Performance of Calcium Hollow Cathode Tube at Various Lamp Currents

Performance of Calcium Hollow Cathode Tube at Various Lamp Currents  

While the first hollow cathode tubes were constructed in such a way that they could be repeatedly flushed with the purified noble gas, the inconvenience connected with such equipment led to the development of permanently sealed tubes. In order to insure a reasonable lifetime of such tubes, they have to be of a certain minimum volume. One of the reasons for the lifetime limits is leakage of air into the tube, but more important seems to be the loss of the filler gas which is slowly absorbed by the metal and the glass surface. Since the lamp operates by the sputtering off of the cathode lining, gradual loss of the latter leads to eventual deterioration of the lamp. Lamps for metals that sputter abundantly, like the alkali metals, or zinc and cadmium, have short lifetimes, mostly well below a hundred hours. 

Although the flame appears to represent the most versatile means to vaporize a sample, it is limited to materials dissolved in a liquid that can be aspirated. Furthermore, some elements fail to produce effective vapors in ordinary flames. Several other methods of vaporization have been described, which are applicable to samples in the solid state and to elements not amenable to measurement in the flame. A short note on the application of atomic absorption to solids has been published (SI) but no details are given. 

The principle of the hollow cathode tube, production of a vapor of atoms by cathodic sputtering, has been employed by Gatehouse and Walsh (Gl) for sample vaporization. The sample is introduced into a vacuum chamber and is made the cathode which produces a cloud of activated atoms. The light of a separate hollow cathode tube is passed through this vapor and absorption is measured in a spectrophotometer. 

The method is said to yield reproducible results, and a straight working curve was obtained with silver. 

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