INSTRUMENTATON METHODS OF ANALYSIS B.PHARM 7TH SEM PHARMACY SUBJECT UV VISIBLE DETECTORS

1. Detectors
• Detectors:
• An infrared detector is a detector that
reacts to infrared radiation.
• It is simply a transducer of radiant
energy.
• Detectors are used to measure the
intensity of unabsorbed infrared
radiation.

3. 1. Thermal detectors:
•
• Can be used for wide range of wavelength.
• Operate at room temperature.
• Slow response time & lower sensitivity hence not
use for FTIR.
• Thermal detectors are better choice except in the
near infrared where photoconductivity cells are
generally used.

4. Thermocouple
• Made up of 2 dissimilar metal like bismuth &
antimony coated by metal oxides which produce
small voltage proportional to temperature of
junction.
• If wires of 2 dissimilar metals joined head to
tail, then a difference in temperature between
head & tail causes a current to flow in the wire.
• This current is proportional to the intensity of
radiation.

6. Cont….
These are also called as thermopile detectors.
• Materials should be thermally active.
• These are used in dispersive instruments.
• Response time is 30 milliseconds.
• They give response for all frequencies.
• The potential difference (voltage) between the
junctions changes according to the difference in
temperature between the junctions.
• Heating capacity of absorbing element is small
as to detect the change in temperature.

7. Cont….
• Potential varies in the 2 junctions with the difference
in temperature.
• One junction is cold junction which is reference
junction kept at constant temperature not exposed to
IR radiations.
• The other junction is hot junction exposed to IR
radiations which increases temperature of junction.
• Temperature difference due to falling IR radiations
generates the potential difference which amounts to IR
radiations.
• Hot junction is generally blackened to improve its
heat absorbing capacity.
• Advantage- Simple & used for short IR wavelength
• Disadvantage- slower response time & sensitivity

8. Bolometers
• Bolometer is derived from a Greek word (bolometron)
Bolo- for something thrown
Metron- measure
Construction-
• It consists of an absorptive element such as a thin layer of
metal
• Most bolometers use semiconductor or superconductor
absorptive elements rather than metals.
• Its resistance changes due to increase in temperature when IR
radiation falls on it.
• It is electrical resistance thermometer which can detect &
measure feeble thermal radiation.
• The electrical resistance increases approx. 0.4% for every
Celsius degree increase of temperature.

9. It is made of 2 platinum strips, covered with lamp
black, one strip is shielded from radiation & one
exposed to it.
• The strips form 2 branches of Wheatstone
bridge.

10. Working
• The circuit thus effectively operate as
resistance temperature detector.
• When IR radiations fall on the exposed strip,
unbalance occur due to change in resistance ,
this causes current to flow through
Galvanometer & the amount of current
flowing is the measure intensity of IR
radiation.
• The response time is 4 seconds.

11. Golay cell
• It consists of a small metal detector closed by a rigid
blackened metal plate (2mm) , flexible slivered
diaphragm at the other end filled with xenon gas.
• Its response time is 20 milliseconds, hence faster
than other thermal detectors
• It is suitable for wavelengths greater than 15μ.
• Also known as golay pneumatic detector.
• Gas filled chamber undergoes a pressure rise when
heated by radiant energy.
• It is based on expansion of gas measuring device.

13. Working
• The radiation falls on the blackened metal plate, this heats the
gas which leads to deformation of flexible silvered diaphragm.
• The light from a lamp inside the detector is made to fall on
the diaphragm which reflects the light on to a phototube.
• The signal seen by the phototube/photocell is modulated in
accordance with the power of the radiation beams incident on
the gas cell.
• Advantage-
• Wavelength is very wide.
• Response time is much faster than bolometer or
thermocouple.

14. Apllications of IR
• APPLICATIONS OF IR SPECTROSCOPY
• Infrared spectroscopy is widely used in industry as well as in
research. It is a simple and reliable technique for measurement,
quality control and dynamic measurement. It is also employed in
forensic analysis in civil and criminal analysis.
• Some of the major applications of IR spectroscopy are as follows:
• 1. Identification of functional group and structure
elucidation
• Entire IR region is divided into group frequency region and
fingerprint region. Range of group frequency is 4000-1500 cm-
1 while that of finger print region is 1500-400 cm-1.
• In group frequency region, the peaks corresponding to different
functional groups can be observed. According to corresponding
peaks, functional group can be determined.
• Each atom of the molecule is connected by bond and each bond
requires different IR region so characteristic peaks are observed.
This region of IR spectrum is called as finger print region of the
molecule. It can be determined by characteristic peaks.

15. 2. Identification of substances
IR spectroscopy is used to establish whether a given sample
of an organic substance is identical with another or not. This
is because large number of absorption bands is observed in
the IR spectra of organic molecules and the probability that
any two compounds will produce identical spectra is almost
zero. So if two compounds have identical IR spectra then
both of them must be samples of the same substances.
IR spectra of two enatiomeric compound are identical. So IR
spectroscopy fails to distinguish between enantiomers.
For example, an IR spectrum of benzaldehyde is observed as
follows.
C-H stretching of aromatic ring- 3080 cm-1
C-H stretching of aldehyde- 2860 cm-1 and 2775 cm-1
C=O stretching of an aromatic aldehyde- 1700 cm-1
C=C stretching of an aromatic ring- 1595 cm-1
C-H bending- 745 cm-1 and 685 cm-1
No other compound then benzaldehyde produces same IR
spectra as shown above.

16. Studying the progress of the reaction
Progress of chemical reaction can be determined by examining
the small portion of the reaction mixure withdrawn from time to
time. The rate of disappearance of a characteristic absorption
band of the reactant group and/or the rate of appearance of the
characteristic absorption band of the product group due to
formation of product is observed.
4. Detection of impurities
IR spectrum of the test sample to be determined is compared
with the standard compound. If any additional peaks are
observed in the IR spectrum, then it is due to impurities present
in the compound.

17. Quantitative analysis
The quantity of the substance can be determined either in pure
form or as a mixure of two or more compounds. In this,
characteristic peak corresponding to the drug substance is
chosen and log I0/It of peaks for standard and test sample is
compared. This is called base line technique to determine the
quantity of the substance.