Hollow Cathode Lamps

Hollow Cathode Lamps

A hollow cathode lamp gives a high intensity, narrow line wavelength of element to be determined

Hollow Cathode Lamp Schematic

Hollow Cathode Lamp

The hollow cathode lamp consists of a glass cylinder filled with an inert gas usually Argon or Neon at low pressure. The cathode is made from metal which is to be determined.. The emission line of the lamp corresponds with the absorption wavelength of the analyte. The end window of the lamp is usually made of Quartz or Pyrex that transmits the spectral lines of the element to be determined.

Following stages are involved in light emission from Hollow cathode lamp:

• Sputtering – filled gas is ionized when potential difference is applied between the anode and the cathode. Positively charged inert gas ions strike the negatively charged cathode and dislodge metal atoms.
• Excitation – sputtered metal atoms are excited to impact with the ionized gas
• Emission – light of wavelength specific to the element comprising the cathode is emitted when the atom decays from the excited state to the normal state

Hollow cathode lamps have a shelf life as well as usage lifetime defined in milliampere hours. Increasing current increases lamp intensity but excessive current reduces lamp life and also results in self absorption broadening ,i.e, atoms in the hollow cathode lamp begin to absorb light emitted from the hollow cathode lamp itself. This leads to lower absorbance and reduction in the linear range of calibration curve.

Multielement Hollow Cathode Lamps

The cathode of multielement lamps is made from alloying compatible elements without overlapping line spectra. Examples of such lengths are Ca-Mg,Cu-Fe-Ni, Cu-Fe-Mn-Zn, etc. All elements of multielement hollow cathode lamps can be determined sequentially without need for change of lamps in between. Multielement lamps provide advantages of cost, speed of analysis but the sensitivity is lower in comparison to individual element determination by single element lamp

Limitations of Hollow Cathode Lamps

• Hollow cathode lamps have a shelf life
• With the exception of multielement lamps the lamp needs to be changed for determination of different elements
• Sputtering deposits metal atoms on sides and end windows which affects lamp life and more so for volatile elements
• Some cathode materials liberate hydrogen on heating which contributes to continuum background emission

Electrodeless Discharge Lamps

For most elements hollow cathode lamp is a satisfactory light source. In case of volatile elements reduced lamp life and low intensity can be overcome by use of high energy throughput electrodeless discharge lamps. Electrodeless discharge lamps are commonly available for Sb, As, Bi, Cd, Cs, Pb, Hg, K, Rb, Sn, Te, etc.

Electrodeless Discharge Lamp Schematic Diagram

Electrodeless Discharge Lamp

An EDL consists of a quartz bulb filled with an inert gas containing the element or a salt of the element for which the lamp is to be used. The bulb is placed inside a ceramic cylinder on which antenna for a RF generator is coiled. When an RF field is applied to the bulb, the inert gas is ionised and the coupled energy excites the vaporized atoms inside the bulb and causes emission of characteristic light. EDL’s offer advantage of lower detection limits. The useful life of an EDL is considerably longer than that of a hollow cathode lamp of same element.