O THER T REATMENTS

2.3.7 O THER T REATMENTS

2.3.7.1 Stir Bar Sorptive Extraction Stir bar sorptive extraction (SBSE) is based on the partitioning of target analytes between

the sample (mostly aqueous-based liquid samples) and a stationary phase-coated stir bar [19]. Until now, only PDMS-coated stir bars are commercially available, restricting the range of applications to the extraction of hydrophobic compounds (organochlorine and organophosphorus pesticides) due to the apolar character of PDMS.

The experimental procedure followed in SBSE is quite simple. The liquid sample and the PDMS-coated magnetic stir bar are placed in a container. Then, the sample is stirred for a certain period of time (30 –240 min) until no additional recovery for target analytes is observed even when the extraction time is increased further. Finally, the stir bar is removed and placed in a specially designed unit in which thermal desorption and transfer of target analytes to the head of the GC column take place.

SBSE is usually compared and proposed as an alternative to SPME. The use of a PDMS-coated stir bar (10 mm length, 0.5 mm coating thickness) results in a significant increase in the volume of the extraction phase from ~0.5 mL for an SPME fiber (100 mm PDMS) to ~24 mL for a stir bar. Consequently, the yield of the extraction process is much greater when using a stir bar rather than an SPME fiber, both coated with PDMS. However, the greater coating area of magnetic stir bars is simultaneously its main drawback since the extraction kinetics are slower than for SPME fibers, and a high amount of interfering matrix compounds are coextracted with target analytes. Nevertheless, the simplicity of operation and its solventless nature make SBSE a very attractive technique, and the development of new stir bars coated with more polar and selective sorbents are expected in the near future.

2.3.7.2 Liquid Membrane Extraction Techniques Liquid membrane extraction techniques (supported liquid membrane, SLME, and

microporous membrane liquid –liquid, MMLLE, extractions) are based on the use a hydrophobic membrane, containing an organic solvent, which separates two immis- cible phases. These extraction techniques are a combination of three simultaneous processes: extraction of analyte into organic phase, membrane transport, and reex- traction in an acceptor phase. Chemical gradient existing between the two sides of

56 Analysis of Pesticides in Food and Environmental Samples the liquid membrane causes permeation of solutes. The compounds present in the

donor phase diffuse across the organic liquid membrane to the acceptor phase, where they accumulate at a concentration generally greater than that in the donor phase. Depending on the sample volume, different membrane unit formats for liquid membrane extraction are applied [20]. The main advantages of liquid membrane extraction over the traditional separation methods are small amounts of organic phases used, mass transfer is performed in one step, and it is possible to achieve high separation and concentration factors.

The distinguishing factor of the use of SLMs or MMLLE is the possibility of connecting them online with an analytical system. MMLLE is easily interfaced to gas chromatography and normal-phase HPLC, whereas SLM is compatible with reversed-phase HPLC. These online connections result in an improvement of the overall reliability of analysis, since the number of steps involved in sample prepar- ation is decreased and allows method automation. Additionally, significant reduction in analysis time is achieved. Till now, SLME and MMLLE have been successfully applied for enrichment of phenoxy acid, sulfonylurea, and triazine herbicides from environmental water samples. In those examples, similar or even better results were obtained in comparison with conventional sample preparation methods.

Thanks to their flexibility, SLME and MMLLE have proved to be interesting techniques to be combined with a second pretreatment technique (e.g., SPE). At this regard, detection limits as low as 30 mg=L have been achieved by combination of SLME and SPE for the determination of atrazine in fruit juices (orange, apple, blackcurrant, and grape) [21].