Light sources used in spectroscopic analysis introduced you to three broad categories of spectral light sources. Unfortunately there is no single source that is suitable over the entire UV– Visible spectral region (190 -800 nm). Spectrophotometers today come with more than one source with automatic changeover at defined wavelength for optimized and interruption free operation.
The desirable features for light sources are:
- Uniform light output over specified wavelength range
- Stability of response over extended time periods
- Long usage life span
The features of common light sources are reviewed here to help you make your choice:
Deuterium lamps are most common light sources for the UV region ( 160 – 375 nm). Excitation of deuterium gas at low pressure by applying a discharge results in generation of UV radiation and for applications in vacuum UV region special lamps operating down to 115 nm are available. The gas envelope and windows are made of quartz as glass absorbs in this region.
Deuterium lamps exhibit superior stability in comparison to the Xenon and Mercury- Xenon lamps .Deuterium lamps also have long service life. However, before operation sufficient warm-up time is necessary and a stable power supply is necessary for maintaining a constant current and the temperature of the lamp during operation.
Xenon sources cover the entire range UV- visible and even beyond (160 – 2000 nm). Emission is intense in the UV region which rapidly levels out in other regions. Such lamps have lower stability than deuterium lamps. Pulsed Xenon lamps provide additional benefits like less heat generation, longer lamp life due to lower duty cycle and reduced decay of sample due to intermittent exposure.
Mercury-Xenon lamps contain a mixture of Xenon gas and Mercury vapour. Such lamps emit a broad spectrum and include sharp peaks in both UV and visible region corresponding to mercury’s spectral lines. Such lamps have increased utility over xenon lamps due to high output intensity, stability and longer useful life.
Xenon or Mercury- Xenon lamps require warm up time of several minutes and cathode erosion starts affecting lamp stability which, however, can be overcome by using superior cathode materials.
Tungsten filament lamp has been conventionally used as a light source in the visible region covering 350 – 2200 nm. It consists of a thin coiled tungsten filament sealed inside an evacuated glass bulb. Power supply should ensure constant voltage to the lamp for a stable response
Tungsten – halogen lamps consist of a quartz envelope filled with an inert gas and a small amount of halogen such as iodine. Iodine forms tungsten iodide which is volatile in nature.The tungsten iodide molecules decompose on coming in contact with hot filament and redeposit tungsten onto the filament. Tungsten – halogen lamps bring the output closer to the UV region (240-2500 nm) and also extend useful lamp life to almost double of normal tungsten lamp.
It is important to make a judicious choice when deciding on the lamp requirement. You should base your choice on the required wavelength range, intensity requirement, stability, requirement of a stable supply and expected useful life.