Hollow-cathode lamp
A hollow cathode lamp ( HKL) is a special design of a neon lamp, a form of gas discharge tube, which primarily as a source of optical radiation in an atomic absorption spectrometer (AAS ) is used in the context of instrumental analysis. The hollow cathode lamp can be used in the atomic absorption spectrophotometer as a reference light source having a certain light spectrum. This precise and well-known spectrum of light is primarily dependent on the material used in the cathode of the lamp.
Construction
An essential component of a hollow cathode lamp is the special design of the cathode, which consists of a small metallic vessel (pot ) is formed, and as shown in the adjacent sectional view at the top is open. As with any Glimmlapme is adjacent to the cathode, the anode, as an additional electrode in a normally filled with a noble gas at subatmospheric pressure vitreous. On one side of the outer glass body is a special lamp base for receiving the electrical connections and the holder in the AAS, on the other side of the covered with quartz glass beam output.
Upon application of a high DC voltage from a voltage source in the range of a few 100 V at 10 mA flows with some occurs between the cathode and anode of a glow discharge. The plasma density in a glow discharge is too low for use as a reference light source in an atomic absorption spectrometer, several orders of magnitude - the necessary high plasma density would occur if the rod-shaped cathode until an arc discharge which would result in a conventional discharge tube for the thermal destruction of the lamp. Remedy represents the structural shape of a hollow cathode
By the formation of the hollow cathode significantly less ions and electrons, can diffuse to the outside of the lamp, and there will be lost due to recombination of the plasma in the inner region of the cathode. Another effect of increasing the plasma density is caused by the change in electric potential distribution in the hollow cathode, which results in a longer residence time of ions in the interior. Primary through these two effects can lead to need inside the hollow cathode high plasma densities without the high power density of a sheet charge.
Application
In the element-specific atomic absorption spectrometry hollow cathode lamps may be used, the cathode is made of the element of the analyte. For this reason are in analyzers usually have a larger number of different hollow cathode lamps, which can be chosen in a turret for each analysis step. Alternatively, electrodeless lamps may be used for induction AAS, for normal hollow cathode lamps to show less than about 300 nm, a significant deterioration in their intensity emission lines.
In the ultra trace range has one using an atomic absorption spectrometer according to ( graphite furnace and hydride atomic spectrometry ) with the graphite furnace atomic absorption spectrometry detection limits of 0.2 ug / l are achieved with tin hollow cathode lamp for example tin. In the hydride, the tin compounds of the sample solution by means of sodium borohydride as gaseous stannane ( Zinnwasserstoff ) are transferred into the quartz cell of the atomic absorption spectrometer. There stannane decomposes at about 1000 ° C in the elements, the atomic tin vapor absorbs the specific tin lines tin- hollow cathode lamp. Here have been reported as detection limit of 0.5 ug / l.