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Instrumentation of UV or Visible Spectroscopy


INSTRUMENTATION

Instruments for measuring the absorption of U.V. or visible radiation are made up of the following components;
1. Sources (UV and visible)
2. filter or monochromator
3. Sample containers or sample cells
4. Detector

1. Radiation source
It is important that the power of the radiation source does not change abruptly over its wavelength range.
The electrical excitation of deuterium or hydrogen at low pressure produces a continuous UV spectrum. The mechanism for this involves formation of an excited molecular species, which breaks up to give two atomic species and an ultraviolet photon
Both Deuterium and Hydrogen lamps emit radiation in the range 160 - 375 nm. Quartz windows must be used in these lamps, and quartz cuvettes must be used, because glass absorbs radiation of wavelengths less than 350 nm.

Various UV radiation sources are as follows
a. Deuterium lamp
b. Hydrogen lamp
c. Tungsten lamp
d. Xenon discharge lamp
e. Mercury arc lamp

Various Visible radiation sources are as follows
a. Tungsten lamp
b. Mercury vapour lamp
c. Carbonone lamp

2. filters or monochromators
All monochromators contain the following component parts;
• An entrance slit
• A collimating lens
• A dispersing device (a prism or a grating)
• A focusing lens
• An exit slit
Polychromatic radiation (radiation of more than one wavelength) enters the monochromator through the entrance slit. The beam is collimated, and then strikes the dispersing element at an angle. The beam is split into its component wavelengths by the grating or prism. By moving the dispersing element or the exit slit, radiation of only a particular wavelength leaves the monochromator through the exit slit.

3. sample containers or sample cells
A variety of sample cells available for UV region. The choice of sample cell is based on
a) the path length, shape, size
b) the transmission characteristics at the desired wavelength
c) the relative expense

The cell holding the sample should be transparent to the wavelength region to be recorded. Quartz or fused silica cuvettes are required for spectroscopy in the UV region. Silicate glasses can be used for the manufacture of cuvettes for use between 350 and 2000 nm.The thickness of the cell is generally 1 cm. cells may be rectangular in shape or cylindrical with flat ends.

4. Detectors
In order to detect radiation, three types of photosensitive devices are
a. photovoltaic cells or barrier- layer cell
b. phototubes or photoemissive tubes
c. photomultiplier tubes

Photovoltaic cell is also known as barrier layer or photronic cell. It consists of a metallic base plate like iron or aluminium which acts as one electrode. On its surface, a thin layer of a semiconductor metal like selenium is deposited. Then the surface of selenium is covered by a very thin layer of silver or gold which acts as a second collector tube.

When the radiation is incident upon the surface of selenium, electrons are generated at the selenium- silver surface and the electrons are collected by the silver. This accumulation at the silver surface creates an electric voltage difference between the silver surface and the basis of the cell.

Phototubes are also known as photoemissive cells. A phototube consists of an evacuated glass bulb. There is light sensitive cathode inside it. The inner surface of cathode is coated with light sensitive layer such as potassium oxide and silver oxide.

When radiation is incident upon a cathode, photoelectrons are emitted. These are collected by an anode. Then these are returned via external circuit. And by this process current is amplified and recorded.

The photomultiplier tube is a commonly used detector in UV spectroscopy. It consists of a photoemissive cathode (a cathode which emits electrons when struck by photons of radiation), several dynodes (which emit several electrons for each electron striking them) and an anode.
A photon of radiation entering the tube strikes the cathode, causing the emission of several electrons. These electrons are accelerated towards the first dynode (which is 90V more positive than the cathode). The electrons strike the first dynode, causing the emission of several electrons for each incident electron. These electrons are then accelerated towards the second dynode, to produce more electrons which are accelerated towards dynode three and so on. Eventually, the electrons are collected at the anode. By this time, each original photon has produced 106 - 107 electrons. The resulting current is amplified and measured.
Photomultipliers are very sensitive to UV and visible radiation. They have fast response times. Intense light damages photomultipliers; they are limited to measuring low power radiation.
 

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