Deuterium and Hydrogen lamps
             Deuterium lamps are of a low-voltage type where an arc is formed between a heated oxide-coated filament and a metal electrode, this produces a continuous spectrum in the  UV region. This mechanism involves an initial formation of an excited molecular species, followed by dissociation of the excited molecule to give two atomic species plus a UV photon.
 For example:  D2 + Ee        D2*        D’ + D” + hv
Where Ee is the electrical energy absorbed by the molecule and D2* represents the excited deuterium molecule. The energetics for the overall process is represented by the equation:  Ee = ED2* = ED’ + ED” + hv
             The heated filament of the lamp provides electrons to maintain a direct current when about 40 V is applied.
             An important feature of the deuterium and hydrogen lamps, is the shape of the aperture between the two electrodes, which constricts the discharge to a narrow path. An intense ball of radiation about  1 to 1.5 mm in diameter is produced. Deuterium produces a larger and brighter ball than does hydrogen.
             Both deuterium and hydrogen lamps produce a continuous spectrum in the region of 160 to 375 nm. At wavelengths greater than 400 nm, the lamps produce emission lines, which are superimposed on the continuous spectrum.
             Quartz windows are used in deuterium and hydrogen lamps, because glass absorbs strongly at wavelength less than 350 nm.

       Hollow-Cathode lamp
             The Hollow-Cathode lamp consists of a tungsten anode and a cylindrical cathode sealed in a glass tube containing an inert gas, such as argon, at a pressure of 1 to 5 torr. It is the most useful radiation source for atomic absorption spectroscopy.
             A potential of 300 V is applied across the electrodes causes ionization of the argon and generates a current of 5 to 10 mA as the argon cations migrate toward the cathode and electrons migrate toward the anode. Sputtering is produced when the argon cations strikes the cathode with sufficient energy to dislodge some of the metal atoms and produce an atomic cloud. Some of the sputtered metal atoms are in excited states and emit their characteristic wavelengths as they return to the ground state. The emission lines in the lamp produced are at a lower temperature than the analyte atoms in the flame. Thus the emission lines from the lamp are broadened less than the absorption peaks in the flame.
             The sputtered metal atoms in a lamp diffuse back to the cathode surface and are deposited. The Hollow-Cathode lamp is widely regarded as the single most important event in the evolution of atomic absorption spectroscopy.

      Electrodeless Discharge lamps
             These are useful sources of atomic line spectra and provide radiant intensities that are usually one to two orders of magnitude greater than their Hollow-Cathode counterparts. The Electrodeless Discharge lamp contains no electrode but instead is energized by an intense field of radio-frequency or microwave radiation. The argon ionizes in this field, and the ions are accelerated by the high-frequency component of the field until they gain sufficient energy to excite the atoms of the metal whose spectrum is sought.
             Their performance appears to be less reliable than the Hollow-Cathode lamp.

  Spectral resolution/bandwidth
             Resolution measures the ability to separate two closely spaced peaks. The greater the resolution, the smaller the difference between two wavelength.
             The bandwidth of a monochromator depends upon the dispersion of the prism or grating and on the width of the entrance and/or exit slits. Variable slits are equipped in monochromators so the bandwidth can be changed. Narrow slits, and thus narrow bandwidth, lead to higher instrument resolution.
             The bandwidth is defined as the width of the band in wavelength units at half-peak height.
             Therefore resolution is defined as how close two bands in a spectrum or a chromatogram can be to each other and still be seen as two peaks. In chromatography, it is defined as the difference in retention times of adjacent peaks divided by their width.
          Monochromators are used to provide narrow bands of radiation. Monochromators have the advantage that the output wavelength can be varied continuously over a considerable spectral range.
             The components of the monochromator typically include an entrance slit, a collimating lens or mirror to produce a parallel beam, a prism or grating to disperse the radiation into its component wavelengths, and a focusing element that projects a series of rectangular images of the entrance slit upon a planar surface called the focal plane.
             Monochromator slits – These partly determine the quality of the instrument. The effective bandwidth of a monochromator depends on the dispersion of the prism or grating and on the width of the entrance and/or exit slits. Most monochromators are equipped with variable slits so the effective bandwidth can be changed. Narrow slits and narrow bandwidths lead to higher instrument resolution.
             The grating monochromator is ruled with a series of closely spaced parallel grooves with a repeat distance d. The grating is coated with aluminium to make it reflective. And a layer of silica (SiO2) is placed on top of the aluminium to prevent the metal surface from oxidizing. When light is reflected from the grating, each groove behaves as a source of radiation.

             The Fourier-transform spectrometer is a system that consists of four optical arms, usually at right angles to each other, with a beam splitter at their point of intersection. Radiation passes down the first arm and is separated by a beam splitter into two perpendicular half-beams of equal intensity that pass down into other arms of the spectrometer. Then the two half-beams are reflected by mirrors back to the beam splitter, where they are recombine and are reflected together onto the detector.
             There are a number of advantages of the Fourier-transform spectrometer; one of the advantages is that the sample is exposed to the entire radiation beam at the same time and absorption effects at all wavelengths modify the final signal (known as multiplex advantage).
             A list of advantages of fourier-transform spectrometers over dispersive instrument is as follows:

Comparison of Dispersive and Fourier transform

            Dispersive instrument
1.  Many moving parts result in mechanical slippage.
2.  Calibration against reference spectra required to measure frequency.
3.  Stray light within instrument causes spurious readings.
4.  In order to improve resolution, only small amount of IR beam is allowed to pass through the slits.
5.  Only narrow-frequency radiation falls on the detector at any one time.
6.  Slow scan speeds make dispersive instruments too slow for monitoring systems undergoing rapid change (e.g. gas chromatographic effluents).
7.   Sample subject to thermal effects from the focused beam.
8.  Any emission of IR radiation by sample will fall on detector.
9.  No multiplex advantage.
 

                                Fourier transform
1.    Only mirror moves during an experiment.
2. Use of laser provides high frequency accuracy (to 0.01 cm-1).
3. Stray light does not affect detector, since all signals are modulated.
4. Much larger beam may be used at all times; data collection is easier.
5. All frequencies of radiation fall on detector simultaneously; improved S/N ratio obtained quickly (Fellgett advantage).
6. Rapid scan speeds permit monitoring samples undergoing rapid change.
7. Beam is not focused, hence sample is not subject to thermal effects.
8. Any emission of IR radiation by sample will not be detected.
9. Simultaneous interference by all wavelengths on all other wavelengths gives more complete spectra (Multiplex advantage).
 

             Two main advantages of interferometer over dispersive spectrometers for infrared spectroscopy are speed and sensitivity. These advantages result from the increased energy throughput (Jacquinot’s advantage) and higher signal-to-noise ratio (Fellgett’s advantage) available from an interferometer.
             The Jacquinot’s advantage is referred to as the advantage gained by using all of the energy of the source, and passing through circular holes rather than slits, while the Fellgett’s advantage is the advantage of collecting the spectroscopic data nearly simultaneously.
 

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