Analytical Spectroscopy by R.P.W. Scott

Introduction to Spectroscopy

Spectroscopy is the study of the absorption and emission of electromagnetic radiation by matter. The electromagnetic radiation of analytical interest ranges from γ rays that have a frequency of about 3x10¹⁸ and wavelength of about 10 pm, to the radio waves used in NMR having a frequency of about of 3x10⁶ and a wavelength of about10 m. This range encompasses UV and visible spectroscopy, Fluorescence spectroscopy, infrared spectroscopy, Raman Spectroscopy, ESR (electron spin resonance spectroscopy) and NMR (nuclear magnetic resonance spectroscopy). There is another very important, so- called analytical spectroscopic technique, called mass spectroscopy (MS), which is somewhat an anomaly, as it does not deal with the adsorption or emission of electromagnetic radiation but the separation of ions of different masses. In its early development, the resolution of different ion masses was included as a spectroscopic technique largely because the curves relating ion intensity to ion mass bore some graphical similarities to true electromagnetic wave absorption spectra.

An electromagnetic wave consists of a sinusoidal electrostatic field acting at right angles to, and in phase with, a sinusoidal magnetic field. A diagram of an electromagnetic wave is shown in figure 1.

Figure 1 An electromagnetic Wave

It is the electric vector that has the major effect on matter; it is the electric vector that activates the cells in the retina of the eye and provides sight; it is the same vector that activates the light sensitive surface of a photoelectric cell, which then responds to the intensity of light falling on it. All electromagnetic radiation travels at the same velocity (c) viz., 2.997925 x 10⁸ ms^-1 which can be approximated to 3 x 10⁸ m/s. electromagnetic radiation has two characteristics, its frequency and its wavelength which are related by the following equation,

(1)

where ((λ) is the wavelength of the electromagnetic wave, and ((ν) is the frequency of the electromagnetic wave.

The relationship between wavelength and frequency for the different regions is shown in figure 2.

Figure 2 The Relationship between wavelength and Frequency and the Different Regions of the electromagnetic Spectrum

The regions given in figure 2 does not help the analytical chemist much as it does not indicate how the different frequencies or wavelengths are related to the physical chemical processes with which they are associated. This relationship can be explained as follows.

γ Rays-These rays, that have frequencies lying between ca 3x10¹⁸ and 3x10²⁰ Herz (100pm-1pm wavelength), are capable of exciting atomic nuclei. γ Ray spectroscopy is not normally used in general analytical work and the energies involved amount to 10⁹-10¹¹ joules per gram atom.
X-Rays - These rays, having frequencies lying between ca 3x10¹⁶ and 3x10¹⁸ Herz (10nm-100pm wavelength), can excite inner electronic transitions in an atom and are used extensively in X-ray crystallography. Inner electronic transitions involve energies that may be as great as ten thousand joules per gram atom
Ultra Violet and Visible Radiation - Ultra violet and Visible radiation, having frequencies lying between ca 3x10¹⁴and 3x10¹⁶Herz (1μ-10nm wavelength), can excite outer electronic vibrations (valence electrons) in atoms and absorption or emission of light of this wavelength range is commonly used in analytical techniques to identify aromatic compounds, olefins and substances having one or more double bonds and unshared electrons. The excitation of a valence electron involves the movement of electronic charges in the molecule from one energy level to another. The energies involved in such transitions amount to some hundreds of kilojoules per mole. Absorption of radiation in this frequency range is also used in certain types of sensing devices that are employed in chromatographic detectors.
Infrared and Microwave Radiation - Infrared and part of the microwave band, having frequencies lying between ca 3x10¹⁰ and 3x10¹⁴ Herz (100μm-1μm wavelength), can excite molecular vibrations and rotations. Absorption of electromagnetic radiation in this wavelength range can be used to confirm the identity of specific compounds and to identify explicit chemical groups in a molecular structure.
Short Wave Radio Radiation -Short wave radio radiation, having frequencies lying between ca 3x10⁸and 3x10¹⁰ Herz is employed in electron spin resonance studies.
Medium Wave Radio Radiation - Medium wave radio radiation, having frequencies lying between ca 3x10⁶ and 3x10⁸ Herz is used in nuclear magnetic resonance studies.

Einstein, Planck and Bohr suggested that electromagnetic radiation could also be considered as a stream of particles in discrete energy packets called quanta and the energy (E) of each particle of frequency (n) was given by,
E = hν (2)
where (h) is Planck’s Constant = 6.62 x 10^-34 Js = 6.62 x10^-27 ergs/sec

Using equation (2), the energy associated with any transition whether electronic, rotational or vibrational, induced by radiation of a specific frequency can be calculated.

About the Author
RAYMOND PETER WILLIAM SCOTT was born on June 20 1924 in Erith, Kent, UK. He studied at the University of London, obtaining his B.Sc. degree in 1946 and his D.Sc. degree in 1960. After spending more than a decade at Benzole Producers, Ltd. Where he became head of the Physical Chemistry Laboratory, he moved to Unilever Research Laboratories as Manager of their Physical Chemistry department. In 1969 he became Director of Physical Chemistry at Hoffmann-La Roche, Nutley, NJ, U.S.A. and subsequently accepted the position of Director of the Applied Research Department at the Perkin-Elmer Corporation, Norwalk, CT, U.S.A.
In 1986 he became an independent consultant and was appointed Visiting Professor at Georgetown
University, Washington, DC, U.S.A. and at Berkbeck College of the University of London; in 1986 he retired but continues to write technical books dealing with various aspects of physical chemistry and physical chemical techniques. Dr. Scott has authored or co-authored over 200 peer reviewed scientific papers and authored, co-authored or edited over thirty books on various aspects of physical and analytical chemistry. Dr. Scott was a founding member of the British chromatography Society and received the American Chemical society Award in chromatography (1977), the M. S. Tswett chromatography Medal (1978), the Tswett chromatography Medal U.S.S.R., (1979), the A. J. P. Martin chromatography Award (1982) and the Royal Society of Chemistry Award in Analysis and Instrumentation (1988).
Dr. Scott’s activities in gas chromatography started at the inception of the technique, inventing the Heat of Combustion Detector (the precursor of the Flame Ionization Detector), pioneered work on high sensitivity detectors, high efficiency columns and presented fundamental treatments of the relationship between the theory and practice of the technique. He established the viability of the moving bed continuous preparative gas chromatography, examined both theoretically and experimentally those factors that controlled dispersion in packed beds and helped establish the gas chromatograph as a process monitoring instrument. Dr. Scott took and active part in the renaissance of liquid chromatography, was involved in the development of high performance liquid chromatography and invented the wire transport detector. He invented the liquid chromatography mass spectrometry transport interface, introduced micro-bore liquid chromatography columns and used them to provide columns of 750,000 theoretical plates and liquid chromatography separations in less than a second. Dr. Scott has always been a “hands-on” scientist with a remarkable record of accomplishments in chromatography ranging from hardware design to the development of fundamental theory. He has never shied away from questioning “conventional wisdom” and his original approach to problems has often produced significant breakthroughs.