Page 378 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Organic Chemistry, Compound Detection 481

FIGURE 10 Infrared spectrum of heroin by diffuse reflectance spectroscopy as a mixture in potassium bromide using
approximately 1 mg of heroin.
powders and crystals, solids with rough surfaces, gem- time-dependent IR signals to yield a spectrum defined by
stones, minerals, plastics, and fibers. The best results are two independent wave numbers. By spreading IR peaks
achieved with the sample in powdered form. The qual- over the second dimension, a complex spectrum consist-
ity of the spectra can be enormously enhanced by mixing ing of overlapped peaks can be substantially simplified,
the sample with potassium bromide in the ratio of 1:10 for and spectral resolution is enhanced. Peaks located on a
organic materials. 2D spectral plane provide information on connectivity and
Many pharmaceuticals can be studied successfully us- interactions among functional groups associated with the
ingthistechnique,forexample1mgofheroininpotassium IR bands. The technique was recently developed by Isao
bromide (Fig. 10). It is possible to monitor the concentra- Noda and his co-workers, and he used it to demonstrate the
tion of heroin in a potassium bromide sample once a cali- knownincompatibilityofpolyethyleneandpolystyreneby
bration graph has been prepared. The DRIFTS technique showing that the two resins are segregated at the molecular
can be applied to quantitative analysis. level (Fig. 11).
Another application of DRIFTS is the identification of
fibers. There are many types of fibers, both synthetic and
3. Chromatographic Optimization
natural, which can be identified by IR spectroscopy but
of Combined GC–FTIR–MS
not so easily by other techniques.
By using a micro sampling cup it is possible to pro- Recent rapid developments of analytical instrumentation
duce diffuse reflectance spectra with a few milligrams have made it possible to obtain simultaneously GC–MS
of the sample, such as less than 1 mg of heroin. The and GC–FTIR information on a sample from one injec-
DRIFTS technique can thus be applied to the screening tion and one GC separation. Combining GC–MS and GC–
of illicit street drugs, such as a mixture of cocaine and FTIR into one system provides a higher confidence quali-
amphetamine. tative result and saves time, laboratory space, and money.
It should be realized that mass spectrometry and infrared
spectrometry are complementary.
2. Two-Dimensional Infrared Spectroscopy
(2D IR) In terms of relative practical sensitivity, the Mass Spec-
trum Detector (MSD) in the scanning mode is approxi-
Two-dimensionalinfrared(2DIR)spectroscopyisarecent mately 10 times better than the Infrared Detector (IRD).
novel technique based on time-resolved IR spectroscopy. Thus, a mass spectrum on a level of 1 ng requires 10 ng
The spreading out of IR spectra over two dimensions sim- for an IR spectrum if the compound is a strong absorber.
plifies complex spectra with overlapping peaks. Corre-
lation of such peaks may identify or eliminate specific
4. Matrix Isolation Fourier Transform
intermolecular and intramolecular interactions. Such cor-
Spectrometry (MI-FTIR)
relations can assist in assigning peaks to particular vibra-
tional modes. In 2D IR, a system is excited by an exter- Matrix isolation techniques are used in the spectroscopic
nal perturbation, which induces a dynamic fluctuation of detection of (a) chemical species that are too reactive,
the IR spectrum. A correlation analysis is applied to the too unstable, or too transient to be investigated under