Question

1) The emission spectrum of a hollow cathode lamp for molybdenum was found to have a sharp peak at 313.3 nm as long as the lamp current was less than 50 mA. At higher currents, however, the peak developed a cup-like crater at its maximum. Explain. 2) Why does the cathode of the hollow cathode lamp have a cylindrical shape? Why is an element lamp required as the source for AA? Which copper line was used for analysis of copper in the CAM experiment?

Answer 1

1). This phenomenon is attributed to self-absorption. Self-absorption is a process observed when a HCL lamp is operated at high currents where sputtering of a large number of atoms takes place. Most atoms do not get enough excitation energy in this process and thus absorb emitted radiation from other excited atoms within the lamp. The result is a splitting of the atomic line where the center of the line would have a minimum exactly at the wavelength of the original line.

2).

The common source of light is a “hollow cathode lamp”. This contains a tungsten anode and a cylindrical hollow cathode made of the element to be determined. These are sealed in a glass tube filled with an inert gas – e.g. argon or neon – at a pressure between 1Nm-2 – 5Nm-2.

When an electrical potential is applied between the anode and cathode, some of the inert gas atoms are ionized. These gaseous ions bombard the cathode and eject metal atoms from the cathode in a process called sputtering (Fig). Some sputtered atoms are in excited states and emit radiation characteristic of the metal as they fall back to the ground state. The shape of the cathode concentrates the radiation into a beam which passes through a quartz window, and the shape of the lamp is such that most of the sputtered atoms are redeposited on the cathode.

The hollow cathode lamp is a type of glow discharge tube that uses a hollow cathode to enhance the emission intensity. Compared to parallel plate electrodes, using a hollow cathode increases the current density by more than 10 times and this is accompanied by a significant increase in light intensity and a lower voltage drop in the lamp. This is known as the hollow cathode effect (or hollow effect).

The light produced by the hollow-cathode lamp is emitted from excited atoms of the same element which is to be determined. Therefore the radiant energy corresponds directly to the wavelength which is absorbable by the atomized sample. This method provides both sensitivity and selectivity since other elements in the sample will not generally absorb the chosen wavelength and thus, will not interfere with the measurement. To reduce background interference, the wavelength of interest is isolated by a monochromator placed between the sample and the detector.