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Organic Chemistry, Compound Detection 479

and CEC is in the nature of the separation. If the separation A. Infrared Spectroscopy
isachievedonthebasisofdifferentialpartitioningbetween
Infrared (IR) spectroscopy is probably the quickest and
two phases, then it is chromatography. If it uses both, it is
cheapest of the spectroscopic techniques in determining
referred to as electrochromatography.
the functional groups of the sample. The samples can be
Compared with HPLC, CEC uses electroosmotic flow
solids, liquids, or gases and can be measured in solution or
(EOF) rather than high pressure to force the mobile phase
as neat liquids mulled with KBr or mineral oil. Compari-
through the capillary. The advantage of EOF for CEC
son of IR spectra of substances of known structure has led
is that the flow profile is flat and thus precludes band
to many correlations between wavelength (or frequency)
broadening by trans-channel of radial diffusion. The re-
of IR absorption and features of molecular structure. Cer-
sult is that the number of plates is at least double that
tain structural features can easily be established. For ex-
of HPLC. Another advantage of CEC is that the pack-
ample, in an organic compound that contains only C, H
ing particles are smaller than those of today’s HPLC
and O, the oxygen can only be present as C O, O H, or
systems.
C O C or a combination of these, such as the ester or
Since there is no back pressure resisting the pump’s
carboxylic acid group.
ability to deliver the solvent at a given flow rate, as in
The presence or absence of absorption in the car-
HPLC, capillaries can be made longer. Thus, it is prac- −1
bonyl region (1730–1670 cm ) or hydroxy region (3700–
tical to utilize packings as small as 1 µm. These can −1
3300 cm ) can serve to eliminate or establish some of
be porous or nonporous, spherical or irregular, coated or
these possibilities. One simple application of IR is to de-
uncoated, pure silica or mixed mode (silica/polymer, sil-
termine whether two samples are identical. If the sam-
ica/alumina, etc.). The combination of longer capillaries
ples are the same, their IR spectra (obtained under identi-
packed with smaller-diameter particles makes it possible
cal conditions) must be the same. If the two samples are
to obtain and use columns with 100,000–500,000 and per-
both pure substances very similar in structure, the dif-
haps even 1,000,000 plates per meter.
ferences in the spectra may be so small that it would
In conventional HPLC, achieving more than about
not be easy to detect them; it may even be beyond the
25,000 plates per meter requires coupling columns and
power of the instrument to detect them. The absorption
long analysis times.
peaks of the spectrum of an impure sample is usually
Using small internal diameter columns and small par-
less intense than those of a pure sample and the spec-
ticle diameters it is possible to realize somewhat higher
trum will show additional peaks. The IR spectra of enan-
plate counts. It took 30 years, but today, capillary columns
tiomers are identical but those of diastereoisomers are
account for nearly 75% of the GC separations.
different.
Simple mixtures can be determined from their IR spec-
tra. For example mixtures of cycloalkanones show charac-
XII. SPECTROSCOPIC METHODS
teristic differences of the carbonyl group: cyclobutanone
−1
−1
(1788 cm ), cyclopentanone (1746 cm ), and cyclohex-
There has been a profound impact on the application −1
anone (1718 cm ). IR spectroscopy has been applied to
of spectroscopic techniques in the detection of organic
deuteration of organic samples. It is found that the C H
compounds especially in the areas of natural products −1
stretch bands disappear from the 3000 cm and the C D
and polymers. The commercially available Fourier trans- −1
stretch bands appear in the 2200 cm region. It is possible
form signal handling techniques are rapidly revolutioniz-
by calibrating pure compounds to determine the isotopic
ing nuclear magnetic resonance (FTNMR), infrared spec-
purity in routine analysis.
troscopy (FTIR), and Raman spectroscopy (FTRS), as
Modern IR analysis utilizing high-performance detec-
well as prototypes of mass spectrometry (FTMS) which −1
tors can generate spectra (4000–200 cm ) in 60 sec. Sam-
are not yet available commercially. Previously organic
ple amounts of 10 µg were found to be sufficient for such
chemists usually required all the spectroscopic data (UV,
spectra.
IR, NMR, and MS) to supplement microanalysis and gen-
eral physical characteristics such as refractive index, den-
sity, and melting and boiling points in order to elucidate
B. Fourier Transform Infrared Spectroscopy
the structure of the organic compound. Furthermore, it
was found that at least 50 mg of the material was needed Fourier Transform IR spectroscopy is widely used because
in order to determine the structure. It is now possible with of its rapidity of providing high-resolution spectra with
the modern available techniques to determine structures samples in the nanogram range. A complete spectrum for
with less than a milligram of material. a sample of 10 −9 g can be obtained in less than 1 sec.