Transport Mechanisms in SLM

In SLM extraction, the transport mechanism is influenced primarily by the chemical characteristics of the analytes to be extracted and the organic liquid in the membrane into which the analytes will interact and diffuse. Analyte solubility in the membrane and its partition coefficient will have the main impact on separation and enrichment. Analyte transport in SLM extraction can be substantially categorized into two major types one is diffusive transport (or simple permeation) and the other covers facilitated transport (or carrier-mediated transport).73 

On the other hand, acidic compounds (A ) in the sample will already be charged and will thus not be enriched. Moreover, the neutral species (N) could be extracted, but not enriched, and the size and charge of macromolecules (if they are ionizable) will prevent them from entering the membrane. But even if some macromolecules do enter the liquid membrane, their low diffusion coefficients will cause a slow rate of transport. Accordingly, enrichment of small basic compounds and 

Schematic representation of selective SLM extraction of a small basic compound (B). 

The other type of transport, facilitated transport, involves an analyte-specific carrier mixed in the organic membrane phase at a certain concentration. The solubility of the carrier in the surrounding aqueous phases has to be very low to prevent leakage, which would prevent specific analyte transport across the membrane. 

FIGURE 4.6 Illustration of the SLM-coupled countertransport of an anionic herbicide (glyphosate) metabo lite, AMPA- (left) and a cationic metal ion, M+ (right). 

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The discussion in Section 4.4.1.3 on transport mechanisms in SLM has manifestly demonstrated another facet of tuning analyte-selective extraction. For example, Figure 4.5 clearly demonstrates the selective extraction of a basic compound—all that is required here is a simple adjustment of the pH on either side of the membrane. Also, Figure 4.6 neatly illustrates the possibility of performing such selective extraction of anionic and cationic species in another transport mechanism that employs selective carriers. Thus, by fine-tuning the chemistry/composition of the sample, membrane liquid, and acceptor phases, analyte-selective extraction can be tailor-made. 

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