Page 369 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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472 Organic Chemistry, Compound Detection

TABLE II Color Reagents Used for Detecting Gas Chromatographic Effluents
Class of
compound Reagent Color
Alcohols Potassium dichromate/nitric acid Blue
Aldehydes 2,4-Dinitrophenylhydrazine Yellow or red precipitate
Ketones Schiff’s reagent Pink
2,4-Dinitrophenylhydrazine Yellow or red precipitate
Esters Ferrous hydroxamate Red
Mercaptans Sodium nitroprusside Red
Isatin Green
Lead acetate Yellow precipitate
Sulfides Sodium nitroprusside Red (primary)
Sodium nitroprusside Blue (secondary)
Nitriles Ferrous hydroxamate/propylene glycol Red
Aromatics Formaldehyde/sulfuric acid Red
Alkyl halides Alcoholic silver nitrate White precipitate

analysis time, reduction of sample weight, and also in- technique is usually integrated with a gas chromatograph,
creases the accuracy of determination. a mass spectrometer, or both. This arrangement enables
pyrolysis products to be analyzed immediately, so that
transfer losses and secondary degradations are minimized.
7. Combination of TLC and Vapor-Phase
The resulting chromatogram or mass spectrum may pro-
Chromatography
vide qualitative information concerning the composition
The combination of TLC and GLC enables a more com- or identity of the sample, quantitative data on its constitu-
plete separation and identification of complex mixtures of tion, or it may enable mechanistic and kinetic studies of
organic compounds. In most cases the mixture is subjected thermal fragmentation processes.
to preliminary separation by TLC followed by a complete The combined Py/GC/MS technique requires a minute
separation of the preseparated compounds of the mixture amount of sample, and the analysis time is very short.
by GC. The TLC–GLC technique has been applied in the Furthermore, such systems are now totally automated and
analysis of citrus and other essential oils and for determi- computerized.
nation of steroids in urine as well as unsaturated hydroxy Analytical pyrolysis has been used successfullyinmany
acid and glycerides which are separated on a thin layer of disciplines such as polymer chemistry, organic geochem-
silica impregnated with AgNO 3 and then subjected to GC. istry, soil chemistry, forensic sciences, food science, en-
The fractions from the chromatoplates can also be altered vironmental studies, microbiology, and extraterrestrial
prior to running a vapor-phase chromatographic separa- studies involving meteorites and lunar samples. A large
tion. Thus, acids are converted to methyl esters, alcohols number of organic substances found in nature are unsuit-
andsterolstoacetates,etc.,priortoGLC.Anapplicationof able for direct analysis by modern techniques such as col-
the combination of TLC with GLC is the direct application umn chromatography and mass spectrometry. This may be
of the compounds to the thin-layer plate as they emerge due to their complex structure and polar and nonvolatile
from the exit tube of the GC. This technique was used for character.
the analysis of fatty acids, steroids, essential oils found A significant step in the evolution of analytical py-
in coffee essence, and many alkaloids. The association of rolysis was the combination of pyrolysis with a sophis-
TLC techniques with GC–MS was described for various ticated physiochemical technique for the efficient sepa-
separations and identifications of organic compounds. ration and/or identification of the fragments. In 1959, a
combined Py/GC system of polymers was introduced. Im-
provementsintheanalysisconditionswereobtainedbyus-
C. Pyrolysis Gas Chromatography (Py/GC)
ing the high-resolution capabilities of capillary columns.
Prolysis is the thermal fission of naturally occurring In 1970, a continued interest in the analysis of extrater-
and synthetic polymers producing a range of smaller restrial samples such as lunar rocks and the Allende me-
molecules. Analysis of these products enables a profile of terorite (found in Mexico) was reported, using the mod-
the original compound to be reconstructed. The pyrolytic ified Py/GC/MS technique. Applying a pyrolysis/mass