The Separation Membrane

The dialytic transport across thin membranes can be described in eqn [33]:

C,MD and ci,MA are the solute concentrations in the membrane at the interfaces with the donor and the

Table 1 Dialysis procedures

Objectives in a microanalytical scale Donor solution Acceptor solution

Table 1 Dialysis procedures

Objectives in a microanalytical scale Donor solution Acceptor solution

Puri fication

Quiescent or slowly flowing, small

Flowing or stirred, large volume

sample volume

Enrichment

Flowing or stirred, large sample volume

Slowly flowing or gently stirred, small volume

Reagent addition

Stirred or flowing, large reagent volume

Quiescent or slowly flowing, small volume

Separation

Quiescent or slowly flowing, small

Quiescent or slowly flowing, small volume

sample volume

Figure 5 Frequently used dialysis set-up: (A) meander cell with a flat membrane, (B) dialysis probe with a flat membrane M, (C) hollow fibre membrane cell, (D) hollow fibre dialysis probe. ID, IA, inletsto the donor and the acceptor chamber; OA, OD, outlets from the acceptor and the donor chamber.

acceptor solutions, respectively. Linear concentration gradients can be assumed in thin membranes.

The separation membrane should be considered particularly with regard to selectively but also with regard to the overall mass transfer kinetics. The membrane material determines the transport mechanism, which influences the selectivity of the separation process in particular. Table 2 gives an overview about the most important membrane materials and the dominant transport mechanisms. Classic dialysis through microporous membranes causes a loss of sensitivity with respect to the following detection or determination procedure. So-called selective dialysis across gas-filled membranes or SLMs enables an analyte enrichment to be performed. The selectivity of the SLM technique can be enhanced by the addition of selectively reacting ligands to the liquid membrane phase. When charged ions are complexed and transported through these membrane systems elec-troneutrality must be maintained. In many cases ion pairs with selected counter ions are transported through the membrane. When the ligand is dissolved in the liquid membrane phase and the counter ion cannot transverse the membrane the analyte ion transport is coupled with a back-diffusion of an ion with the same electric charge. A similar situation can be found in ion-exchange membranes, which are used to enrich ions by Donnan dialysis.

Gas dialysis through hydrophobic and micropor-ous membranes is a fast transport process compared with the other transport mechanisms. The diffusion constants in the gas phase are several orders of magnitude greater than in liquid and solid phases. The selectivity of the membrane transport is determined by the ratio of the partial pressure p, of the analyte to the total pressure p in the membrane pores. In small pores the condensation and adsorption kinetics of the gases also have to be taken into account.

Gas dialysis across homogeneous membranes is generally more selective. The different solubilities of the gases in the membrane material are additional selection factors. The mass transport rate is considerably smaller than those in microporous membranes.

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