3.6.1 Diffusive Enrichment Through Membranes
As an early contribution to this research field, Hinze et al. (1990) conducted liquid
membrane separations. The supported enantiomer-selective membrane was formed
by dipping cellulose or other filter paper into an aqueous solution containing the
chiral OIM-6 ionene. A “blank” membrane was also prepared in an analogous
manner by dipping the paper in a solution that did not contain the ionene. The
experiments were conducted by placing 10 mL of ether containing the racemate of
the chiral compound in the left-hand side of the chamber and 10 mL of neat ether on
the opposite side. Both sides were stirred at about 30 rpm with magnet stirrers. The
amount of each isomer that permeated the supported liquid membrane was determined by HPLC analysis. The experiment was subsequently also carried out in a
similar fashion using the “blank” membrane. Significant enantiomeric excess
(EE) was determined for several of the analytes.
A slightly modified approach was used by Armstrong (1987), Armstrong and Jin
1987) and Möller (1993) who employed cyclodextrins as mobile carrier in an
aqueous phase for diffusive enantiomer separation through an impregnated
membrane. Applying this technique, Möller was the first to investigate the
enantiomer-selective separation through an impregnated membrane of the environmental pollutant α-HCH. In this case, the supported membrane was formed by
dipping a filter paper (50 μm cut-off, Schleicher & Schüll, Germany) into an aqueous
solution containing β-cyclodextrin (0.7 M). The impregnating solution consisted of
an aqueous 0.7 M β-cyclodextrin solution, 125 mM urea and 37.5 mM NaOH. The
latter two compounds were added in order to improve the solubility of β-cyclodextrin in water. The results obtained with 1 and 0.5 mM rac-α-HCH solutions during an
experimental period of 14 days are summarised in Table 3.2.
The most effective enantiomer separations were achieved by the experimental
approach used in Experiment 2, followed by Experiment 1, while the application of
glass fibre filters turned out to be less successful. Furthermore, it is worth noting that
a preferential permeation of (+)-α-HCH through the liquid membrane was observed
by Möller, where the maximum enantiomeric ratio was encountered after about
6 days. As Möller aimed at a complete enantiomer separation, which was not
accessible by this technique, no further optimisation was carried out. Therefore,
the actual potential of the liquid membrane technique is not clear yet.
However, more recent developments in the field of enantiomer-selective separation on membranes illustrate the strong potential for this research area. Especially,
the non-diffusive, dialytic separation of enantiomers through membranes is used
frequently for the separation of enantiomers, especially in pharmaceutical research
and development, which even led to the development of a sensor for online chemical
and biological detection (Piacentini et al. 2017; Tran et al. 2017; Yuan et al. 2017).
However, suitable methods for application in environmental pollutant research have
not yet been reported, although these methods seem promising and further research
in this field should be encouraged.
44
3 Enantiomer-Selective High- and Ultra- High-Performance Liquid Chromatography
As an early contribution to this research field, Hinze et al. (1990) conducted liquid
membrane separations. The supported enantiomer-selective membrane was formed
by dipping cellulose or other filter paper into an aqueous solution containing the
chiral OIM-6 ionene. A “blank” membrane was also prepared in an analogous
manner by dipping the paper in a solution that did not contain the ionene. The
experiments were conducted by placing 10 mL of ether containing the racemate of
the chiral compound in the left-hand side of the chamber and 10 mL of neat ether on
the opposite side. Both sides were stirred at about 30 rpm with magnet stirrers. The
amount of each isomer that permeated the supported liquid membrane was determined by HPLC analysis. The experiment was subsequently also carried out in a
similar fashion using the “blank” membrane. Significant enantiomeric excess
(EE) was determined for several of the analytes.
A slightly modified approach was used by Armstrong (1987), Armstrong and Jin
1987) and Möller (1993) who employed cyclodextrins as mobile carrier in an
aqueous phase for diffusive enantiomer separation through an impregnated
membrane. Applying this technique, Möller was the first to investigate the
enantiomer-selective separation through an impregnated membrane of the environmental pollutant α-HCH. In this case, the supported membrane was formed by
dipping a filter paper (50 μm cut-off, Schleicher & Schüll, Germany) into an aqueous
solution containing β-cyclodextrin (0.7 M). The impregnating solution consisted of
an aqueous 0.7 M β-cyclodextrin solution, 125 mM urea and 37.5 mM NaOH. The
latter two compounds were added in order to improve the solubility of β-cyclodextrin in water. The results obtained with 1 and 0.5 mM rac-α-HCH solutions during an
experimental period of 14 days are summarised in Table 3.2.
The most effective enantiomer separations were achieved by the experimental
approach used in Experiment 2, followed by Experiment 1, while the application of
glass fibre filters turned out to be less successful. Furthermore, it is worth noting that
a preferential permeation of (+)-α-HCH through the liquid membrane was observed
by Möller, where the maximum enantiomeric ratio was encountered after about
6 days. As Möller aimed at a complete enantiomer separation, which was not
accessible by this technique, no further optimisation was carried out. Therefore,
the actual potential of the liquid membrane technique is not clear yet.
However, more recent developments in the field of enantiomer-selective separation on membranes illustrate the strong potential for this research area. Especially,
the non-diffusive, dialytic separation of enantiomers through membranes is used
frequently for the separation of enantiomers, especially in pharmaceutical research
and development, which even led to the development of a sensor for online chemical
and biological detection (Piacentini et al. 2017; Tran et al. 2017; Yuan et al. 2017).
However, suitable methods for application in environmental pollutant research have
not yet been reported, although these methods seem promising and further research
in this field should be encouraged.
44
3 Enantiomer-Selective High- and Ultra- High-Performance Liquid Chromatography