1. Prepared and presented by:
Pinak R. Patel
Assistant Professor
Department of pharmaceutical chemistry
Dharmaj degree pharmacy college, Dharmaj

2. Photo acoustic spectroscopy is in which an impinging light
part of a class of photo beam is absorbed and alters
thermal techniques the thermal state of the
sample
This "thermal state" can manifest One method of detection is to
itself as a change in temperature, experimentally measure the
density, or other measurable temperature or density of the absorbing
property of the sample material. This is referred to as
thermometric detection.
However, if the incoming light is modulated, the absorbing sample
Actual warms and cools in a cycle. If the cycle is so fast that the sample
phenomena does not have time to expand and contract in response to the
modulated light, a change in pressure develops.
This pressure "wave" can lead to the production of a sound wave. These sound
waves can be detected by a sensitive microphone, piezoelectric devices, or optical
methods (for example, deflection of highly collimated light reflecting from the
surface). These techniques are more properly called photo acoustic techniques.

3. • The photo acoustic effect of matter was first discovered by Alexander
Graham Bell in 1880. He found that if he aimed a strong light source at a
surface, and pulsed the light by turning it on and off, an acoustic signal with
similar frequency was emitted from the surface.

4. • Photo acoustic spectrometry (PAS) is also referred to as opto acoustic
spectrometry.
• It uses modulated UV-VIS, infrared or microwave radiation which impinges
upon gaseous, liquid or solid sample.
• It excite sample during absorption.
• After excitation some of the molecules returns to the ground state by
radiation less process. (but heat is released).
• The thermal energy that is emitted during the relaxation will cause expansion
of the gaseous or liquid sample or expansion of filler gas above the liquid or
solid sample in the cell.
• Because the incident radiation is modulated, the gas periodically expand and
contracts at a modulated frequency.
• So the pressure waves which are generated are measured with the help of
sound detector.

5. If the incident radiation is in UV region Now relaxation can occur by
and is absorbed by the sample it will fluorescence, phosphorescence, or
cause the excitation of the molecule to radiation less process
the excitation state
Now during the radiation less relaxation if
heat is emitted as a form of energy it does So if the incident radiation us
contribute to the PAS signal. in the IR or microwave region ,
the molecule can absorb
radiation by excitation to
Heat H which is formed during the radiation higher vibration and rotational
less relaxation is directly proportional to the levels rather than emission of
absorptivity of the sample a and the intensity photon as in case of
of the radiation source absorption after UV radiation.
H= al So only heat is produced
- Absorptivity refers to the amount of the
energy absorbed by the system.
- High intensity sources are required for the
generation of heat which is required for the
generation of PAS signal

6. Regardless of the wavelength of incident radiation, the radiation is modulated at a
frequency that causes a pressure wave to be formed. (mtlb k[ j λ[ n) light hs[ a[ j
pressure waves n) frequency hs[ )
This modulation is achieved by using pulsed source e.g. pulsed LASER or by using
continuously operated source and a chopper.
The frequency of the modulation must be slow relative to the rate of relaxation in the
molecule in order to allow the measured PAS signal to oscillate at same frequency as that
of modulated incident radiation. So generally modulation frequencies between range of
20 Hz and 100 Hz is generally used.
The modulation frequency is adjusted to a value that is different from any natural
environment oscillations that might interfere with the analysis and to a value that
maximize the intensity of the measured signal.

7. Now mechanism by which heat from the sample is converted to a pressure wave depends
upon the physical state of the sample
If the sample is gas or liquid that fills the cell, the heat that is emitted during the radiation
less relaxation process increases the temperature of the sample. This increased
temperature causes increased pressure within the fixed volume cell. (for gases ideal gas
law is used to predict the increased pressure.)
When radiation is not striking the sample, the temperature and pressure decreases
Pressure waves developed in the sample at a frequency to that of incident radiation
frequency, and now it can be heard as sound waves.
For gaseous sample generated pressure waves are monitored with the help of
microphone and for liquid samples piezoelectric detectors are used.
If solid or liquid sample does not fill the entire volume of the cell a non absorbent filler
gas is used in the cell. Usually the filler gas is air or any other inert gas. The gas is chosen
so that it will nor absorb the given radiation ,the from the sample is transferred to the
inert gas resulting in its expansion and generation of pressure waves.

8. Intensity of PAS signal for a sample that does not fill the entire cell depends upon
absorptivity of the sample and thermal diffusion length of the sample (thermal
diffusion length: it is the distance in the sample through which
thermal energy can be conducted during a fixed period.)
CONCEPT OF TDL (Thermal diffusion length)
Thermally thick sample: relatively small diffusion length
Thermally thin samples: relatively large diffusion length
For thermally thick samples radiation is absorbed in 1st thermal diffusion length
Which has effect on PAS signal
Though radiation is absorbed significantly deep. They can not penetrate rapidly to warm
the surface.
It means that if TDL increases, more radiation
is absorbed in to interior of the sample and
can travel the surface of sample and warm
the gas.

9. This TDL also depends upon modulation frequency of incoming radiation.
So as modulation frequency is increased there is increase in TDL
For thermally thin samples:
Heat formed in the sample can rapidly travel the surface and affect the PAS signal.
So finally we can say that opaque solid or liquid that is,
thermally thick and for which TDL is less than distance of optical penetration,
PAS signal is proportional to optical absorptive coefficient and to TDL.
that opaque solid or liquid that is, thermally thin and for which
TDL approaches or exceeds distance of optical penetration (sample length)
PAS signal is inversely proportional to modulation frequency and is independent
of optical absorptive coefficient