2. INTRODUCTION-
The measurement of light energy absorbed or emitted in matter and it’s interpretation
for obtaining the structural information or concentration of the molecular or atomic
species is called Spectroscopy.
Ultraviolet spectroscopy is the measurement of alteration of a beam of light after it
passes through a sample or after the reflection from a sample surface.
The range of U.V. Visible spectroscopy can be divided in to
0-180nm – Vaccum U.V.
180-380nm – U.V.(Normal)
380-750nm - Visible region.
INSTRUMENTATION OF U.V. VISIBLE
SPECTROSCOPY-
U.V. INSTRUMENT-
SCHEMATIC DIAGRAM OF SINGLE BEAM U.V.
SPECTROPHOTOMETER –
3. SCHEMATIC DIAGRAM OF DOUBLE BEAM U.V.
SPECTROPHOTOMETER-
COMPONENTS OF U.V. VISIBLE SPECTROSCOPY-
i-Source
ii-Filter of monochromator
iii-Sample cell
iv-Detector
i-Source-
4. It must be stable having sufficient intensity called light transmission at desired
wavelength.
It must supply light over the entire wavelength region of analysis.
Tungsten lamp is the light source of choice in the visible region but in the U.V.
region ditorium or hydrogen pump are required.
In tungsten lamp only 15% of radiant energy falls with in the visible region and above
28000 c but to remove the unwanted near I.R. light emitted monochromators are
employed to select the desired wave length.
For U.V. region light source like hydrogen discharge lamp(120-350nm),xenon arc
lamp,mercury arc lamp,Tungsten lamp are used.
Mercury Arc Lamp
Hydrogen discharge lamp
5. Xenon Arc Lamp
Tungston Lamp
ii-Filters of monochromators-
Filter is a device which allow light of desired wave length to pass and absorb or
distract the unwanted light.
Filters are generally of two types i.e. Absorption filter and Interference filter.
(Interference filter)
6. (Absorption filter)
Monochromators are also two types i.e. Prism and grating.
The main advantage of prism is that they undergo dispersion giving wavelength which
donot overlap but the disadvantage is dispersions are non linear.
For gratings linear dispersion is obtained but a suffer overlap of different spectral
order.To remove this problem filters are used to reduce radiation of different order
and straight radiation.
(PRISM MONOCHROMATOR)
(GRATING MONOCHROMATOR)
iii-Sample cell
Material of the cell should be non reactive.
7. In double beam spectrophotometer two cells are needed one for the reference and
other for the sample.This two cells comes in pairs.So,that the absorbance are almost
identical to minimize the error of measurement.
A>Barrier layer cell-
The detector has a thin film metallic layer coated with silver or gold and acts as an
electrode.
It has also a metal base plate which acts as another electrode.
These two layers are separated by a semiconductor layer of selenium.
(Barrier layer cell)
B>Photo cell-
It consists of light sensitive cathode in the form of half cylinder of metal contained in
a evaquated tube.
The anode is also present in the tube ,fixed along the axis of the tube.
When light falls on Cathode electrons are liberated and collected by Anode.There by
electricity is generated.
(Photo Cell)
* Photo multiplier tube-
In a vacuum tube, a primary photo-cathode is fixed which receives radiation
from the sample.
Some eight to ten dynodes are fixed each with increasing potential of 75-100V.
8. (Photo multiplier tube)
iv-Detectors-
After passing through the sample and references the light beam are focused together
on to the detector.
Any change in intensity of light of sample cell in comparison to the reference cell
produces a pulsing current which get converted to absorbance or transmittance unit.
APPLICATION-
Detection of Impurities- UV absorption spectroscopy is one of the best methods for
determination of impurities in organic molecules.
Structure elucidation of organic compounds- UV spectroscopy is useful in the
structure elucidation of organic molecules, the presence or absence of unsaturation,
the presence of hetero atoms.
Quantitative analysis- UV absorption spectroscopy can be used for the quantitative
determination of compounds that absorb UV radiation.
Qualitative analysis- UV absorption spectroscopy can characterize those types of
compounds which absorbs UV radiation. Identification is done by comparing the
absorption spectrum with the spectra of known compounds.
REFERENCE-
1-Principles of Instrumental Analysis, Skoog.
2-Instrumental methods of chemical analysis, Gurdeep R. chatwal. Page no2.116-2.122.
3-Elementary organic analysis, Principles and chemical applications , Y R Sharma, page
no12-14.
