Page 381 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN0011A-541 July 25, 2001 17:27

484 Organic Chemistry, Compound Detection

available. This technique has the potential to be more ver- As technology advances, it provides new opportuni-
satile than conventional laser Raman spectroscopy. The ties. The gas-filled optical system technology opens up
technique in NIR-FTRS is to excite the spectrum with near the VUV for spectrochiral analysis. The terminology “far
IR laser, pass the scattered light through filters to remove ultraviolet” might be more appropriate since it includes
the reflected and Rayleigh scattered excitation radiation, access to this wavelength region by gas-filled optical
and then to process the Raman scatter on an FTIR instru- techniques.
ment. Being infrared excited, the spectra were essentially
clear of fluorescence, while color due to absorption was
E. Mass Spectrometry (MS)
far less of a problem in near infrared than with the conven-
tional techniques. For example, rubrene, which is deeply Mass spectrometry provides a means for studying sam-
colored and intensely fluorescent, produced a well-defined ples at the molecular level. Although MS is basically a
Raman spectrum at 1064 nm. Recently FTRS spectra of al- structure-identifying tool, it is not as specific for the de-
kaloids such as heroin, codeine, and morphine have been tection and determination of functional groups (IR) or rel-
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obtained, but because of the complexities of their spec- ative positions protons or C nuclei (NMR) spectroscopy.
tra they have not yet been fully analyzed. It is obvious Mass spectrometry, like UV, IR, and NMR spectroscopy,
that within the next few years group frequencies char- works best with pure samples. Structural elucidation of
acteristic of structural features in complex organic and mixtures is not possible without some means of separat-
bioorganic compounds will be determined. It appears that ing each constituent in the mixture as in GC–MS.
NIR-FTRS is the choice method in obtaining spectra of Besides being a useful structure-elucidating tool, MS
biochemical substances such as enzymes, proteins, and can be applied to detect very low levels of specific com-
polypeptides because such molecules can be studied in pounds and elements. Accurate determination of masses
aqueous media. Interpretation of the complex nature of can also be determined. An important advantage of the MS
these biomolecular spectra will also take time. Commer- technique is its high sensitivity and accuracy. MS is able
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cial instruments offering mid-infrared (400–4000 cm ), to provide more specific information per given amount of
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near infrared (4000–10,000 cm ), and Raman spectra all material than any other analytical technique. Furthermore,
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on the same spectrometer will be available in the near even with minute amounts of material (10 –10 −9 g), this
future. information can be provided in a reproducible and accu-
rate manner. In the 1940s MS was an important analyt-
ical technique for characterizing complex fuel mixtures.
D. Vacuumless Spectrochemistry
About 25 years ago interest was centered on a system-
in the Vacuum Ultraviolet
atic study of ionic fragmentation mechanisms in order
The emission wavelength region below 200 nm has tradi- to provide a set of rules that could be applied to eluci-
tionally been called the vacuum ultraviolet (VUV). This date structures of organic compounds. In the late 1960s
region contains the best spectral lines for many indus- mass spectrometers were coupled to gas chromatographs.
trially important elements, such as carbon, phosphorus, This provided such dramatic improvements in selectiv-
sulfur, and boron, in steel and cast iron by spark excita- ity and sensitivity that MS has since become one of the
tion, and the halogens for the inductively coupled plasma most generally useful analytical techniques for identifica-
(ICP) analysis of oils and fuels. tion and quantitation of organic substances at ultratrace
Wavelengths below 200 nm are absorbed by some com- levels. Chemical ionization mass spectrometry (CIMS)
ponents of air, primarily oxygen and water vapor. There- uses reagent ions rather than electrons to ionize a sample.
fore, the usual approach to spectrochemistry in this wave- Field desorption and rapid heating techniques are extend-
length region is to evacuate the optical system. ing mass spectrometry to high-molecular-weight, low-
The use of a nitrogen gas-filled system provides im- volatility compounds. During the past decade mass spec-
portant advantages for both the industrial and academic trometry has undergone a number of significant changes
spectroscopist. This technology opens up the low wave- that have far-reaching contributions especially in biol-
lengths of the emission spectrum which contains many ogy and medicine. The mass range of mass spectrometers
important spectral lines for elements of interest for practi- has been extended by approximately an order of magni-
cal spectrochemistry. It should be emphasized that none of tude in the past decade. Certain types of mass analyz-
the wavelengths below 140 run have been achieved with ers have been used to reach higher masses (150,000 in
a conventional vacuum spectrometer. some quadrupole experiments). Commercial instruments
One of the economic advantages of ICP analysis of are now available with mass ranges of 7500 compared
halogens in oils, fuels, and other matrices is that it is faster with 1000 about ten years ago. Another improvement is
than other methods such as X-ray fluorescence (XRF). desorption ionization [including fast atom bombardment