Prepared and presented by: Pinak R. Patel Assistant Professor Department of pharmaceutical chemistryDharmaj degree pharmacy college, Dharmaj
Photo acoustic spectroscopy is in which an impinging lightpart of a class of photo beam is absorbed and altersthermal techniques the thermal state of the sampleThis "thermal state" can manifest One method of detection is toitself as a change in temperature, experimentally measure thedensity, or other measurable temperature or density of the absorbingproperty of the sample material. This is referred to as thermometric detection. However, if the incoming light is modulated, the absorbing sampleActual warms and cools in a cycle. If the cycle is so fast that the samplephenomena 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 soundwaves can be detected by a sensitive microphone, piezoelectric devices, or opticalmethods (for example, deflection of highly collimated light reflecting from thesurface). These techniques are more properly called photo acoustic techniques.
• 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.
• 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.
If the incident radiation is in UV region Now relaxation can occur byand is absorbed by the sample it will fluorescence, phosphorescence, orcause the excitation of the molecule to radiation less processthe excitation stateNow during the radiation less relaxation ifheat is emitted as a form of energy it does So if the incident radiation uscontribute to the PAS signal. in the IR or microwave region , the molecule can absorb radiation by excitation toHeat H which is formed during the radiation higher vibration and rotationalless relaxation is directly proportional to the levels rather than emission ofabsorptivity of the sample a and the intensity photon as in case ofof the radiation source absorption after UV radiation. H= al So only heat is produced- Absorptivity refers to the amount of theenergy absorbed by the system.- High intensity sources are required for thegeneration of heat which is required for thegeneration of PAS signal
Regardless of the wavelength of incident radiation, the radiation is modulated at afrequency that causes a pressure wave to be formed. (mtlb k[ j λ[ n) light hs[ a[ jpressure waves n) frequency hs[ )This modulation is achieved by using pulsed source e.g. pulsed LASER or by usingcontinuously operated source and a chopper.The frequency of the modulation must be slow relative to the rate of relaxation in themolecule in order to allow the measured PAS signal to oscillate at same frequency as thatof modulated incident radiation. So generally modulation frequencies between range of20 Hz and 100 Hz is generally used.The modulation frequency is adjusted to a value that is different from any naturalenvironment oscillations that might interfere with the analysis and to a value thatmaximize the intensity of the measured signal.
Now mechanism by which heat from the sample is converted to a pressure wave dependsupon the physical state of the sampleIf the sample is gas or liquid that fills the cell, the heat that is emitted during the radiationless relaxation process increases the temperature of the sample. This increasedtemperature causes increased pressure within the fixed volume cell. (for gases ideal gaslaw 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 fillergas is used in the cell. Usually the filler gas is air or any other inert gas. The gas is chosenso that it will nor absorb the given radiation ,the from the sample is transferred to theinert gas resulting in its expansion and generation of pressure waves.
Intensity of PAS signal for a sample that does not fill the entire cell depends uponabsorptivity of the sample and thermal diffusion length of the sample (thermaldiffusion length: it is the distance in the sample through whichthermal energy can be conducted during a fixed period.)CONCEPT OF TDL (Thermal diffusion length)Thermally thick sample: relatively small diffusion lengthThermally 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.
This TDL also depends upon modulation frequency of incoming radiation.So as modulation frequency is increased there is increase in TDLFor 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