Wednesday, July 24, 2013

Spectrophotometry - UV-Visible Spectrophotometry

We have seen the basic instrumentation of spectrophotometer in the last post. Here, we will discuss the instrumentation in more details. In this post, we will discuss one of the types of spectrophotometry - UV-Visible spectrophotometry.

Components of UV-Visible Spectrophotometer:
We will discuss the following as the components of UV-visible spectrophotometer.

  • Source
  • Monochromator
    • Prism
    • Diffraction Grating
  • Cuvettes
  • Detectors
    • Photomultiplier
    • Silicon Diode
    • Diode Array 
1. Source
The source should be stable during the measurement period, i.e.; the intensity of the radiation that is emitted should not fluctuate. Also, the radiation source should not change abruptly over its wavelength range.
Source of UV radiation:
Ultra-violet (UV) light is generally derived from a deuterium arc that provides emission of high intensity and adequate continuity in the range of wavelength 180-380nm. The mechanism for this involves the formation of an excited molecular species, which breaks up to give two atomic species and an UV photon. A quartz or silica envelope must be used in these lamps and the cuvettes should also be of the same material because of the heat generated and also to transmit the shorter wavelengths of UV radiation (glass absorbs radiation of wavelengths less than 350nm). However, the limiting factor is the lower limit of the atmospheric transmission at about 190nm.
Source of visible radiation:
The main source of visible radiation is tungsten filament lamp. This type of lamp uses the wavelength 350nm-2500nm. Recently, in modern instruments, the source is tungsten-halogen (also described as quartz iodine) lamp. In this type, lamps contain a small amount of iodine in a quartz ‘envelope’ which also contains tungsten filament. Here, iodine reacts with tungsten (gaseous state formed by sublimation) thereby producing the compound WI2 which is volatile. When the WI2 molecules hit the filament they decompose, redepositing tungsten back on the filament. The lifetime of a tungsten/halogen lamp is approximately double than that of an ordinary tungsten filament lamp. These lamps are very efficient. 

2. Monochromator:
Coming to the next component of UV visible spectrophotometry. The function of a monochromator is to produce a beam of 'monochromatic radiation' meaning 'single wavelength radiation' that can be selected from a wide range of wavelengths.
The essential components of the monochromator are:
  • Entrance Slit
  • Collimating device (which produces parallel light)
  • Dispersing system (prism or dispersion grating)
  • Focusing lens or mirror
  • Exit slit
Polychromatic radiation (radiation of more than one wavelength) enters the monochromator through the entrance slit. The beam is collimated with the help of collimating device and then strikes the dispersing device (which can be prism or grating which is explained below) at an angle. The beam is split into its component wavelengths by the grating prism. By moving the dispersing element or the exit slit, radiation of only a particular wavelength leaves the monochromator through the exit slit. Below are explained two types of dispersing system as prisms and diffraction gratings.

a. Prisms:
A prism is a material that provides a continuous spectrum in which the component wavelengths are separated in space. It improves the definition of light entering the source through the prism by using an entrance slit which defines the incident beam. This beam then strikes the collimator which is used to produce a parallel beam at the prism as can be seen in the diagram showing a typical prism monochromator. Once the dispersion occurs, the spectrum is focused at the exit slit which scans the beam to isolate the required wavelength. Mirrors are used instead of lenses in UV-visible systems for the sake of efficiency and also is cheaper.

b. Diffraction Grating:
Diffraction grating is another means for producing monochromatic light. It consists of a series of parallel grooves (or lines) on a reflecting surface. These grooves are considered as separate mirrors from which the reflected light interacts with light reflected from neighbouring grooves to produce interference, and so to select preferentially the wavelength that is reflected when the angle of grating in the incident beam is changed.

Advantages of gratings over prisms:

There is better resolution and linear dispersion. Hence, the constant bandwidth and simpler mechanical design for wavelength selection. 

3. Cuvettes:
The container in which the sample or the reference solution is placed is called the cuvette. It must be transparent to the radiation which will pass through it. The material of which the cuvette is made is generally quartz or fused silica for spectroscopy in UV region. Silicate glasses can also be used for the manufacture of cuvettes for its use between 350 and 2000nm. 

4. Detectors:
There are four principal detectors as the photoconductive cell, the photomultiplier, the silicon diode, and the diode array. Of these four, the photoconductive cell is so severely restricted in both wavelength response and sensitivity and hence is never found in instruments these days. The most commonly encountered detectors are the last three which we will very briefly discuss here one by one.
a. Photomultiplier:
Light causes emission of electrons from a photocathode which accelerate past a series of dynodes. Electrons striking the first dynode release a secondary emission that is stronger than the original beam and so on through the series of the dynodes to produce a cascade effect. The electron density released by the final dynode to the anode can be many orders of magnitude greater than that from the cathode. The photomultipliers have an internal amplification that gives them great sensitivity and a wide spectral range.
b. Silicon Diode:

These are mechanically robust (as it is a solid-state device) and electronic benefits include reduced power supply and control circuit requirements.
c. Diode Array:
Diode array is an assembly of individual detector elements arranged in a linear manner or matrix-form which can be mounted so that the complete spectrum is focused onto an array of appropriate size. 

This was about the UV-visible spectrophotometry.