Additional hints for worksheet 4.1: How to construct an absorption photometer on your own (1/4)
Questions:
- How can the layout of a photometer be improved over and above the information on worksheet 4.1?
- What measurement errors may occur and how can they be reduced or avoided?
The light source
- The spectrum of an absorption coefficient over a large spectral range shall be determined.
A broadband light source is needed. Its light will be separated into its spectral parts by a monochromator. Halogen lamps are the preferred light sources thanks to their luminance in ranges of 500 to 3000 nm. Their brightness is stable if they are used with a stabilised DC power supply. Operation on alternating current (with a transformator) or a non-stabilised plug adapter is unfavourable: running on 50 Hz, halogen lamps flicker on 100 Hz, because the filament is heated and cooled down at every halfwave, and the emission depends on the temperature of the filament. This can degrade the quality of the measured data.
The brightness of halogen lamps in the blue range is low and in the ultraviolet it is non-existent. Photometers in laboratories use deuterium lamps in addition to the halogen lamps for that reason. Deuterium lamps emit light at short wavelengths of 200 to 450 nm, but they are relatively expensive and require a special power supply. Their UV radiation is dangerous for the human eye and skin.
As an alternative, one could confine the trials to a smaller amount of wavelengths. LED or laser pointers are handy in that case because they can be easily exchanged one for another in the optical setup. This has to be possible in a reproducible manner to avoid casual measurement errors through inaccurate alignment. A monochromator is not necessary any more. Their brightness is stable, though it is dependent on temperature. The spectral bandwidth of LED is between 10 nm in the infrared and red and about 50 nm in the blue. This may lead to systematic measurement errors with substances having a highly variable absorption characteristic within the bandwidth of a LED.
White LED are blue emitting LED in fact: A part of their blue light is converted by a phosphor into a broad emission band at green, yellow and red wavelengths, whereby the impression of white is given. With a white light emitting LED a monochromator has to be used.
Laser pointers (in contrast to LED) are very bright and have a very small spectral bandwidth. Their radiation is well parallelised by an integrated lens. This is of great advantage because it makes the lens in front of the cuvette and the monochromator expendable.
