Page 391 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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494 Organic Chemistry, Compound Detection

HPLC extends the range of species which can be carried XIII. X-RAY CRYSTALLOGRAPHY
in the analyte stream, but it suffers from relatively low
column efficiency. Chromatography by itself provides ex- The purpose of X-ray analysis of crystal structures is to
tremely limited qualitative information. The most produc- provide information on the positions of the individual
tive approach is the combination of an instrument having atoms of a molecule, their interatomic distances, bond
high separating power and one having high discriminating angles, and other features of molecular geometry such
power. One measurement method with high discriminat- as the planarity of a particular group of atoms, the
ing power is supersonic jet (SJ) spectroscopy. Kantrowitz angles between the planes, and torsion angles around the
and Grey originally proposed supersonic expansions as a bonds. The resulting three-dimensional representation
means of increasing the intensity of molecular beams in of the atomic contents, rapidly determined by modern
the early fifties, but it was in the mid-seventies that the SJ computerized techniques, of the crystal establishes the
techniques were popularized for spectroscopic studies. It complete conformational structure and geometrical
was only in the eighties that the analytical application to details hitherto unknown.
the study of complex samples were recorded. The combi- This information is of primary interest to chemists and
nation of SJ spectroscopy with chromatography promises biochemists who are concerned with the relation of struc-
to be a powerful method for detailed analysis. Among tural features to chemical properties. It was only X-ray
all the line-narrowing spectroscopic techniques, SJ spec- analysis that finally determined the double helix of two
troscopy is the one which can be most directly coupled to DNA molecules held together by hydrogen bonds that
chromatography. Advantages of the SJ approach include verified Watson and Crick’s proposed model for DNA.
elimination of all matrix interferences, such as spectral Thus, X-ray analysis of organic compounds provides an
shifts, site inhomogeneous broadening, phono-coupling, unambiguous complete three-dimensional picture of the
all other extrinsic fluorescence quenching, and Raman molecule, whereas other chemical and physical methods
scatter and background from the matrx. The need to iden- of structure analysis involved different aspects which col-
tify a suitable matrix is avoided. No complex cryogenic lectively one can deduce the number and nature of the
equipment is required. A disadvantage of SJ is the very atoms bonded to each atom present (the topology of the
short time which sample molecules spend in the obser- molecule) or, for relatively simple molecules, provide
vation volume, limiting the amount of signal averaging some quantitative information from which geometrical
which can be done if only a small quantity of a sample is details can be derived. There are instances whereby only
available or if it is to be detected in the narrow time span X-ray analysis will decide absolutely the spatial arrange-
during which it elutes as a chromatographic band. Also, ment of groups of atoms in simple organic molecules. It
although the spectroscopic detection method may be non- was X-ray analysis that confirmed that the cyclic sulphite
destructive itself, the sample is usually lost. Among all (Fig. 17) had a boat–chair conformation with the S O
the line-narrowing spectroscopic techniques, supersonic group trans to the cyclopropane ring and in an equatorial
jet spectroscopy is the one which can be most easily used position.
for real-time detection in chromatography. The apparent disadvantages of X-ray analysis is that
The equipment required for SJ detection is less expen- it is time consuming even with the use of automated
sive than that for either MS or MI. Furthermore, the SJ ap- computerized techniques. Further-more, X-ray analysis
proach allows for adding on downstream detectors, such can only be carried out on pure crystalline materials.
as especially MS; the MS can be used in 100% and even The crystals must be a certain size and regularity in
weakly absorbing molecules should be detected. order that a good X-ray structure of the compound is
obtained.
1. Gas Chromatography–Supersonic Jet
Spectroscopy (GC–SJ Spectroscopy)
XIV. CHEMICAL ANALYSIS
It was in 1982 that the analytical potential of SJ spec-
troscopy was used to selectively detect the components The requirement for information on chemical composi-
of a mixture of naphthalene and α- and β-methyl naph- tion extends into most sectors of industry. The chemical
thalenes. Trace impurities of fluorene in biphenyl at less measurement analytical market is extremely diverse.
than 1 ppm could be detected. Once the analytical advan- Chemical analysis is a key process in drug discovery, in
tages of SJ were realized, it would seem that coupling it assessing the quality of feedstock materials and end prod-
to GC would be a narrow extension. A 100-ng detection ucts, in monitoring chemical processes, in demonstrating
has been recently achieved for perylene, and it has been regulatory compliance, in disease diagnostics, and in en-
estimated that 1-pg detection limit can be attained. vironmental conservation.