Photoacoustic Spectroscopy

By

Rajesh M KambleT. P. No. 27

History

Photoacoustic effect discovered by Alexander Grahm Bell in 1880

Bell found that when light was focused on to thin diaphragm, sound was emitted

Bell also studied the sounds produced by the irradiation of various solid samples in a brass cavity sealed with a glass window

The original experiment carried out by Alexander Bell

Introduction

PAS or Optoacoustic spectroscopy was developed in 1973 which provides UV, Visible and IR absorption spectra of solids, semisolids and turbid liquids

Obtaining of spectra for above kind of samples by ordinary methods is usually difficult because of light scattering and reflection

It is non-destructive technique

Minimal or no sample preparation

Applied for opaque and scattering samples

Used for qualitative and quantitative experiments

Used to detect defects on the surface of thin films

The Photoacoustic effect

PAS is based upon a light absorption effect

In PAS the gas to be measured is irradiated by a chopped beam of light of a pre-selected wavelength

The gas molecules absorb some of the light energy and convert it in to an acoustic signal which is detected by a microphone

If the frequency of the light coincides with an absorption band of the gas in the cell, then the gas molecules will absorb part of the light

The higher the concentration of gas in the cell, the more light will be absorbed

As the gas absorbs energy, it is heated and therefore expands and causes a pressure rise

As the light is chopped, the pressure will alternately increase or decrease and an acoustic signal is thus generated

Only the absorbed light is converted to sound

The acoustic signal is detected by microphone

The electrical output signals from the microphone are added in an amplifier before they are processed

Photoacoustic effect in solids

In PAS studies of solids, the PAS effect is observed by from periodic heat flow from the solid to the surrounding gas

The periodic heat flow produces pressure fluctuation in the gas of the cell and are detected by the microphone

The power of resulting sound is directly related to the extent of absorption of light by solid

The analog signal from microphone recorded as a function of wavelength of incident light

The radiation reflected or scattered by the sample has no effect on microphone and thus does not interfere

Possible PA generating mechanisms

Conventional PAS setup

Instruments

A single beam PAS the spectrum from the lamp is first recorded digitally followed by the spectrum for the sample

The stored lamp data then used to correct the output from the sample for variations in the lamp output as a function of wavelength

Double-beam instrument is equipped with a pair of matched cells (and transducers), one contains a sample and the other reference material such as finely divided carbon

ApplicationsBulk studies :

- PAS provide optical data for solids which are not highly reflective, highly opaque or highly scattering (e.g. Insulator, semiconductor & metallic systems)

-Many solid & semi solid biological systems can be studied by PAS

Surface studiesDe-excitation studies

UV/visible PA spectroscopy

PAS permits spectroscopic

studies of blood without

separation of blood cells,

protein & lipid molecules

The whole blood does not yield satisfactory spectra by conventional spectroscopy because of highly scattering

properties of large molecules

PAS spectra of smears of blood and blood components

Non-biological studies Spectrum: (a) PAS of Cr2O3

powder in the region 200-900 nm

(b) Optical absorption

spectrum on a 4µ thick Cr2O3

crystal

(c) Diffuse reflectance spectrum on Cr2O3 powder

The two crystal field bands of Cr3+ ion at 600 & 460 nm are clearly resolved in PA spectrum as they are in absorption spectrum of Cr2O3

crystal

PAS spectra of Cr2O3

Surface studies Adsorbed & chemisorbed molecular

species on the surfaces of metals, semiconductors and insulators can be studied by PAS

PAS offers a simple & highly sensitive means for performing non-destructive compound identification directly on the TLC plates

Conventional spectroscopic techniques are unsuitable because of the opacity & light scattering properties of silica gel adsorbent on the TLC plates

The compounds are: (A) p-nitroaniline (B) benzylidene acetone(C) salicylaldehyde(D) 1-tetralone and (E) fluorenone

De-excitation (Fluorescent)studies

PA effect measures non-radiative de-excitation processes in a system after it has been optically excited

This selective PAS technique applied to study of florescent (or phosphorescent) & photosensitive materials

Fluorescent Ho2O3: Ho3+ have strong fluorescent energy levels & these tend to de-excite through the emission of photon rather than phonon or heat excitation

PAS spectra of Ho2O3

References

1. Principles of Instrumental Analysis, 5th Edn., Skoog and West

2. Photoacoustics and Photoacoustic Spectroscopy, Allan Rosencwaig (Chemical Analysis , Vol.57)

3. Rosencwaig A. Photoacoustic Spectroscopy: A New Tool for Investigation of Solids, Anal. Chem. 1975, 47(6), 592 A-604 A.

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