Design Of The Different Mesco Formats

10.3.1 PDMS-coated fibre enclosed in an LDPE membrane

As a precursor of the MESCO [5] we tested membrane bags (13 x 2.5 cm) of 100 mm thick low-density polyethylene (LDPE) tubing (PolymerSynthesewerk Rheinberg, Germany), heat-sealed at both ends, in combination with a 100 mm PDMS-coated SPME fibre (Supelco, Deisenhofen, Germany) as collector phase (VS — 0.68 mL) and 25 mL of a 40/60 iso-propanol/water mixture (v/v) as inner fluid. LDPE is the membrane material also used in construction of SPMDs [6] and the PDMS-coated fibre is a rational tool for solid-phase microextraction of analytes from aqueous samples, and provides high enrichment factors for more hydrophobic substances [7]. Figure 10.1 shows the design of this permeation sampler. The coil spring (of stainless steel) prevents the fibre coating from a direct contact with the membrane. A serious shortcoming of this sampler is that the polymer-coated quartz glass fibre tip is fragile and difficult to handle during removal from and re-inserting into the steel needle of the commercial SPME syringe device.

10.3.2 PDMS-coated stir bar enclosed in a dialysis membrane bag (MESCO I)

This type, first described by Vrana et al. in 2001 [2,8], uses the PDMS-coated stir bar as collector phase. The stir bar is known under the trademark Twister™ (Gerstel, MUlheim/Ruhr, Germany) and is commonly used for solvent-free microextraction using the same principle as

■ PDMS-coated SPME fibre

■ LDPE membrane

■Fluid filling

Fig. 10.1. Construction of MESCO precursor [5].

Fluid filling

Fluid filling

Dialysis membrane

_ PDMS-coated Twister bar

Closure

Dialysis membrane

_ PDMS-coated Twister bar

Closure

an SPME fibre, but with a larger extraction capacity. Figure 10.2 shows a diagram of the sampler. Specifically, we tested dialysis membrane bags made of regenerated cellulose (Spectra/Por 6) with molecular weight cut-off of 1 kDa (3 x 1.8 cm), sealed at each end with a 35 mm Spectra/Por closure, in combination with Twister bars of 15 mm length coated with a 500-mm-thick layer of PDMS (VS - 24 mL) and 3mL bi-distilled water as membrane bag filling. Regenerated cellulose is a porous hydrophilic membrane material that enables widening the applicability to a broader polarity range of pollutants, including low-hydrophobic substances (log KOW<4). Unfortunately, this material has relatively low chemical and thermal stability and is subject to microbial degradation, which potentially leads to the damage of the sampler in natural surface waters during prolonged exposure of several weeks.

10.3.3 Silicone material enclosed in an LDPE membrane (MESCO II)

This sampler type combines [8,9] the advantages of a high-capacity collector phase with that of a more stable membrane material, LDPE. These membranes are hydrophobic, resistant to solvents and biodegradation and they can be heat-sealed. Furthermore, the relatively expensive and fragile Twister bar is substituted by a cheap silicone material (pieces of a tube or rod) as collector phase. Figure 10.3 shows the schematic design of the sampler. Additional investigations have shown the usefulness of these materials for an effective pre-concentration of several classes of persistent organic pollutants from water samples and the applicability of thermodesorption-GC-MS analogously to the processing of Twister bars [9,10]. The significantly enhanced volume of the collector phase

LDPE membrane

Fluid filling

Silicone rod/tube

Fig. 10.3. Schematic design of MESCO II [8].

LDPE membrane

Fluid filling

Silicone rod/tube

Fig. 10.3. Schematic design of MESCO II [8].

(> 100 mL) increases the maximum exposure time of the passive sampler in the field. A practical drawback of silicone tubes, when used as collecting phase in combination with water as fluid filling, is that remaining water droplets (inside the tube) can disrupt the GC-MS analysis.

Since 2004 we have focused our work on improvement of the promising MESCO II format with silicone rods enclosed. Several thicknesses of LDPE membrane were tested as well as other membrane materials, such as the dense polypropylene bag, usually used for membraneassisted solvent extraction of water samples in the laboratory [11]. Interestingly, it turns out in a preliminary laboratory study that this latter material is not useful for MESCO devices because it obviously prevents the transfer of substances to the inner receiving phase (sili-cone rod).

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