, butylated hydroxytoluene, tert-butylhydroquinone, and tris(2-butoxyethyl) phosphate) (Figure 2) that can leach into alcoholic drinks and vinegas [25]. They investigated extraction efficiency at NaCl concentrations of 0, 50, 100, and 200 g/L. They found that, in most cases, increasing the analyte's NaCl concentration, resulted in an overall increase in extraction efficiency. However, the increase observed varied considerably between the compounds investigated. For butylated hydroxyanisole, extraction efficiency increased by a factor of 3.5. There was an increase of 4 for butyl paraben, 6 for methyl paraben, 7.5 for propyl paraben, and 8 for ethyl paraben. However, for butylated hydroxytoluene, there was little change in extraction efficiency between 0 and 100 g/L NaCl, while at 200 g/L NaCl, there was in fact a decrease in extraction efficiency of 50%. SPME has also been used to quantify off-flavors from corks (referred to as wine taint) used in wine bottles in order to assess wine quality. [PERSON] et al. developed a method for the analysis of compounds in corks responsible for cork-tainted wine [26]. HS-SPME, with a DVB/CAR/PDMS fiber combined with GC-MS, was used for analysis. The compounds analyzed consisted of a number of chlorinated anisoles (2,4,6-trichloroanisole, 2,3,4,6-tetrachloroanisole, 2,4,6-tribromonaisole, and pentachloroanisole), pyrazines (2-isopropyl-3-methoxypyrazine, 2-methoxy-3,5-dimethylpyrazine, and 2-isobutyl-3-methoxypyrazine), and miscellaneous compounds, 2-methoxyphenol, 2-methylisobormeol (a monoterpene) and geosmin (a sesquiterpenoid). These compounds have a very low odor threshold of perception (around 5 to 25 ppt). They can leach out from the cork, and impart a woody, dank, acrid aroma and flavor to the wine. The addition of NaCl at a concentration of 300 g L\({}^{-1}\) increased the extraction of chlorinated compounds by a factor of four. Extraction for the other compounds was even greater with an increase in extraction by a factor of 14. [PERSON] et al. optimized extraction for a number of aldehydes from pale malts for analysis using HS-SPME [27]. These aldehydes are important compounds that affect the flavor of beer. The marker aldehyde compounds analyzed were 2-methylpropanal, 2-methylbutanal, 3-methylbutanal, hexanal, furfural, methional, phenylacetaldehyde and trans 2-nonneal (Figure 3). A 65 \(\upmu\)m PDMS/DVB SMPE fiber was used for the extraction. Extraction conditions that were investigated included the amount of the sample, extraction time, extraction temperature, and ionic strength. When 0.5 to 2.0 g of NaCl was added to the GC vial, no significant effect on the amount of aldehydes extracted was observed. The type of salt used is also an important factor to consider in optimizing extraction efficiency. The effect of a number of salts/salt mixtures (NaCl, Na\({}_{2}\)SO\({}_{4}\), (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\), and NaH\({}_{2}\)PO\({}_{4}\)) on the extraction of free fatty acids (C2-C10) in cheese and wine samples was investigated by [PERSON] et al. [28]. The HS-SMPE analysis involved the use of a  
Figure 2: Preservatives and antioxidants used in food packaging investigated by [PERSON] et al. 50/30 \(\upmu\)m DVB/CAR/PDMS fiber. Extraction was performed at 35 \({}^{\circ}\)C with constant stirring, with incubation and extraction times of 30 and 60 min, respectively.