You are hereAN UPDATE ON FUNDAMENTALS OF UV- VISIBLE SPECTROSCOPY

AN UPDATE ON FUNDAMENTALS OF UV- VISIBLE SPECTROSCOPY


Modern Instrumentation of UV-Visible Spectroscopy
Schematic diagram of a double-beam UV-Vis. spectrophotometer;


Instruments for measuring the absorption of U.V. or visible radiation are made up of the following components;

  1. Sources (UV and visible)
  2. Wavelength selector (monochromator)
  3. Sample containers
  4. Detector
  5. Signal processor and readout

Sources of UV radiation
It is important that the power of the radiation source does not change abruptly over it's wavelength range. The electrical excitation of deuterium or hydrogen at low pressure produces a continuous UV spectrum. 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

Sources of visible radiation
The tungsten filament lamp is commonly employed as a source of visible light. This type of lamp is used in the wavelength range of 350 - 2500 nm. The energy emitted by a tungsten filament lamp is proportional to the fourth power of the operating voltage. Tungsten/halogen lamps contain a small amount of iodine in a quartz "envelope" which also contains the tungsten filament. The iodine reacts with gaseous tungsten, formed by sublimation, producing the volatile compound WI2. When molecules of WI2 hit the filament they decompose, redepositing tungsten back on the filament. Tungsten/halogen lamps are very efficient, and their output extends well into the ultra-violet. They are used in many modern spectrophotometers.

MONOCHROMATORS
Monochromators are devices that can selectively provide radiation of a desired wavelength out of the range of wavelengths emitted by the source. These are of two types; the prism and grating monochromators. These are described in the following paragraphs.

Prism Monochromators: This occurs dueto the refraction of the light when it passes through the prism. The radiations ofdifferent colours having different wavelengths are refracted to different extent due tothe difference in the refractive index of glass for different wavelengths. Shorterwavelengths are refracted more than longer wavelengths as depicted in Fig.


If a prism is rotated, different wavelengths of the radiation, coming out after refracting through it, can be made to pass through the exit slit. In a prism monochromator, shown in Fig. A fine beam of the light from the source is obtained by passing through an entrance slit. This is then collimated on the prism with the help of a lens. The refracted beams are then focused on an exit slit. The prism is then rotated in a predetermined way to provide the desired wavelength from the exit slit.

Grating Monochromators: A grating is made by cutting or etching a series of closely spaced parallel grooves onthe smooth reflective surface of a solid material as shown in Fig. The surfaceis made reflective by making a thin film of aluminium on it and the etching is done with the help of a suitably shaped diamond tool. The intensity of radiation reflected by a grating varies with the wavelength, the wavelength of maximum intensity being dependent on the angle from which the radiation is reflected from the surface of the line of the grating as shown in Fig. In grating monochromator, a fine beam of the light from the source falls on a concave mirror through an entrance slit. This is then reflected on the grating which disperses it. The dispersed radiation is then directed to an exit slit. The range of wavelengths isolated by the monochromator is determined by the extent of dispersion by the grating and the width of the exit slit. Rotation of the grating in a predetermined way can be used to obtain the desired wavelength from the exit slit.


DETECTORS USED IN UV- VISIBLE SPECTROSCOPY
Three types of detectors are generally used in UV Visible spectrophotometry.

Photovoltaic Cell: Beams are allowed to fall on a transparent silver anode under which selenium backing is present which is connected by another silver cathode. Due to the beam heating on the surface of the selenium electrons flows through the electrodes and intensity is recorded. Signals can’t be amplified in this technique. Signal is limited to only blue and red colour beam. It is a less efficient technique than photomultiplier detectors.

Photomultiplier tubes: The photomultiplier tube is a commonly used detector in   UV-Vis spectroscopy. It consists of aphotoemissive 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.

Phototubes: These types of detectors are simplified version or the previous version of photomultiplier detector. Instead of dynodes single anodes is used and it is less sensitive that photomultiplier type of detector.


Detectors of UV-VIS radiation; a) Phototube and b) Photomultiplier tube

SINGLE BEAM SPECTROMETERS
As the name suggests, these instruments contain a single beam of light. The same beam is used for reading the absorption of the sample as well as the reference. The schematic diagram of a typical single beam UV- Visible spectrometer is given in Fig. The radiation from the source is passed through a filter or a suitable monochromator to get a band or a monochromatic radiation. It is then passed through the sample (or the reference) and the transmitted radiation is detected by the photodetector. The signal so obtained is sent as a read out or is recorded. Typically, two operations have to be performed – first, the cuvette is filled with the reference solution and the absorbance reading at a given wavelength or the spectrum over the desired range is recorded. Second, the cuvette is taken out and rinsed and filled with sample solution and the process is repeated. The spectrum of the sample is obtained by subtracting the spectrum of the reference from that of the sample solution.

DOUBLE BEAM SPECTROMETERS
In a double beam spectrometer, the radiation coming from the monochromator is split into two beams with the help of a beam splitter. These are passed simultaneously through the reference and the sample cell. The transmitted radiations are detected by the detectors and the difference in the signal at all the wavelengths is suitably amplified and sent for the output. The general arrangement of a double beam spectrometer is shown in Fig. There could be variations depending on the manufacturer, the wavelength regions for which the instrument is designed, the resolutions required etc.


Comparison between single and double beam spectrophotometer:

1. There is beam splitter in double beam spectrophotometer, no splitting of beam is necessary for single beam spectrophotometer.
2.
We can analyze two samples by D.B.S at a time but S.B.S is not so much efficient.
3.
D.B.S can be used for both qualitative and quantitative analysis. S.B.S is only for qualitative measurement.
4.
D.B.S is coasty compared to the S.B.S 

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