instrumentation of photoacoustic spectroscopy.

1. PHOTOACOUSTIC
SPECTROMETRY….
ISTRUMENTATION.
prepared by: Shweta singh
guided by: Dimal shah
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3. SOURCE:-
 The source depends upon the wavelength range that is
required for the study.
 Intense source are normally used in order to maximize the
output signal and to increase the signal-to-noise ratio.
 Tungsten filament lamp have been used in the spectral range
from about 600 to 1000nm.
 The most popular is high pressure(50 to 70 atm) xenon arc
lamp in spectral range of 250 to 2500nm.
 Hydrogen and dueterium arcs are useful from about 165 to
250nm.
 Krypton lamp can be used from 125 to 165nm.
 Open carbon arc lamp from 350 to 700nm can be used but the
disadvantage is that carbon electrode in the source are
consumed during emission and must be replaced periodically.
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4.  Lasers are available for use throughout most of the
spectral range used for PAS.
 the lasers can be operated in either the continous
or pulsed mode.
 both lasers that emit light and lasers that emit
continuum are used.
 E.g argon ion laser, nitrogen laser, Nd:YAG laser,
co2 laser, co laser etc.
 The intensity of the source is usually modulated, at
a single wavelength, either by pulsing the source or
by using a constant- intensity source with a
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chopper.

5. WAVELENGTH SELECTOR:-
 In instruments that use a continuous source, such
as a xenon-arc lamp, the radiation is usually made
monochromatic with a diffraction grating
monochromator.
 Sources that emit line radiation, such as some
lasers, can use a monochromator or filter to select
the proper line.
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6. CELLS:-
 Cell design depends on the physical state of sample.
 The cell must posses one transparent window through
which incident radiation can pass and acoustically
insulated to prevent interference from external signals.
 The body is constructed from stainless steel or polished
aluminum, that has large thermal mass.
 A substance that has a large thermal mass requires
absorption of a relatively large amount of energy before
its temperature is significantly raised.
 In addition cells are also constructed from copper, silver,
and gold.
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7.  Because of large thermal mass, scattered incident
radiation that strikes the cell body causes an
insignificant temperature rise and PAS signal
relative to those that occur when radiation strikes
an absorbing samples.
 The cell windows are constructed from material that
is transparent.
 Sapphire and quartz window are most commonly
used in UV region.
 In the visible region glass and plastic windows have
been used.
 In IR region – ZnSe is used.
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8.  When analyte is a gas, the cell is most often
cylindrical.
1. Single-pass cells:
 In those cells that have two windows on the end of
the cylinder, radiation makes a single pass through
the cell.
 In some cells acoustic baffle are used to decrease
noise within cell.
2. Multiple-pass cells:
 In this type both ends of the cylinder are coated
with a multilayer of a highly reflective dielectric
material.
 A hole through the dielectric substance allows
radiation to enter the cell. 8

9.  The radiation is reflected through the sample
several times by dielectric material.
 This type can be used for assays at lower
concentrations than single-pass cells.
3. Differential cell:
 Often consist of two sequentially connected cells
within the same cylinder.
 Each cell use separate detector. The incident
radiation sequentially passes through the cell.
 The combined cell and detector is a spectrophone.
 PAS in the IR region has been used as detector for
gas chromatography.
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11.  The distance between the sample and the window
should be greater than the length the length of thermal
diffusion in order to prevent loss of thermal energy from
the sample through the window.
 The volume of the filler gas within the cell is kept as
small as possible in order to maximize the PAS signal.
 The filler gas chosen to be non absorbing. Often air or
an inert gas is used. Used of an inert gas generally
results in greater analytical sensitivity.
 If the sample is a thin layer of a thermally thin
substance, incident radiation can strike the sample
without first passing through the filler gas because heat
can rapidly travel to inner surface of the sample.
 If the sample is not a thin layer of thermally thin
substance, radiation usually strikes the sample from 11
above after passing through a filler gas.

12. DETECTORS:-
 Microphone and piezoelectric transducer(PZT) have
been used as detector for PAS.
 Of the several type of microphones, the condenser
microphone is most common used.
1. Condenser microphone:
 Consist of circular, thin metallic diaphragm that is
mounted by its edge near a rigid stationary metallic
plate.
 The two plate serve as the plate of capacitor in an
electric circuit. The thin diaphragm is forced closer to the
stationary plate whenever a pressure wave strikes it.
 When pressure is no longer exerted on the diaphragm
by the wave, the diaphragm is returned to its initial
position by tension in the diaphragm.
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13.  The result is diaphragm that vibrates at the frequency of
the modulated radiation that strikes the sample.
 The distance through which the diaphragm modulates is
the function of amplitude of pressure wave that is
emitted from the cell.
 Microphone cannot be used in liquids.
 Piezoelectric transducer are primarily used for detecting
PAS signal in solutions.
2. Piezoelectric detector:
 Is a solid material such as lead zirconate titanate which
has a structure that is altered with application of
pressure.
 Structural alteration causes a rearrangement of charge
within a substance and an alteration of the potential
difference across the material.
 A piezoelectric transducer changes a pressure wave
into an electric signal which subsequently is amplified
and monitered.
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