Hollow cathode lamp (HCL):

It consists of a sealed cylindrical glass tube along with a quartz window at one end and a hollowed cylindrical cathode together within an anode wire made of tungsten.  The cathode is fabricated from the analyte element and the lamp is filled along with an inert gas such as argon or neon under vacuum (100-200 Pa). The schematic diagram of a hollow-cathode lamp is displays in Figure.  While a voltage of ~300 V corresponding  to 5-50 m A current is applied among the two electrodes,  a low  pressure  glow  discharge   confined  to  the  within  of  the  cathode  material  is generates.

Figure: Schematic diagram of hollow cathode lamp illustrating different components

Basic function of the gas within the tube is to bombard the cathode and vapourise the atoms from the cathode surface. For that the gaseous cations are accelerated towards the cathode.   Collision energy is enough to cause some atoms of the cathode to be transformed within gaseous atoms in a procedure known as sputtering. These metal atoms are  then  excited  through collisions  along with  electrons  and  ions  therefore  emitting  features emission lines.

The emission spectrum  of the cathode  material  involves  a number  of intense,  sharp lines  because of  transitions  among  excited  states  and  the  ground  state,  frequently  known as resonance lines. Intensity of resonance lines from an HCL rises with increasing current.  As of now, HCL for over 60 elements are available.  Therefore, these days multielement cathode lamps are more in use for routine determinations, by their performance is not extremely reliable. In that case, cathode is made up from alloys of metals having  same  melting  point  such  as  Ca-Mg,  Ag-Au,  Cu-Fe,  Zn-Cd,  etc.  While elements  having various  melting points are used, more volatile element is lost first resulting  within gradual  weakening  of its  spectrum  and  degeneration  within  one  element lamp.