Page 66 - Subyek Encyclopedia - Encyclopedia of Separation Science

Sepsci*21*TSK*Venkatachala=BG
I / CHROMATOGRAPHY 61

The retention mechanism for MEKC strongly duces unacceptable peak shapes and poor resolution.
resembles that of RPC with two important However, SFC Rnds applications in many areas
differences. Surfactants used to generate the where GC and LC are unsatisfactory, for example
pseudo-stationary phases provide a different type in the separation of middle molecular weight
of sorption environment to solvated chemically compounds, low molecular weight synthetic poly-
bonded phases and, therefore, different selectiv- mers, fats and oils, enantiomers, and organometallic
ity. The intrinsic efRciency of MEKC is signiRcantly compounds.
greater than that of LC and enhances resolution,
although the peak capacity is lower owing to the
Rnite migration window for MEKC. A signiRcant Instrumentation
number of RPC-type applications are now performed Modern chromatographic methods are instrumental
by MEKC, indicating that the method can compete techniques in which the optimal conditions for the
favourably with RPC for some separations. MEKC is separation are set and varied by electromechanical
inherently a microcolumn technique, providing ad- devices controlled by a computer external to the col-
vantages in coupling to other chromatographic sys- umn or layer. Separations are largely automated with
tems and for the analysis of samples only available in important features of the instrumentation being con-
small amounts. Disadvantages include sample intro- trol of the Sow and composition of the mobile phase,
duction problems, limited dynamic sample concen- provision of an inlet system for sample introduction,
tration range, and poor limits of detection for trace column temperature control, online detection to
analysis (because of the very small sample sizes in-
monitor the separation, and display and archiving of
volved). Selectivity optimization is determined largely
the results. Instrument requirements differ signif-
by the choice of surfactant and the use of mobile- and
icantly according to the needs of the method em-
stationary-phase additives.
ployed. Unattended operation is usually possible by
Supercritical Suids have solvating properties that
automated sample storage or preparation devices for
are intermediate between those of gases and liquids.
time-sequenced sample introduction.
In addition, supercritical Suids are compressible so
that their density and solvating power can be varied
by changing external parameters, such as pressure Gas Chromatography
and temperature. This feature is unique to supercriti- For GC a supply of gases in the form of pressurized
cal Suids and represents a major approach to selectiv- cylinders is required for the carrier gas and perhaps
ity optimization. Temperature not only affects also for the detector, for operating pneumatic valves,
density, but may also inSuence the vapour pressure of and for providing automatic cool-down by opening
low molecular weight solutes, promoting some GC- the oven door. To minimize contamination, high pu-
like character to the retention mechanism. The most rity gases are used combined with additional puriRca-
common mobile phase is carbon dioxide, a relatively tion devices. Each cylinder is Rtted with a two-stage
nonpolar Suid. More-polar Suids, such as water, am- pressure regulator for coarse pressure and Sow con-
monia or methanol, tend to have unfavourable criti- trol. Fine tuning is achieved using metering valves or
cal constants or are highly corrosive to column or by electronic pressure control combining electro-
instrument components, limiting their use. Mixed mechanical devices with sensors to compensate auto-
mobile phases can be used to vary selectivity, such as matically for changes in ambient conditions. The col-
carbon dioxide}methanol mixtures, but miscibility umn oven is generally a forced air circulation thermo-
problems and high critical constants for the mixed stat heated resistively and capable of maintaining
mobile phases may restrict the range of properties a constant temperature or of being programmed over
available. SFC can provide faster separations than time. The detector and sample inlet are generally
LC, but it is more restricted than LC in the choice of thermostated separately in insulated metal blocks
mobile phases and retention mechanisms to vary sel- heated by cartridge heaters. The most common
ectivity. SFC is compatible with most detection op- method of introducing samples into a GC inlet is by
tions available for both GC and LC. All practical means of a microsyringe (pyrolysis, headspace and
applications of SFC occur signiRcantly above ambient thermal desorption devices can be considered special-
temperature, which is unsuitable for the separation of ized sample inlets). For packed-column injection
some thermally labile compounds and most bio- a small portion of (liquid) sample is introduced
polymers. Supercritical Suids such as carbon dioxide by microsyringe through a silicone septum into
are unable to mask active sites on typical column a glass liner or the front portion of the column,
packings, resulting in unsatisfactory separations of which is heated and continuously swept by carrier
polar compounds owing to adsorption, which pro- gas. The low sample capacity and carrier gas Sow