Chemistry Reference
In-Depth Information
gasoline. If all the alcoholic eluent could be recycled and reused as alternative energy, the mobile phase after
the LC analysis is a large source of alcohol fuel.
10.2.2 Ionic liquids
An ionic liquid, also called liquid electrolyte or ionic fluid, is a salt that could be used as a solvent. Because
of the property of low vapor pressure, it reduces the release of substances in the environment and can be used
as a green solvent [3, 4]. Ionic liquids also could be used as clean media for green sample extraction [5, 6].
They contain many merits in analytical chemistry, such as negligible vapor pressure, good thermal stability,
tunable viscosity and miscibility with water and organic solvents [7]. Recently, ionic liquids have been used
as additives in LC to improve the peak shape by pairing ion mechanism [8-12]. For green solvent selection,
an ionic liquid could be a potential candidate for LC [13]. The physicochemical and solvation properties of
ionic liquids are dependent on the combination of cations and anions; many such combinations of cations and
anions of ionic liquids have been reported and their properties are well documented [14-19]. General ionic
liquids used in separations are listed here and shown in Figure 10.1 [18]:
(a) alkylimidazolium
(b) dimethyldinonylammonium bromide
(c) (R)- N,N,N-trimethyl-2-aminobutanol-bis(trifluoromethanesulfone)imidate
(d) phenylcholine-bis(trifluoro-methanesulfone)imidate
(e) ethylcholine-bis(trifluoromethanesulfone)imidate
(f) undecenoxycarbonyl-L-leucinol bromide (L-UCLB)
(g) undecenoxycarbonyl-L-pyrrolidinol bromide (L-UCPB)
(h) N-alkyl-N-methylpyrrolidinium bromide
(i)
tetraalkyl-ammonium tetrafluoroborate
Ionic liquids are widely used in many analytical areas, such as water sample extraction [20], separation of
ephedrines [21], extraction of anabolic androgenic steroids [22], determination of aromatic analytes [23, 24],
determination of active compounds from plants [25-27], quantification of phenoxyl acid herbicides and
phenols [28], and analysis of pharmaceutical components [29-31].
10.2.3
Supercritical Fluid Chromatography (SFC)
In the past, scientists tried to find green solvents for separation, extraction and purification purposes. Water
is the best choice for green LC, but using water for LC as the mobile phase is an ideal condition. For practical
experimentation, it is difficult to use pure water as the eluent. Hence, scientists are attempting to discover a
new mobile phase in LC. Gas chromatography (GC) is a kind of chromatography that uses inert gas as the
carrier gas. Could gas be used as a mobile phase for LC? This is an amazing concept for LC. Supercritical
fluid has special properties between gas and liquid, so widely used for desired analytes extraction [32-36].
Supercritical fluid chromatography (SFC) is a special technique that combines the advantages of LC and GC.
SFC usually uses carbon dioxide as the mobile phase and this technique is nearly a solvent-free method with
the exception of adding methanol, ethanol or isopropyl alcohol as the modifiers. Because of the physical and
chemical properties, such as low viscosity and high diffusivity, SFC coupled with switching valve is suitably
used in chiral separation [37-42]. Different additives can give the best resolution to chiral compounds and
these compounds are affected obviously by charged additives [42]. Switching valve is a smart design to use
as the modifier or column selector. Choosing different modifiers coupled with chiral columns to separate
different chiral compounds can obtain good separation efficiency (see Figure 10.2).
