Preparation of hollow fibers
Abstract Currently, there is no dedicated equipment or apparatus commercially available for HF/LPME. The extraction system needs to be built manually. This is, however, not very complicated, and will be described in detail in this section.
This article was written by Stig Pedersen-Bjergaard and Knut Einar Rasmussen, University of Oslo, Norway.
The equipment required for HF/LPME is very simple, and for manual extractions it consists of the following:
- Hollow fiber
- Sample vial
The hollow fiber is of key importance since it works as a support for the supported liquid membrane (SLM) and as a container for the acceptor liquid. Most HF/LPME work in the literature has been accomplished with polypropylene hollow fibers.
Polypropylene appears to be an excellent material for the purpose:
- It is resistant to organic solvents
- it provides the required flexibility and mechanical strength
- organic solvents of low polarity are strongly immobilized in pores of polypropylene
- no serious adsorptive problems are known between target analytes and the polymer itself.
The most popular one has been a 600 µm ID hollow fiber with 200 µm walls, 0.2 µm pores, and with a porosity of approximately 70 %. With this internal diameter, narrow microsyringe needles can be inserted into the lumen, to inject and to remove the acceptor liquid.
Polypropylene hollow fibers with IDs in the range 1.0-1.2 mm ID can also be used. The larger internal diameter simplifies handling of the acceptor liquid, and this type of hollow fiber can be used in combination with automation for the same reason.
The wall thickness of 200 µm is appropriate, and is a compromise between mechanical stability and extraction performance. A thinner wall is advantageous for the extraction performance since this provides a very thin SLM and therefore faster mass transfer, but this sacrifices the mechanical stability. The pore size of 0.2 µm also appears to be appropriate; organic solvents of low polarity are strongly immobilized in this type of hydrophilic porous structure, and the relatively open structures support efficient diffusion of target analytes.
For 2-phase HF/LPME in combination with GC, it is recommended to clean the hollow fibers in acetone to remove polymeric additives and impurities that can be present. This can effectively reduce the background observed in the gas chromatogram. For 3-phase applications in combination with LC or CE, it is normally not required to do any cleaning of the hollow fiber.
Different configurations of the hollow fiber can be used as illustrated:
Each end of the U-shaped hollow fiber is connected to a guiding needle. One needle is use for introduction of the acceptor liquid, the second one is used for removal of the acceptor liquid. Advantage of the U-shape configuration is the large contact area between sample and the hollow fiber.
For automation, a rod-like configuration with a 1.0-1.2 mm ID hollow fiber is more appropriate as illustrated. Here, a syringe in an autosampler or in a robotic system can be inserted directly to the bottom of the hollow fiber to inject and remove the acceptor phase.
Another very interesting hollow fiber configuration is the attachment of a small piece of a hollow fiber to a microsyringe needle as illustrated. In this configuration, the syringe serves to introduce the acceptor liquid, to support the hollow fiber, and as injection syringe for the subsequent chromatographic analysis. The hollow fiber can be sealed in the bottom to enable 3-phase extractions, or open at the bottom for 2-phase extractions and for 3-phase extractions.