3. Results and discussion
3.1. Carrier-mediated counter-transport SDME
In the three-phase SDME, an analyte is first extracted from an aqueous donor phase (a1) into an organic layer (o) and then backextracted into an aqueous acceptor drop (a2). The EF at equilibrium is expressed as follows [31]:
equation(1)
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with the distribution coefficients D1 and D2 defined respectively as
equation(2)
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Here Ci,eq is the equilibrium analytical concentration of the analyte in phase i of volume Vi, as denoted by the subscript i. These two steps of extraction/backextraction are usually driven by controlling the pH of the two aqueous phases. For example, an acidic compound in a neutral form in a donor with a low pH can be enriched into a basic acceptor at a high pH, where it takes a negatively charged form. However, when analytes such as arsenic compounds are very hydrophilic or they have charges [32] and [33], the first step of extraction into the organic layer from the aqueous donor phase becomes difficult. Thus, the effectiveness of SDME can be significantly hampered. Fig. 2 shows that, without a carrier, very little extraction of the arsenic compounds occurred with 10-min SDME from the donor phase of pH 2 to the acceptor drop of pH 13, whereas fluoresceinamine was enriched by nearly 270-fold. One interesting finding was that DMA (pKa = 6.2) having two hydrophobic methyl groups, was extracted less than MMA (pKa1 = 4.1). We speculate that this occurred because DMA has more of a tendency toward protonation (at least partially) at pH 2 than MMA, as supported by quantum mechanical calculations using density functional theory (data not shown).