Page 372 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN0011A-541 July 25, 2001 17:27

Organic Chemistry, Compound Detection 475

FTIR is a powerful and highly specific detection tech- lose, alumina, polyamides and ion-exchangers, consider-
nique. The combination of SFC with FTIR has tremendous able versatility is available in the type of substances that
scope because of the possibility of separating and identi- can be separated.
fying compounds which are not amenable to GC–FTIR TLC is a microanalytical procedure and provides for
analysis. separations and at least tentative identification of sub-
SFC–FTIR is a very useful technique in characterizing stances in the milligram microgram, nonogram, and even
chemical additives in polymers. Many of these additives, picogram (pg) range. Adsorption TLC is very sensitive to
which include slip agents, plasticizers, UV absorbers, light differences in configuration that affect the free energy of
stabilizers and antioxidants are thermally labile or have a adsorption onto the layer surface and is, therefore, well
high molecular mass which makes them difficult to ana- suited to the separation of structural isomers. Quantitative
lyze by other methods. estimation of the separated compounds is carried out in
The biological or pharmacological activity and effec- situ by densitometric estimation of the TLC plates.
tiveness of chiral molecules depend largely on their con- TLC has recently been transformed by the introduction
figuration. Often only one of the enantiomers is pharma- of new and better plates, accurate and precise autospotters,
cologically beneficially active, while the other enantiomer new development techniques, and rapid scanning densit-
may not only be inactive but also toxic. Most of the chiral ometers into a sophisticated, instrumentalized, and quan-
stationary phases for TLC, GC, and LC of the chiral- titative technique.
cavity type act by allowing selective occlusion or interca- With respect to the TLC plates, there are two major
lation of one enantiomer into chiral cavities in the phase advances. The first was the introduction to high perfor-
matrix. mance TLC (HPLC) plates produced from silica gel of
between 5 and 10 µm compared to the 12 to 25 µm
commonly used for standard TLC plates. This resulted
B. Superficial Fluid Chromatography (SCF)
in more rapid and efficient chromatographic separation
and SCF/MS for Analysis of Complex
and reduced bond broadening, thus producing higher sen-
Hydrocarbon Mixtures
sitivity. Second were a large number of bond-phases, of
The physical properties of supercritical fluid mobile the type originally produced for HPLC, such as C 2 ,C 8 ,
phases allow chromatographic separations of many com- C 12 ,C 18 , aminopropyl, cyanopropyl, diphenyl, and the so
pounds that are generally separated by LC. The density of called chiral phase which are used in TLC.
a supercritical fluid is typically 100 to 1000 times greater The chiral plates are used extensively for the enan-
than that of a gas, which imparts “liquid-like” solvating tiomeric separation of racemic amino acids and their
properties. However, the viscosities and diffusivities of su- derivatives by means of a ligand exchange mechanism.
percritical fluids remain similar to those of gases. Conse- DetectionandquantitationofanalysisontheTLCplates
quently, high-solvating power mobile phases that behave have progressed considerably in recent years. A range of
similarly to gases define the advantages of SFC. These microprocessor-controlled scanning densitometers is now
properties also allow successful interfacing to MS. The available, capable of providing UV and fluoresence de-
immediate application of SFC/MS in complex hydrocar- tection at the nanogram level. Chromatographic processes
bon analysis is for mixtures containing nonpolar to mod- have been reported to be filmed on video.
erately polar compounds that are thermally labile or of
relatively high molecular weight.
A. High-Performance Thin-Layer
Chromatography
VIII. THIN-LAYER CHROMATOGRAPHY High-performance TLC enables us to carry out the most
complicated separations. The HPTLC plates are prepared
Thin-layer chromatography (TLC) is a subdivision of liq- from optimized (e.g., particle size and particle size distri-
uid chromatography (LC) in which the mobile phase is a bution) adsorbent layers and extremely even surfaces. The
liquid and the stationary phase is situated as a thin layer HPTLC plates offer greater separating efficiency (plate
on the surface of a flat plate. TLC is sometimes grouped number and resolution) through smaller plate heights than
with paper chromatography under the term planar liquid the conventional TLC plates. Shorter analysis time, de-
chromatography because of the flat geometry of the paper tection limits in the nanogram range with UV adsorption
or layer stationary phases. TLC is a simple, rapid, ver- detection and in the picogram range with fluorometric de-
satile, sensitive, inexpensive analytical technique for the tection are additional advantages.
separation of organic substances. Since numerous station- The HPTLC plates may typically deliver some 4000
ary phases (sorbents) are available, such as silica, cellu- theoretical plates over a distance of 3 cm in 10 min. This