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Study Notes: UV-Vis Radiation Source

The best light source would be one that gave good intensity with low noise across all ultraviolet and visible wavelengths and also provided stability over a long period of time. This ideal however does not exist and so two separate sources are commonly used in a spectrophotometer.

Noise relates to unwanted signal fluctuation that is inherent in an instrument to some extent or other. It arises from various uncontrolled sources such as temperature variation and voltage variation in the power supply. Too much noise can affect the accuracy, precision and sensitivity of an analysis. Signal to noise ratios are often quoted to indicate the performance of an instrument - the higher the better.

A high noise spectrum as shown by much random fluctuations in the scan compared to the same spectrum with low noise where a smooth scan results.

A tungsten filament lamp is commonly used as the visible light source - this is just like a normal light globe. The wavelength range for this type of lamp extends across the entire visible range and extends into the ultraviolet (300 nm to 3000 nm). Characteristics include very low noise and a long service life of typically 10,000 hours. A tungsten filament lamp

Ultraviolet light is normally supplied by a deuterium arc lamp made from quartz. It produces an intense UV output in the range from about 180 nm to 380 nm (with some useful intensity into the visible region). Glass is not used in the manufacture of UV lamps as it is not transparent to ultraviolet light whereas quartz is.

A deuterium filled lamp Deuterium lamps have low noise characteristics (that can be the limiting factor when it comes to noise performance in a spectrophotometer). Intensity decreases steadily over time such that a typical service life is 1,000 hours.

Some spectrophotometers may require the lamps to be manually switched on, while for others the lamps function automatically. In any event, a 30 minute warm up time is required to allow lamp intensity to become constant (during this time the detector is also warming up to maximum sensitivity).

Spectrophotometers are very reliable instruments in general. The most common problem will be lamp failure. Remember this when troubleshooting!

Note that tungsten and deuterium lamps are known as continuous sources as they emit radiation over a wide wavelength range with only low variation in intensity. Line sources, on the other hand, emit a limited number of discrete radiation bands covering a limited range of wavelengths.

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