= m
= eV
= Hz


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UV Light
Beer's Law
Diode Array
Deuterium Discharge Lamp
Low Pressure Zinc Discharge Lamp
High Pressure Mercury Discharge Lamp
Low Pressure Cadmium Lamp
Tungsten Halogen Lamp
Lens and Window Material in Spectrometers
Fluorescence Reagents
Diffraction Grating
Fourier Transform IR Spectrometer
Halide Disks
Mull Samples
Film Samples for IR Spectroscopy
Light Pipes
Attenuated Total Reflectance Spectroscopy
Multiple Internal Reflectance
External Reflectance
Specular Reflectance
Diffuse Reflectance
Photoacoustic Spectroscopy
Beam Splitter
Raman Scattering
Rayleigh Scattering
Raman Spectroscopy
Atomic Spectroscopy
Atomic Emission Spectroscopy
Atomic Absorption Spectroscopy
The Inductively Coupled Plasma Torch
The Helium Plasma Torch
Emission Spectrometer
Atomic Absorption Spectrometry
Flame Atomic Absorption Spectrometer
Flame AA
Hollow Cathode Lamp
Electrothermal Atomization
Graphite Furnace
L’vov Platform
Electron Paramagnetic Resonance
Zeeman Effect
Continuous Wave
Electron Paramagnetic Resonance
Pulsed EPR
Electron Spin Echo
Multple Resonance Spectroscopy
Magnetic Resonance Spectroscopy
Nucleus Spin Decoupling in NMR
Superconducting Magnets
NMR Microcells
Electron Impact Ionisation
Chemical Ionization
Inductively Coupled Plasma Ionization
Secondary Ion Mass Spectrometry
Fast Atom Bombardment
Plasma Desorption Mass Spectrometry
Laser Desorption Mass Spectrometry
Matrix Assisted Desorption mass Spectrometry
Field Desorption Ionization
Thermospray Ionization
Electrospray Ionization
Atmospheric Pressure Ionization
Particle Beam Interface
Permeable Membrane Interface
Sector Mass Spectrometer
Quadrupole Mass Spectrometer
Ion Trap Mass Spectrometer
Time of Flight Mass Spectrometer
Optical RotationCircular Dichroism
Circularly Polarized Light
Verdet Constant
Faraday Effect

Scanning by Photo Acoustic Spectroscopy

In photo acoustic spectroscopy, the energy absorbed from the infrared radiation is measured by sensing the mechanical vibration produced, and by employing an appropriate acoustic measuring device. A diagram representing the photo acoustic spectroscopic sensing system is shown in figure 23.

Figure 23. Diagram of the Photo Acoustic Sensing System

The incident radiation is allowed to fall on the sample contained in a suitable enclosure. When the modulated infrared radiation is absorbed by the sample, the substance heats and cools in response to the radiation received. Situated in the enclosure is an acoustic sensing device, which may be a simple microphone or a piezoelectric sensor. The sensor detects the acoustic pulses (caused by the heating of the surrounding gas) as they are generated by the different IR frequencies that are absorbed. The advantage of this type of IR measurement is that it can be used effectively with very black or highly absorbing samples.

Lloyd et al [6] employed a simple micro phonic detector as the sensor, to scan a TLC plate. The thin layer sheets were either aluminium or poly(ethylene terephthalate) backed, and both silica and alumina were used as a thin layer about 250 μm thick. 1 cm diameter discs were excised from the plates and placed in the sample compartment of the microphonic cell. The cell was sealed in a glove bag after purging for 15 minutes with helium The cell itself was fabricated from polished stainless steel with a sodium chloride window, and was supported on vibration-free mounts. It had a total volume of about 0.4 cm3. The IR output from a Nicolet 7199 FTIR spectrometer was focused through the sodium chloride window onto the plate surface. The acoustic waves were detected with a Brüel and Kjoer 4165 microphone, which was exposed to the helium in the cell, through a pipe 10 mm long and 1 mm I.D. Some spectra of tetraphenylcyclopentadienone taken by this procedure are shown in figure 24.

Figure 24. The FTIR/PS Spectra of 300 μg of Tetraphenylcyclopenta-Dienone from Different TLC Plates

The figure shows the actual spectra as taken, the background spectra of the plate, and the difference spectra of the sample alone. It is seen, by comparison with the spectra obtained from the KBr pellet sample, that reasonably fine structure is disclosed. However, as might be expected, the signal to noise ratio is not very satisfactory, and consequently the absolute sensitivity is not as good as that obtained by other scanning procedures.


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.

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