Hollow-cathode lamp
The cathode is made of the pure metal of interest or a mixture of metals containing the metal of interest.   It is important that the cathode be stored under a vacuum to avoid any kind of contamination. Contamination of the cathode compromises the purity of the metal of interest and the data obtained for that metal. The material of the window is specifically selected in order to get the best transmission of spectral lines for the cathode element.