Photoacoustic SpectroscopyPhotoacoustic Spectroscopy

Introduction

photoacoustic the measurement of the effected spectroscopy of absorbed electromagnetic energy (particularly of light) on matter by means of acoustic detection.

The discovery of the photoacoustic effect dates to 1880 when Alexander Graham Bell,

Next Bell showed that materials exposed to the non-visible portions of the solar spectrum can also produce sounds.

Bell photoacoustic effectThe photoacoustic (PA) or optoacoustic (OA)

effect, i.e. the generation of acoustic waves due to the absorption of modulated electromagnetic waves, is an old effect, discovered by Bell in 1880.

This effect is weak; only a very small fraction (<1ppm) of the absorbed optical energy is converted into acoustic energy.

Principle of Photoacoustic Spectroscopy

A photoacoustic spectrum of a sample can be recorded by measuring the sound at different wavelengths of the light.

This spectrum can be used to identify the absorbing components of the sample. The photoacoustic effect can be used to study solids, liquids and gases.

Sample+Light-electon.

How it works

Sample.Far IR.Electron Excitation.Acoustic Waves.Microphone.Spctra analaysis. Result.

Electron ExcitationAs light is absorbed electrons are excited either

electronically or vibrationally. When looking at electronic excitation, electrons jump to a higher energy level.

As they drop back to its ground state, the extra energy is given off as heat. Collision deactivation, another form of heat formation, involves the colliding of atoms. The collision of atoms give off energy in the form of heat.

However in the case of electronic excitation, the energy can also be dissipated through chemical reactions or radiative emissions as seen in figure.

Acoustic Waves  

With the formation of heat, thermal expension heat, also occurs. The expansion of heat creates localized pressure waves and in turn, can be measured as an acoustic wave.

With acoustic waves sent after every pulse of light, a sensor can then measure the waves.

Acoustic wave can be measured and plotted to form a spectrum of the material.

Importance of Photoacoustic Spectrometry

PAS may seem similar to other infrared techniques such as Fourier transform infrared spectroscopy(FTIR).

PAS does not measure the effect relative to the background but directly from the sample, making it extremely accurate.

Types 1. The Gas-Coupling Method,

2. The Direct Coupling Method.

Applications

PAS is used for both qualitative and quantitative analysis of sample.

PAS is used for analysis of biological gases sample.PAS is use for durg properties analysis (Polystyrene ,

Aspirin, Paracetamol etc).All sample use this PAS (solid,gas,liguid).Monitoring of gas and water qualities enviroments.Monitoring and inspection of foods and agricultural products.Monitoring human helath condition .

By Maruthupandi M , M.sc chemistry ,vivekanada college,Madurai.