S OLID -P HASE M ICROEXTRACTION As it has been stated previously, SPE has demonstrated to be a very useful procedure

2.3.5 S OLID -P HASE M ICROEXTRACTION As it has been stated previously, SPE has demonstrated to be a very useful procedure

for the extraction of a great variety of pesticides in food and environmental analysis. However, although in a lower extent than LLE, this technique still requires the use of toxic organic solvents and its applicability is restricted to liquid samples. With the aim of eliminating these drawbacks, Arthur and Pawliszyn introduced SPME in 1989 [16]. Its simplicity of operation, solventless nature, and the availability of commercial fibers have made SPME to be rapidly implemented in analytical laboratories.

As depicted in Figure 2.8, the SPME device is quite simple, and just consists of a silica fiber coated with a polymeric stationary phase similar to those used in gas chromatography columns. The fiber is located inside the needle (protecting needle) of a syringe specially designed to allow exposure of the fiber during sample analysis. As in any SPE procedure, SPME is based on the partitioning of target analytes

Sample Handling of Pesticides in Food and Environmental Samples 53

1. Introduce needle

2. Expose fiber 3. Retry fiber

in sample vial

to sample

Syringe

Protecting needle

Silica fiber coated with a sorbent

FIGURE 2.8 SPME device and typical mode of operation.

between the sample and the stationary phase and consists of two consecutive steps, extraction and desorption. An intermediate washing step can also be performed.

2.3.5.1 Extraction The extraction step can be performed both by exposure of the fiber to the head-

space (restricted to volatile compounds in liquid or solid samples) or by direct immersion of the fiber into the sample (aqueous-based liquid samples). As described in Figure 2.8, the experimental procedure is very simple. Firstly, the fiber is inside the protecting needle which is introduced into the sample vial. Then, the fiber is exposed to the sample to perform extraction by sorption of the analytes to the stationary phase. Finally, the fiber is retried inside the needle for further desorption and the whole device removed.

Obviously, a proper selection of the SPME sorbent is a key factor in the success of the analysis. In general, the polarity of the fiber should be as similar as possible to that of the analyte of interest. In this sense, there are nowadays a great variety of fibers commercially available that covers a wide range of polarities (i.e., carbowax=DVB

for polar compounds or polydimethylsiloxane [PDMS] for hydrophobic compounds). In addition, both the fiber thickness and the porosity of the sorbent will influence the final extraction efficiency. Besides, other physical and chemical parameters such as tempera- ture, exposition time, agitation, pH, or ionic strength (salting-out effect) of the sample can be optimized. As an example, it can be mentioned the extraction of dinoseb, an alquil-substituted dinitrophenol, in waters. The SPME of this compound can be favored

54 Analysis of Pesticides in Food and Environmental Samples by using a polyacrylate fiber and by adding 10% of NaCl at pH ¼ 2 due to the produced

salting-out effect and the lower ionization of dinoseb at low pH values. Finally, concerning extraction, it is interesting to mention that from the math- ematical model governing SPME, it can be concluded that when sample volume is much higher than the fiber volume, the extraction efficiency becomes independent of the sample volume. Although it is not applicable for laboratory samples (low volumes), this earlier fact makes SPME a very interesting tool for in-field sampling procedures, since the fiber can be exposed to the air or directly immersed into a lake or a river regardless of the sample volume.

2.3.5.2 Desorption Desorption can be performed thermally in the injection port of a gas chromatograph,

or by elution of the analytes by means of a suitable solvent. In the latter case, desorption can be carried out in a vial containing a small volume of the solvent to be further analyzed by chromatographic techniques or eluted with the mobile phase on an especially designed SPME –HPLC interface.

Thermal desorption of the analytes in the injector port of the GC instrument is based on the increase of the partition coefficient gas fiber with the increasing temperature. In addition, a constant flow of carrier gas inside the injector facilitates removal of the analytes from the fiber. The main advantage of the thermal desorption is the fact that the total amount of extracted analytes is introduced in the chromato-

graphic system and analyzed, thus compensating the low recoveries usually obtained in the extraction step. However, unfortunately, thermal desorption cannot be used for nonvolatile or thermolabile compounds, thus necessary to use desorption with solvents. The procedure is similar to SPE elution but, in this case, the fiber is immersed in a small volume of elution solvent and agitated or heated to favor the transfer of the analytes to the solvent solution. A fraction of this extract or, for some applications, an evaporated and redissolved extract, is subsequently injected into the chromatographic system.

Recently, there are commercially available interfaces allowing the direct coup- ling of SPME to liquid chromatography. The coupling is similar to that described earlier in Figure 2.7 for SPE –HPLC but placing a specially designed little chamber instead of a precolumn in the loop of a six-port injection valve. This interface allows desorption of the analytes by the chromatographic mobile phase, where the total amount of compounds extracted introduced in the chromatographic system.