Hollow cathode lamp
A hollow cathode lamp (HKL) is a special design of a glow lamp , a form of gas discharge tube , which is primarily used as an optical radiation source in an atomic absorption spectrometer (AAS) within the scope of instrumental analysis . The hollow cathode lamp functions in the atomic absorption spectrometer as a reference light source with a defined light spectrum . This precise and known light spectrum depends primarily on the material used on the cathode of the lamp.
construction
An essential component of a hollow cathode lamp is the special design of the cathode , which is formed from a small metallic vessel (pot) and, as shown in the adjacent sectional view, is open at the top. As with every glow lamp, the anode is located next to the cathode as an additional electrode in a glass body which is usually filled with noble gas at negative pressure. On one side of the outer glass body there is a special lamp base for receiving the electrical connections and the holder in the AAS, on the other side the beam exit covered with quartz glass .
When applying a high DC voltage from a voltage source in the range of a few 100 V with currents of a few 10 mA , a glow discharge occurs between the cathode and the anode . However, the plasma density of a glow discharge is too low by a few powers of ten for use as a reference light source in an atomic absorption spectrometer - the necessary high plasma density would only occur with a rod-shaped cathode during an arc discharge , which would lead to thermal destruction of the lamp with a conventional glow lamp. The remedy is the shape of a hollow cathode.
The formation of the hollow cathode means that significantly fewer ions and electrons can diffuse into the outer area of the lamp in the inner area of the cathode and are lost there by recombination for the plasma. Another effect to increase the plasma density is due to the changed electrical potential distribution in the hollow cathode, which leads to a longer residence time of ions in the interior. Primarily through these two effects, high plasma densities can arise inside the hollow cathode without the high power density of an arc discharge being required.
application
Element- specific hollow cathode lamps are used in atomic absorption spectrometry; the cathode consists of the element of the analyte. For this reason there are usually a large number of different hollow cathode lamps in analysis devices, which can be selected in a turret device for the respective analysis step. Alternatively, electrodeless induction lamps can also be used for AAS , because normal hollow cathode lamps show a significant decrease in the intensity of their emission lines below about 300 nm .
In the ultra-trace range , an atomic absorption spectrometer with a tin hollow cathode lamp z. B. tin according to (graphite tube and hydride technology of atomic spectrometry ), with detection limits of 0.2 µg / l being achieved with the graphite tube atomic absorption spectrometry . In hydride technology, the tin compounds in the sample solution are transferred as gaseous stannane (tin hydrogen) into the quartz cuvette of the atomic absorption spectrometer using sodium borohydride . There, the stannane breaks down into the elements at approx. 1000 ° C, with the atomic tin vapor specifically absorbing the tin lines of a tin hollow cathode lamp. Here 0.5 µg / l was given as the detection limit.
Web links
Individual evidence
- ^ Hollow cathode glow discharge. Fraunhofer IST, 2010, accessed February 8, 2014 .
- ↑ Markus Enders, Beate Termin: Atomic Absorption Spectroscopy. (No longer available online.) Uni-Heidelberg, 2002, archived from the original on March 19, 2013 ; Retrieved February 8, 2014 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ K. Cammann (Ed.), Instrumentelle Analytische Chemie , Spektrum Akademischer Verlag, Heidelberg-Berlin, 2001, pp. 4-47.
