A Spectrophotometer is a device which measures the absorption of a specific wavelength of light by the sample which is kept in the beam path. White light comprises of a combination of colours and absorption of the unresolved beam is hardly of any benefit to the analytical chemist.
Light is dispersed into its components by means of a dispersing element such as a prism or a grating. Before the advent of dispersion devices absorption studies were carried out using coloured glass filters in the beam path which isolated the selected colour beam for passing through the sample and the instrument was called a colorimeter.
Filters are transparent coloured glass or polymer films which are manually mounted in the beam path or fixed permanently on the circular mount in the beam path to achieve required wavelengths selection. The colour of the filter is complimentary colour of the test solution.
|Colour of solution||Complimentary colour||Wavelength band|
|Red||bluish green||490 – 500 nm|
|Orange to yellow||blue||435 – 490 nm|
|Yellowish green||violet||400 – 435 nm|
|Bluish green||red||650 – 700 nm|
|Violet yellowish||green||560 – 580 nm|
|Purple||green||500 – 560 nm|
Advantages of Spectrophotometers
Speed of Analysis
The spectrophotometer automatically scans the selected wavelength range whereas in case of filter colorimeter: the filter choice decides the band selection. This approach is suitable for measuring absorbance at selected wavelengths but if a spectrum is required point by point absorbance readings are needed to plot the absorbance graph. This is a time consuming process.
A combination of light dispersing devices in spectrophotometers such as a grating and the monochromator provides a narrow band isolation whereas the filters provide a broadband selection only. Narrow bands result in higher resolution.
Accuracy of Results
Filters used in colorimeters do not retain their transmission properties over usage as these properties are affected by gradual fading by the light beam from the source and also loss of transmission due to carelessness in handling and exposure to laboratory environment.
Apart from absorption in the visible region a large number of organic compounds absorb specifically in the UV region. Filters are generally restricted to absorption over visible wavelength of light
The only advantage of filter colorimeters is their lower cost in comparison to spectrophotometers but accuracy and reproducibility of results gets compromised. For these reasons spectrophotometers have replaced filter colorimeters even in school laboratories.