Methods using volatility of molecules

Many methods to extract and quantify the organic compounds in solid matrices have been established thanks to their volatility. Between them, dynamic headspace (HS) purge-and-trap and direct HS methods are mainly used. The first one allows the analysis of low concentrations by enriching volatile components on a polymer bed and therefore improve the sensitivity, whereas the second one is largely used in samples with high concentration of aroma compounds (Kataoka et al. 2000).

The headspace methods allow measuring the transfer or release of volatile compounds in solid matrix, by static or dynamic ways. The static headspace method uses a closed system sampling the headspace at thermodynamic equilibrium, whereas the dynamic headspace method uses an open system and the measurements are taken when the equilibrium is not reached.

Figure 1.10. Schematic diagram of trapping of VOCs on Tenax^TM, from (Hodgson et al.

1998).

The dynamic headspace extraction system is a purge and trap technique proceeding with a pre-concentration step on adsorbent for sampling VOCs. Figure 1.10. shows a schematic

28 diagram of this technique. Its principle is the adsorption of the volatile molecules on a chemically inert sorbent material, generally Tenax^TM (2,6-diphenyl-p-phenylene oxide polymer), followed by thermal desorption of the adsorbent and its subsequent analysis by GC-MS (Hodgson et al. 1998).

In 1981 - 1982, Kolb, Auer and Pospisil presented the theoretical principles of multiple headspace extraction, the purpose of which was to eliminate the matrix effect produced by the calibration phase when different matrix standards are used. This method is a dynamic gas extraction carried out stepwise on a same sample, allowing quantify the volatiles compounds in solid or complex liquid samples. At each extraction step, the overall equilibrium of the analyte in the system must be first established then analyte present in the headspace volume is removed and analyzed as GC area. Various extractions of the same sample, generally five, are performed in order to reach an exponential decay of the analyte present in the sample. MHE calculates, thanks to a mathematical extrapolation, the total peak area which is proportional to the total amount of the analyte present in the original sample, which can be determined by external calibration (Kolb and Ettre 1991).

Multiple headspace extraction has been used for determination of volatile compounds in solid matrixes such as polymers (Kolb et al. 1981) and cellulose based packaging materials (Wenzl and Lankmayr 2000). This method has also been used for determination of monomer solubilities (i.e. styrene and acylonitrile) in water (Chai et al. 2005).

Solid Phase MicroExtraction (SPME), being intermediate method between dynamic and static headspace technique, was introduced in 1990 by Arthur and Pawliszyn as a solvent-free sample preparation technique (Arthur and Pawliszyn 1990). This technique uses a small volume of sorbent dispersed on the surface of small fibres to isolate and concentrate analytes from sample matrix either by immersion in the liquid matrix or in the headspace above solid

29 matrix (HS-SPME). Depending on the nature of the fibre coating, the compounds are absorbed or adsorbed by the fibre until the equilibrium is reached in the system (Pawliszyn 2000). The Figure 1.11 shows the design of a commercial SPME device. Because the amount of analyte extracted by the fibre is linked to the partition coefficient HS/fiber and is not a mirror picture of the molecules present in sample, SPME is mainly used as non-quantitative extraction technique. Internal or external calibration techniques are used to quantifying, however they can be affected by the matrix effects, causing significant differences in the partition coefficients and release rates for different compounds and consequently satisfactory results are rarely produced. Despite this difficulty of calibration, SPME has proven a performing technique for the trace analysis of volatiles offering more sensitivity and selectivity than a simple static headspace technique.

Figure 1.11. Design of the commercial SPME device (Harmon 1997).

In the field of food science, SPME technique has been used to investigate the volatile organic compounds in packaging materials (Hakkarainen 2008) and coupled to mass spectrometry (MS) has been reported as a powerful tool for screening purposes concerning volatile compounds (Felix et al. 2012). Additionally, SPME has also been used for the analysis of

30 components and contaminants including flavors, off-flavors, pesticides and other contaminants. Many different food types have been studied by SMPE, such as fruits, vegetables, plants, beverages and dairy products (Kataoka et al. 2000). For example, Van Aardt et al. (2001) reported the use of CAR/PDMS solid phase microextraction in static headspace at 45 °C for 15 min as an effective method for the recovery of acetaldehyde in milk and water media with detection levels as low as 200 and 20 ppb, respectively (van Aardt et al.

2001).

In the last decade, with the purpose of quantifying analytes in different matrices and increasing the sensitivity, multiple headspace extraction was coupled with solid phase microextraction. The theory of this method was presented by Ezquerro et al. (2003). Since the amount of compound extracted by the fibre is proportional to the initial amount and the peak area decays exponentially with the number of extractions of the same sample, it is possible to estimate the total peak area carrying out three or four successive extractions by HS-SPME.

The total peak area allows the quantification of analyte using an external calibration. This method eliminates the matrix effect in the quantitative analysis of solid samples and it is a solvent-free method. Its use to quantifying the volatiles compounds in polymers packaging materials (Ezquerro et al. 2003), in cork stoppers (Ezquerro and Tena 2005), in wine (Carrillo and Tena 2006), in tomatoes (Serrano et al. 2009) has already been reported.