Radiation Source Use in Spectroscopy
There are many radiation sources used in atomic
spectroscopy. Some of them are given below:
Hollow Cathode Lamp
It emits the spectrum of that metal which is used in the cathode. A different hollow cathode lamp is used for each element to be
analyzed.
Hollow Cathode Lamp Principle:
A light source that emits exactly the wavelength required
for the analysis without using any additional monochromator.
Instrumentation of Hollow Cathode Lamp
A cylindrical hollow glass tube that contains a
cathode made of analyte metal or coating of the metallic compound and an anode
containing a tungsten wire.
It has a sealed glass tube containing inert gas Ne/Ar
at low pressure (1-5) torr.
Its lamp window is made of Quartz or Pyrex.
When the lamp is
turned on DC Voltage of 300V is applied, and M(atoms) are supplied with energy
which causes them to elevate to M*. Upon returning M* M˚ exactly the same wavelength is emitted.
Working Mechanism of Hollow Cathode Lamp
Its working mechanism is divided into three
different steps
Ø Sputtering
Ø Excitation
Ø Emission
When the lamp is on, atoms are ionized, and Neon is bombarded
toward the surface of the cathode. Atoms of that element are sputtered M˚ and
become excited.
When Excited atoms return to ground state M˚ to M*
they emit radiation of the same wavelength.
Electrodeless Discharge lamp
Electrodeless discharge lamps (EDLs) consist of an element or a salt of the element. The outer tube is evacuated. The bulb is contained in a ceramic cylinder on which an RF coil is wound. When an RF field of 500-1000V power is applied, the inert gas Ar is ionized and the coupled energy vaporizes the element and excites the atoms inside the bulb, resulting in the emission of the characteristic spectrum sealed in a quartz bulb that contains an inert gas atmosphere.
Comparison
of HCL and EDL
Hollow cathode lamp |
Electrodeless Discharge Lamp |
|
Ø
Less stable. Ø
Intensity of light is less so they emit less
radiation. Ø
Produce smaller images. Ø
As, Pb, Sn, Bi, and Hg are volatile elements and
cannot ionize through a Hollow cathode lamp. |
Ø
More stable. Ø
High intensity and sensitivity. Ø
Produce a larger image. Ø
It can also ionize more than 15 volatile elements
which cannot be ionized by HCL. |
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