Chemistry Reference
In-Depth Information
Infrared spectroscopy
Non saline waters:
free cyanide.
Raman spectroscopy
Non saline waters:
nitrate and nitrite.
Waste waters:
free cyanide and nitrate.
A more recent development is Fourier transform infrared analysis.
Fourier transform Infrared spectrometry
Fourier transform infrared spectrometry, a versatile and widely used analytical technique,
relies on the creation of interference in a beam of light. A source light beam is split into
two parts and a continually varying phase difference is introduced into one of the two
resultant beams. The two beams are recombined and the interference signal is measured
and recorded, as an interferogram. A Fourier transform of the interferogram provides the
spectrum of the detected light. Fourier transform infrared spectroscopy, a seemingly
indirect method of spectroscopy, has many practical advantages, as discussed below.
A Fourier transform infrared spectrometer consists of an infrared source, an
interference modulator (usually a scanning Michelson interferometer), a sample chamber
and an infrared detector. Interference signals measured at the detector are usually
amplified and then digitised. A digital computer initially records and then processes the
interferogram and also allows the spectral data that result to be manipulated. Permanent
records of spectral data are created using a plotter or other peripheral device.
The principal reasons for choosing Fourier transform infrared spectroscopy are: first,
that these instruments record all wavelengths simultaneously and thus operate with
maximum efficiency; and, second, that they have a more convenient optical geometry
than do dispersive infrared instruments. These two facts lead to the following advantages.
• Fourier transform infrared spectroscopy spectrometers achieve much higher signal-to-
noise ratios in comparable scanning times.
• They can cover wide spectral ranges with a single scan in a short scan time, thereby
permitting the possibility of kinetic time-resolved measurements.
• They provide higher-resolution capabilities without undue sacrifices in energy
throughput or signal-to-noise ratios.
• They encounter none of the stray light problems usually associated with dispersive
spectrometers.
• They provide a more convenient beam geometry—circular rather than slit shaped—at
the sample focus.
Conventional Raman spectroscopy cannot be applied directly to aqueous extracts of
sediments and soils, although it is occasionally used to provide information on organic
solvent extracts of such samples. Fourier transform Raman spectroscopy, on the other
hand, can be directly applied to water samples. The technique complements infrared
spectroscopy in that some functional groups, eg unsaturation, give a much stronger
response in the infrared. Several manufacturers (Perkin-Elmer, Digilab, Bruker) now
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