Continuing our discussion on modern extraction techniques for Pesticides residue analysis, we will be looking now at Solid Phase Microextraction (SPME) and Microwave Assisted Extraction (MAE) techniques.
3. Solid phase microextraction (SPME)
Solid phase microextraction, one of the newest extraction techniques, is widely used in the pesticide residues analysis in samples from different origins, due to the fact that purification and concentration of the sample extract (analytes of interest) are running simultaneously.
The basic part of the SPME system is the SPME syringe which visually resembles a chromatographic syringe, except for the fact that it contains a 1 cm long fiber located within a syringe needle, which is made of an appropriate polymer deposited on the holder of fused silica. Microextraction process is based on the redistribution of analytes between microextraction fiber and sample matrix, i.e. on the selective sorption of target analytes in the active layer of the fiber and direct desorption in the chromatograph injector (thermal in the case of the GC, i.e. by solvent elution in the case of LC – liquid chromatography). The basic principle of analytes microextraction from the solution is shown in Figure 2.
. Fig. 2. Procedure for microextraction of analytes from solution
Before the analysis, the fiber is drawn into a metal tube of the SPME syringe. After breaking through the vial containing the sample, the fiber is pulled out from the syringe i.e. it is exposed to the sample by lowering the syringe plunger. After specific time, the fiber with the sorbed analytes is drawn into the needle, which is then pulled out from the vial. Analytes desorption from the fiber is performed by introducing a SPME syringe needle into the injector of the chromatographic system.
SPME is an equilibrium technique, where analytes are distributed between the three phases: sample, gas phase and fiber. The fiber does not extracts all analytes present in the sample, but by the proper calibration, this technique can be used for successful quantification. The amount of analytes that would be adsorbed on the fiber will depend on the thickness and polarity of the active fiber layer, sampling mode (direct sampling – microextraction from solution, »DM-SPME« and headspace sampling -microextraction from gas phase, »HS-SPME«), the nature of the sample and the analyte (analyte polarity, its molecular weight, pH value, nature of matrix), the mode and speed of the sample mixing, the SPME duration, the temperature at which it is performed, and so on. Today, about thirty different fiber types are in use (different types of polymers and their thickness), so when selecting the fiber it is necessary to take into consideration several factors: molecular weight, structure and polarity of the analyte molecules, the polarity of fibers, the mechanism of extraction (used sampling mode), the detection limit and range of linearity that is desired to be achieved. In order for a fiber to extract specific compounds from a given matrix, it must have a much higher affinity for the given analytes than the matrix, where the general rule applies: non-polar analytes are more efficiently extracted by non-polar active fiber layer. The research in the field of pesticide residues has indicated that, in the most of the cases, fibers with extremely non-polar polydimethylsiloxane (PDMS) and highly polar polyacrylate (PA) active layers are most effective in the analysis of samples of different origin . After fiber selection, it is necessary to determine optimal conditions for analytes transfer in the chromatographic system. Adsorbed analytes are desorbed from the fiber by introducing the SPME syringe needle into the injector. Defining the parameters of desorption involves determination of the optimal injector temperature, flow of the carrier gas and desorption time in the case of GC, i.e. proper choice of elution solvent, its flow rate and desorption time, in the case of HPLC.
There are a significant number of SPME applications in pesticide residues analysis.For example, by using mixed PDMS/DVB (polydimethylsiloxane/divinyl-benzene) fiber, Beltran et al. (2003) provided satisfactory analytical parameters for SPME determination of pyrethroids in tomato samples; Wennrich et al. (2001) for determination of OCPs in kohlrabi, lettuce and tomato; Parrilla Vázquez et al. (2008) for determination of pyrethroids in cucumber; Vega Moreno et al. (2006) for determination of OCPs in soil, while Ravelo-Pérezet et al. (2008) developed a method for determination of pesticides belonging to different chemical groups in tomatoes.
4. Microwave-assisted extraction (MAE)
Microwave-assisted extraction (MAE) is a technique based on usage of the microwave energy, and where compounds can be extracted more selectively and rapidly, with similar or better recovery compared to conventional extraction processes.
The MAE effects a direct migration of the desired components out of the matrix, as a result of selective energy application into the matrix. High method efficiency is a result of the matrix macrostructure destruction. During the MAE of plant material, microwave rays travel freely through the solvent and interact selectively with the free matrix water causing localized heating. The result is non-uniform temperature rise with more pronounced effects where the free water is in larger proportions. The result is a volume expansion within the systems. The walls of these systems cannot accommodate the high internal pressures and rupture spontaneously, allowing the organic contents to flow freely toward the relatively cool surrounding solvent that solubilizes them rapidly.
Considering the complexity of plant material and its non-uniformity regarding different amount of free water, the MAE advantage can be noticed. Particularly, by providing different microwave energy levels, it is possible to selectively rupture some systems over others, thus it is possible to develop schemes that will effect the selective extraction of the given systems contents. MAE is also a promising technique for soil samples, particularly owing to its possibility to control temperature, pressure and microwave energy, as well as to perform a few extractions simultaneously. For method optimization, several variables such as volume and solvent composition, extraction temperature and time, are usually studied.
In MAE, In order to heat a solvent, part of it must be polar with high dielectric constant to absorb microwave energy efficiently. Nonpolar solvents with low dielectric constants can be also used, by adding certain amount of polar solvent that absorbs the microwave radiation and passes it on to other molecules. For example, hexane and toluene can be modulated by the addition of small amounts of acetone or methanol. The first use of MAE technique for pesticide residues determination (parathion and bromophos in maize, soya bean, fava bean, walnut, cotton seed and soil), was reported by Ganzler et al. (1986). In 1993, Onuska & Terry evaluated the extractability of various pesticides in sediment samples, and in the same year Steinheimer (1993) extracted atrazine from soil samples. These works were followed by an extensive paper by the group of Lopez-Avila (1994), presenting procedures for extraction of organic compounds such as OCPs, among others, from soils and sediments. In 1997, Pylypiw et al. used a MAE for determination of several pesticides in beet, cucumber, lettuce, pepper and tomato. The results show that MAE is a viable alternative for determination of atrazine and OPPs in orange peel (Bouaid et al., 2000), carbendazim, diethofencarb, azoxystrobine, napropamide and bupirimate in strawberries (Falqui-cao et al., 2001), fenitrothion in beans (Diagne et al.,2002) and pyrethroids in strawberries (Sanusi et al., 2004). In 1994, 20 OCPs were extracted from six marine sediments and soils (Lopez-Avila et al.,1994), and in a subsequent study the list of compounds was expanded to nearly a hundred OCPs and OPPs (Lopez-Avila et al., 1995).
From economical and practical aspects, MAE is a strong competitor to other recent sample preparation techniques. The main MAE advantages are the low temperature requirement, high extraction efficiency, complete automation, and the possibility of extracting different samples at the same time without interference. The main disadvantage of MAE seems to be the lack of selectivity resulting in the co-extraction of significant amounts of interfering compounds. Additional clean-up is therefore needed before chromatographic analysis. Apart from that, the poor efficiency of microwaves when either the target compounds or the solvents are non-polar, or when they are volatile, can be regarded as another disadvantage. Besides, it is important to notice that the application of microwave energy to flammable organic compounds, such as solvents, can pose serious hazards in inexperienced hands, thus an extraordinary level of safety and attention to details when planning and performing experiments must be used by all personnel dealing with microwaves.
TO BE CONTINUED
Rada Ðurović and Tijana Ðorđević : Modern Extraction Techniques for Pesticide Residues Determination in Plant and Soil Samples; Institute of Pesticides and Environmental Protection, Belgrade
Posted by Muyiwa Adebola