Chemistry Reference
In-Depth Information
stationary phase with THF/hexane as the mobile phase, suggesting that multipoint interactions are required for
measurable solute retention. Chromatographic separation of phenols, crown ethers and estrogens on water-ice
is possible. The ice surface is expected to provide two different adsorption sites coming from the OH and O
dangling bonds. Although the solute partition into the quasiliquid layers is also considered, the dependence of
the retention times of the THF concentration implies that the interaction of solutes with the water-ice surface
rather than the partition into the quasiliquid layers is responsible for solute retention. A retention model
suggests that the number of adsorption sites for a crown ether depends on its ring size, whereas two sites are
involved for the retention of phenols having two hydroxyl groups. Although hydroxyl groups can act as both
a hydrogen bond donor and an acceptor, the interaction with the ice OH sites, which are exposed to the
surroundings in comparison with the ice O sites, is more important. More recently, they developed chiral ice
chromatography and accomplished the chiral separation of hexobarbital or 1,1
′
-bis-2-naphthol. Water-ice
particles simultaneously doped with
-cyclodextrin and a salt enable chromatographic separation of
enantiomers without synthetic processes, and enhanced chiral recognition occurring in the liquid-water phase
coexistent with the solid-phase [51]. Water-ice has various interesting physical properties that other solid
materials do not possess. Its low refractive index is a representative one. A liquid-core wave guide has been
successfully fabricated on a water-ice slab and used for a flow-injection cell [52]. It is expected that ice
chromatography on such a lab-on-ice may provide another pathway to novel analytical methodologies.
β
20.10
High-temperature liquid chromatography
High-temperature operation in RP-HPLC provides an opportunity to reduce the quantity of organic solvents
used in a mixed organic-water mobile phase, and has promise for Green Analytical Chemistry [53-55]. High-
temperature liquid chromatography (HTLC) increases the analytes mass-transfer rates, and decreases the
column back pressure and total analysis time. High-temperature separation (100-200°C) has been shown to
improve the analyte resolution by decreasing the mobile phase viscosity, and by increasing the diffusion rate
of the sample species, thus increasing mass transfer of the analytes to the stationary phase and, thereby,
decreasing the peak width. High temperature, as an optimization parameter in the separation process of the
RPLC, has been widely investigated due to a recent finding of an alternative stationary phase, which has a
high thermal stability at high temperature. Among the various stationary phases, zirconia-based one has
received a great deal of attention, since it is extraordinary stable under a tremendous thermal and chemical
condition. Sanagi
et al
. established a preliminary study on the separation of triazole antifungal agents using a
polybutadiene-coated zirconia column at 100-150°C [56]. Clear separations were achieved when using 100
%
purified water as the organic-free eluent. Excellent limits of detection down to pg order were achieved for the
separation of antifungal agents under the optimum conditions. Van't Hoff plots for the separations were linear,
suggesting that no changes occurred in the retention mechanism over the temperature range studied. Boer
et
al
. studied the potential of HTLC on-line combination with a screening system for bioactive compounds
against the enzyme cathepsin B [57]. Samples were separated by HTLC utilizing a zirconia-based stationary
phase, and subsequently analyzed by an on-line continuous-flow enzymatic assay. Detection was performed
by electrospray ionization mass spectrometry, revealing both the bioactivity and the molecular mass of the
bioactive compounds. Compared to conventional RPLC, the amount of methanol necessary for separation
could be decreased to only 10
%
, which improved the compatibility of LC with a biochemical assay.
20.11 Ionic liquids
Ionic liquids (ILs), considered to be 'green' chemicals are widely used in many aspects of science and
technology [58, 59]. Instead of an organic solvent, ILs have received much attention as ecologically friendly
