Spectrophotometry is the name given to the study of electromagnetic spectra. Several different type of unit are used in schools when studying this field – from full size desktop versions costing from £1000 to smaller, cheaper USB devices which use CCD chips similar to digital cameras.
A spectrophotometer is essentially a photometer (a light intensity meter) that can measure intensity as a function of the color, or more specifically, the actual wavelength of light. Spectrophotometry deals with more than just the physical spectrum, it also quantifies near-ultraviolet and near-infrared wavelengths.
Units usually consist of two separate instruments. One for producing light of any selected colour (wavelength), and a photometer for measuring the intensity of light. The two are arranged so that liquid in a cuvette can be inserted between the spectrometer and the photometer. The amount of light passing through the sample is measured by the photometer. The photometer delivers a voltage signal to a display device or PC. The signal changes as the amount of light absorbed by the liquid changes. In smaller CCD devices, a tiny chip, similar to those used in digital cameras is used in place of the photometer. These type are becoming more popular as their costs dwindle. They are also smaller and generally require less maintenance.
When purchasing a spectrophotometer, several factors should be looked at: The technique used to measure the wavelengths, how the spectrum is acquired and the levels of intensity variation they are designed to measure. Most lab suppliers should provide datasheets which will tell you if the device is suitable for your intended application.
The working sequence of a spectrophotometer is usually as follows:
- The light from the source passes through the sample.
- The sample absorbs light.
- The detector detects how much light the sample has absorbed.
- The detector then converts how much light the sample absorbed into a number.
- The numbers are either plotted straight away, or are transmitted to a computer to be further manipulated (e.g. curve smoothing, baseline correction)
Benchtop units are very expensive for most schools but modern USB powered devices ar commonplace in lab supply catalogues and provide more than an adequate level of accuracy for most applications in secondary sciences.