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Organic Chemistry, Compound Detection 461

monochromatic light as a function of frequency of the compounds. In modern technology this involves the use of
scatteredlight.TheRStechniqueprovidescomplemen- chromatography (paper, thin-layer, gas–liquid, high-pre-
tary information to infrared spectroscopy. Generally ssure liquid); spectroscopy (infrared, Raman ultraviolet
strong vibrations in the Raman absorption are weak in and visible, nuclear magnetic resonance); mass spec-
the infrared absorption. trometry; and reaction chromatography (chemical reac-
Reaction GLC Certain chemical reactions of organic tions on thin-layer plates or gas chromatographic columns
compounds (such as reduction, oxidation, dehydration) which can be carried out prior to, during, or immedi-
that take place in a gas–liquid chromatographic col- ately after the chromatographic separation). Pyrolysis and
umn. The products are detected by means of the con- X-ray crystallography of organic compounds furnish im-
ventional detectors. portant structural information on the partial structures or
Spectroscopy Instrumental method of assigning struc- on the whole molecule, respectively. The combination
tural features and functional groups to organic or in- (and computerization) of chemical, chromatographic, and
organic molecules; such features are displayed by in- spectroscopic techniques has become a more efficient tool
tensities and patterns of spectroscopic signals. for the detection and identification of organic compounds
Supercritical fluid chromatography (SFC) Technique than any of these techniques.
in which the mobile phase (fluid) is maintained at tem-
peratures somewhat above its critical point. The mo-
bile phases used in SFC are gases such as freon, ethy- I. INTRODUCTION
lene, or carbon dioxide. It has superior solution prop-
erties and enhances the chromatographic separation Itwasonlyabout40yearsagothatchemistshadthetedious
of higher molecular weight compounds. The column task of identifying and characterizing unknown organic
packings used in SFC are the same as those used in compounds especially in the area of natural products. This
HPLC. may involve degradation of the molecule followed by syn-
Supersonic jet spectroscopy (SJ) Method which mea- thesis involving many steps. For example Woodward elu-
sures with high discriminating power utilizing laser- cidated the structure of strychnine in 1947 and seven-years
induced or lamp-induced fluorescence spectroscopy. later successfully synthesized this compound.
Thermal chromatography Volatilization of organic The advent of computers and Fourier transform com-
compounds at high temperatures and their separation pletely revolutionized the detection and identification of
by chromatographic techniques such as TLC. organic compounds. Modern automated instruments al-
Thin-layer chromatography (TLC) Chromatographic low very small samples in the nanogram (10 −9 g) range to
technique in which the mobile phase is a liquid and be characterized in a very short time. The application of
the stationary phase is a thin-layer (usually 0.25 mm Fourier transform nuclear magnetic resonance (FTNMR)
thick) of an adsorbent (silica gel, alumina, cellulose) and Fourier transform infrared (FTIR) allows recovery of
spread homogeneously on a flat plate (usually a glass the sample in contrast to mass spectrometric (MS) deter-
plate) of various dimensions. mination which is a destructive but quite often necessary
Two-dimensional NMR spectroscopy (2D-NMR) Two- technique.
dimensional NMR refers to spectral data that are col- Modern methods especially in the separation of com-
lected as a function of two time scales—evolution and plex organic mixtures utilizing gas–liquid chromato-
detection. graphy (GLC), high-pressure liquid chromatography
Vacuumless spectrochemistry in the vacuum ultravi- (HPLC), and droplet counter-current (DCC) chromatog-
olet (VUV) Technique in which the emission wave- raphy can separate samples rapidly and efficiently in the
length region is below 200 nm. “Far ultraviolet” is more picogram range which until fairly recently has been im-
appropriate, since it includes access to this wavelength possible. Coupling the chromatographic instruments to
region by gas-filled optical techniques. spectrometers enables a partially automated analysis in
X-ray crystallography Method for determining the even less time. The following coupling of chromato-
molecular structure of crystalline compounds which graphic instruments has been performed: GC–MS, GC–
provides information on the positions of the indi- FTIR, GC–MI–FTIR, GC–UV–VIS, HPLC–MS, HPLC–
vidual atoms of a molecule, their interatomic dis- FTIR, HPLC–FTNMR and MS–MS. (Fig. 1).
tances, bond angles, and other features of molecular These semi-automated systems of analyzing and char-
geometry. acterizing small samples are vital to the natural product
organic chemist and biochemist for detection of highly
active substances in extremely low concentration in living
COMPOUND DETECTION in organic chemistry refers organisms. A typical example is in the field of pheromones
to the methods of separation and identification of organic which includes insect sex attractants which differ quite