“A man is but the product of his thought, what he thinks, he becomes”
— M.K. Gandhi
Light sources are generally of two types. You’d be familiar with ‘continuum light sources’ such as Sun or a light bulb which emit electromagnetic radiation in the wavelength range from about 250 to 700 nm in the visible region which we see as normal white light. The white light comprises of several different wavelengths which constitute the colours of the rainbow. The other type of light sources are ‘line sources’ which emit light of a specific wavelength and it is such light sources which are used in Atomic Absorption Spectroscopy. Now you shall be introduced to such light sources
Hollow Cathode Lamps
A hollow cathode lamp gives a high intensity, narrow line wavelength of element to be determined
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.
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.
The next module will introduce you to flame automization of samples.