Chemistry Reference
In-Depth Information
the fluids also limit their application. The polarity and solvating power of carbon dioxide, the most popular
fluid, are too low for many common analytes. This substantially limits the use of supercritical CO
2
as a
method that could universally replace Soxhlet extraction. However, supercritical extraction (SFE) of non-
polar analytes is very rapid and efficient, reducing energy consumption and the use of volatile organic
solvents.
Supercritical fluid chromatography (SFC) using liquid carbon dioxide as an eluent is the most popular
green alternative to traditional liquid chromatography (LC). It is used mainly as a preparative mode in
chromatography. Supercritical fluid chromatography typically uses carbon dioxide above or near its critical
temperature and pressure bar, combined with an organic modifier such as methanol or ethanol. Most of the
available SFC analytical instrumentation is also LC-compatible. The pumps and mixing chambers are already
at high pressure; however, the pump heads must be cooled. The major alteration is the need to keep the UV
detector at high pressure. SFC provides faster separations than LC and uses minimal amounts of organic
solvents. SFC mobile phases are carbon dioxide-based, but may contain up to 15
ethanol, methanol or
acetonitrile. Theoretically, mixed-phase SFC solvents are less environmentally benign than single-phase CO
2
,
but they are significantly easier to dispose of or recycle than mixed organic-aqueous LC solvents. Capillary
SFC provides high-resolution chromatography at much lower temperatures than gas chromatography and
facilitates fast analysis of thermolabile compounds [26]. This type of SFC has been viewed as an extension
of gas chromatography (GC) in which some of the thermal energy required for mobilizing the solutes is
replaced by solvation energy. The use of capillary SFC grew swiftly, mainly because of the novel combination
of supercritical mobile phases and open tubular fused silica column technology. GC detectors such as flame
ionization, electron capture, nitrogen phosphorus, and sulfur chemiluminescence were popular with SFC.
SFE and SFC are particularly advantageous when they use a carbon dioxide-based phase. Carbon dioxide
gas does cause global warming but the carbon dioxide used in these procedures contributes no new chemicals
to the environment, because it is usually a by-product of other chemical reactions or is obtained directly from
the atmosphere, and the gas is usually recycled when it is used in large quantities.
In liquid chromatography, when elevated temperatures are required for efficiency, using only aqueous
streams is often proposed. S. Heinisch and J-L. Rocca recently reviewed elevated temperature liquid
chromatography [27]. They found that, in addition to the non-toxicity advantage of superheated water, as
discussed above, it is also possible to use flame ionization, low wavelength UV, inductively coupled plasma
mass spectrometry and even nuclear magnetic resonance spectroscopy detectors, with hot deuterium oxide as
the mobile phase [28, 29, 30]. Temperature programming instead of gradient elution could be practical in
some situations. For example, for separations using micro- or nanocolumns, when gradient elution is difficult
to operate at very low flow-rates in a particular system, temperature programming is generally recommended
with the capillary columns because heat transfer is more effective with small inner diameters. However,
temperature programming is sometimes problematic. A wider temperature interval than ambient to 200°C is
usually required for programming. Commercially available equipment is not yet able to cover such a wide
range of temperatures. The required temperature ramp is often too steep (>20°C min
−1
) to be applied to HPLC
separations because heat transfer in liquids is very slow compared with heat transfer in gas. Heating a steel
HPLC column (a material with very low thermal conductivity) can be too slow as well. Temperature
programming is mainly used to improve isocratic isothermal analysis rather than to replace gradient elution.
A frequently overlooked aspect of elevated temperature chromatography is the fact that it consumes more
energy. G. van der Vorst
et al
. [31] performed an
exergetic
life cycle analysis of a chromatographic separation
of enantiomers of a racemic mixture of phenyl acetic acid derivatives in order to compare preparative HPLC
with SFC. Exergy refers to the maximum work that is required to bring a system into equilibrium during a
process [32]. The energy required to produce a chromatogram can be substantial for both HPLC and SFC.
Considering instrumentation alone (in their terminology - '
%
α
system boundary'), the exergy consumption of
a preparative HPLC technique is about 25
%
higher than that of SFC, due to its inherently higher use of
