What you need to know about Hollow Cathode Lamps
Hollow Cathode lamps are line sources designed for use primarily in atomic absorption spectrophotometers. They are gas discharge devices where the discharge is constrained within the cathode of a specific metal. This results in a line output where the wavelength and intensity is dependant on the metal
Single Element vs Multi-ElementHollow Cathode lamps are available as either single element or multi-element depending on the application. Multi-element lamps can be used to test for different elements and give multiple lines but there will be a proportionate decrease in the intensity.
Each cathode contains the element of choice in a very pure form (99.99%) where possible. Alloying is used where the element of choice may not be available in a suitable form (such as mercury, sodium etc.) or to reduce the cost where the metal in the pure form is expensive. In all of these cases the metal used to make the alloy will be carefully selected to ensure that the wavelengths do not overlap or interfere
Glass Type and Fill GasThe type of glass used to make the lamp envelope and the fill gas inside the lamp is optimised to achieve the highest sensitivity.
The choice of glass will depend on the principle wavelength. Borosilicate glass is normally used for wavelengths over 400nm, UV transparent glass is used for 240-400nm and quartz is used for wavelengths shorter than that.
The fill gas (normally neon) is always chosen so that the spectral output of the cathode is free from interference. Argon and other inert gases are also used where appropria
DimensionsUnlike Deuterium lamps, hollow cathode lamps are almost universal in shape. Lamps are available with a 50mm (2inch) diameter for Perkin Elmer instruments and with a 37mm (1.5inch) diameter for fitting into all other commercially available instruments
Data Coding and Self-Reversal (Smith-Hieftje)All variants are available in standard, data coded and self-reversal (Smith-Hieftje) background correction versions. Data coded bases permit some instruments to automatically identify the lamp when it is plugged in. (This option is only available to Varian, Perkin Elmer and Unicam instruments).
The self-reversal (Smith-Hieftje) correction lamps are mainly used in older spectrophotometers as a method of measuring the background absorption. This involves passing a high current momentarily through the cathode to produce a dense cloud of neutral atoms to block the lamp output. This enables the background to be measured and subtracted from the sample signal. These lamps have extra insulation to cope with the high voltage pulse and can be used in normal instruments (not all elements can be used with this technique).
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