Mr. Sohan S. Chitlange(M.Pharm)

Photoacoustic spectroscopy (PAS) is an unusual form of spectroscopy using Light and Sound. PAS is based on the absorption of electromagnetic radiation by analyte molecules.

The absorbed energy is measured by detecting pressure fluctuations in the form of Sound waves or shock pulses. It is a non-destructive technique that is applicable to almost all types of samples. PAS offers minimal or no sample preparation, the ability to look at opaque and scattering sample and the capability to perform depth profiling experiment. These features mean that PAS can be used for on-line monitoring of various gases and also in depth-resolved characterization of materials. PAS is also finding wide application in analysis of biological materials such as blood, skin, tumors, etc.

1. Introduction

Photoacoustic spectroscopy (PAS) is based on the photoacoustic effect. The discovery of the photoacoustic effect dates to 1880-1881, when Alexander Graham Bell¹ showed that thin discs emitted sound when exposed to a beam of sunlight that was rapidly interrupted with a rotating slotted disk. Later Bell showed that materials exposed to the non-visible portions of the solar spectrum (i.e., the infrared and the ultraviolet) can also produce sounds. By measuring the sound at different wavelengths, a photoacoustic spectrum of a sample can be recorded that can be used to identify the absorbing components of the sample.In 1973, PAS was rediscovered by A. Rosencwaig and by A. G. Parker who gave the general theory for the effect commonly referred to as the RG Model.² Photoacoustic spectroscopy provides a means for obtaining ultraviolet, visible, and infrared absorption spectra of solids, semisolids or turbid liquids³.

2. Theory of Pas ^4-6

Photoacoustic spectroscopy is based on the absorption of electromagnetic radiation by analyte molecules. Non-radiative relaxation processes (such as collisions with other molecules) lead to local warming of the sample matrix. Pressure fluctuations are then generated by thermal expansion, which can be detected in the form of acoustic waves.

Fig.1 Principle of photoacoustic experiment.

In gases similar mechanism is followed but in solids the commonly accepted mechanism is called RG theory- the main source of the acoustic wave is the repetitive heat flow from the absorbing condensed- phase sample to the surrounding gas, followed by propagation of the acoustic wave through the gas column to microphone based detector.

2.1 Experimental Conditions^{3, 6-7}

Radiation source can be output from a laser, a monochromator furnishing radiations in UV, IR, or a FT-IR spectrometer. All radiation must be pulsed at an acoustical frequency 50-1200Hz. PA cell is filled with transparent gas often air or helium and  cell volume is kept small, less than 1cm³ in order to preserve the strength of the acoustical signal.

Fig. 2 Simple PAS spectrometer.

A photoacoustic cell is generally available as an accessory from most major suppliers of FT-IR instruments.

2.2Depth Profiling ^{3, 5}

One main advantage of PAS is the ability to get information about the depth in the sample of the absorption. The amount of the sample contributing to the PA signal is proportional to the thermal diffusion depth. This thermal diffusion depth µ, is inversely proportional to the modulation frequency f. Figure 3 shows a model sample that has a thermally thin surface layer (thickness << µ) on a bulk substrate. After the light has been absorbed, the heat has to diffuse from the point of absorption to the surface of the sample to be detected. Since this thermal diffusion is a slow process relative to the light absorption and non- radiative decay, an absorption in the bulk will have a phase lag between the time of absorption and the thermal signal. However, a surface absorption should not have a phase lag since the heat doesn't have far to travel to generate the detected pressure change in the transfer gas.

Fig.3.Photoacoustic depth profiling

3. Advantages of PAS ^5-6

• The sample does not have to be dissolved in some solvent or embedded in a solid matrix, it is to be used as its.
• Conventional absorption spectroscopy is based on excitation by electromagnetic radiation with intensity I and the measurement of reflected or transmitted light intensity I. Thus, the absorbencies derived indirectly from transmittance or reflectance, whereas in PAS pressure waves are detected which are generated directly by the absorbed energy.
• PA signal is not influenced by the scattering particles.
•  PAS allows the determination of absorption coefficients over several orders of magnitude. This analytical technique can be applied to the measurement of weak absorption using PA cells with relatively small path lengths, allowing compact and mobile set-ups
• PA signal depends on the incident radiation power hence the sensitivity can be tuned to desired range by choosing an appropriate radiation source (for example, a lamp versus a laser).
• PAS is useful for sample that are powered, amorphous or otherwise not conductive to reflective or transmission form of optical spectroscopies.

4. PAS Techniques

In this section, various schemes for the excitation, generation, and detection of PA signals are presented.

4.1 Excitation

In order to generate acoustic waves, which can be detected by pressure sensitive transducers, periodic heating and cooling of the sample is necessary to generate pressure fluctuations. In principle, there are two ways to realize PA pressure fluctuations: modulated and pulsed excitation.^8-10. In modulated excitation schemes, radiation sources are employed whose intensity fluctuates periodically example chopped or modulated lamps or IR sources from commercial spectrometers are used for the determination of spectra of opaque solids, modulated continuous wave (cw) lasers are common sources for PA gas phase analysis. In pulsed PAS, laser pulses with durations in the nanosecond range are usually employed for excitation. Since the repetition rates are in the range of a few Hz, this leads to a fast and adiabatic thermal expansion of the sample medium resulting in a short shock pulse frequency.

4.2 Signal Generation

Induction of an acoustic wave by modulated or pulsed irradiation inside a gaseous, liquid or solid sample is termed direct PA generation. Here, detection takes place inside or at an interface of the sample using microphones² .In indirect PA generation^8-11; heat is generated by modulated illumination inside a solid or liquid sample and transported to an interface. Subsequently, sound waves are generated and detected in the gas phase adjacent to the sample using a microphone (see Fig. 5). 

Fig. 5 Indirect generation of photoacoustic waves for the analysis of solid and liquid samples.

4.3 Signal Detection

Sound waves generated directly or indirectly in the gas phase are detected usually by condenser or electrets microphones.^9-11 Detection of sound waves by microphones in condensed matter is typically not suitable. Due to high acoustic impedance mismatches, less than 10^-4 of the acoustic energy is transferred from a solid sample to the adjacent gas phase. Therefore, piezoelectric transducers are employed in many cases for the detection of ultrasonic pulses in liquid and solid samples.

5. Applications of Photoacoustic Spectroscopy

5.1 Gas Phase Analysis^13-18

 In recent years, the development of new PA setups for on-line gas monitoring has been achieved through new developments in diode lasers. Atmospheric pollutants that can be detected by PA measurement techniques includes sulfur oxides (such as SO₂), nitrogen oxides (NO_x), carbon oxides (CO and CO₂), hydrogen sulfide, ammonia¹⁷, methane, and aerosol particles (such as soot)^19,20.

5.2 Analysis of Condensed Matter

The main application of UV/Vis.–PAS is the characterization of semi conducting materials. As the PA signal depends on heat diffusion, the thermal diffusivity can also be determined, which is strongly sensitive to the structural quality of the semi conducting material. Furthermore, packaging materials have been characterized by PA measurements in the UV/Vis range. Using depth-resolved PAS, it was possible to estimate the thicknesses and moisture contents of varnish layers on base paper.

PA analysis of highly concentrated textile dyes¹²

The concentrations of these dyes are in the range of more than 5 g L ^-1, resulting in absorption coefficients of 10³ cm ^-1. The combination of extremely high absorption and scattering particles in the dye solution makes a classical transmission spectroscopic analysis impossible. PA spectroscopy is a viable approach to overcome the problems ^18.

5.3 Depth Profiling of Mammalian Cells for Localization of Ligands¹⁹

It describes an application of this principle to determine the depth profiles of ligands and antitumor agents in mammalian cells. Measurements of the in-phase and quadrature components of the photoacoustic spectra (which yield information from the surface and the interior, respectively) of a tumor cell line, AK-5, treated with the antitumor agent coralyne chloride have been made. They clearly show that the drug accumulates in the cell interior and is not seen on the cell surface, providing in situ evidence for the localization of this drug.

5.4 Analysis of Biological Material^20-22

Conventional spectroscopy does not yield satisfactory spectra because of the string light scattering properties of the blood cells, protein and lipid molecules present. PAS permits spectroscopic studies of blood without the necessity of a preliminary separation of these large molecules.

6. Conclusion

Photoacoustic spectroscopy is based on optical absorption and subsequent detection of pressure fluctuations. The main advantages of this technique over conventional absorption spectroscopy are   

1. Measurement of high absorption coefficients, even in opaque samples, without sample dilution.
2. Determination of absorption spectra of solid samples, even in the form of powder, chips, or large objects.
3. Less influence from light scattering particles.
4. Depth profiling of layered samples.

These features, allowing on-line measurements without sample pretreatment are advantageous in process analysis. In the characterization of industrial products, additional benefits include the potential to determine absorption spectra of opaque solids and depth profiles of layered materials.

References

1. Bell AG. The Manufacturer and Builder. July 1881. Available: http://cdl.library.cornell.edu/cgi-bin/moa/moa-cgi?notisid=ABS1812-0013-416.

2.Rosencwaig A. Photoacoustic and Photoacoustic Spectroscopy. Florida: R.E. Kreger Publishing Company; 1980.

3.Gregoriou V. Photoacoustic Spectroscopy. Available: http://nte-serveur.univ–lyon1,fr/spectroscopie/gbdospedagogiques.html

4.Life Science Trace Gas Facility and Trace Gas Research Group. Photoacoustic Spectroscopy.Available:http://ru.nl/tracegasfacility/traceresearch/laser_spectroscopy/phtoacous....

5. Schmid T. Photoacoustic Spectroscopy for Process Analysis. Analytical and Bioanalytical Chemistry 2006; 384:1071-1086.

6. Ball DW. The Baseline Photoacoustic Spectroscopy, Spectroscopy 2006; 21:14-16.

7.Willard, Meritt, Dean,Settle. Instrumental Methods of Analysis. 7th edition. Delhi: CBS Publishers; 181-183.

8.Patel CKN, Tam AC. Pulsed Optoacoustis Spectroscopy of Condensed Matter. Physical Online Review Archive 1981; 53:517-550.

9.Kinney JB, Staley RH. Applications of Photoacoustic Spectroscopy. Aaaual Review of Material Science 1982; 12:295-321.

10.Tam AC. Application of Photoacoustic Sensing Techniques. Physical Review Online Archive 1986; 58:381-431.

11. Jackson W, Ama NM. Journal of Applied Physics 1980; 51:3343-3353.

12.Nelson ET, Patel CKN. Response of Piezoelectric Transducers used in Pulsed Optoacoustic Spectroscopy. Optics Info Base 1981; 6:354-356.

13.Photoacoustic Spectroscopy for Quantitation of Trace Gases in Air. Available:http://cstl.nist.gov/projects/fy05/iais05gills2.

14.Patel CKN, Burkhardt EG, Lambert CA..Spectroscopic Measurements of Stratospheric Nitric Oxide and Water Vapor.Science 1974; 184:1173–1176.

15.Sigrist MW. Air Monitoring by Spectroscopic Techniques. New York: J. Wiley & Sons; 1994.

16.Frans et al. Photoacoustic Spectroscopy in Trace Gas Monitoring. Encyclopedia of Analytical Chemistry. New York: J. Wiley & Sons; 2000.

17. Schmohl A, Miklós A, Hess P. Detection of ammonia by photoacoustic spectroscopy using Semiconductor lasers. Application Optics 2002;41:1815<-1823.

18. C.Haisch and R. Niessner, Light and Sound- photoacoustic spectroscopy. Spectroscopy Europe 2002; 14:11-15.

19.  Narayanan K,  Chandani S, Ramakrishnan T and  Rao CM.Depth Profiling of Mammalian Cells for Localization of Ligands. Centre for Cellular and Molecular Biology, Hyderabad, India.

20.Laufer JG, Delpy DT, Elwell CE, Beard PC. Quantitative spatially resolved measurement of tissue chromophore concentrations using photoacoustic spectroscopy: application to the measurement of blood oxygenation and haemoglobin concentration. Physics in Medicine and Biology 2007; 52:141168.

21.Rockley M, Davis D, Richardson H. Fourier-transformed infrared photoacoustic Spectroscopy of biological materials. Available : http://.sciencemag.org/cgi/content/abstract /181/4100/657

22.Skoog, Holler, Nieman; Principles of Instrumental Analysis, 5th edition, Thomson Publishers, 341-351.

About Authors:

Mr. Sohan S. Chitlange (M.Pharm)
Dr. D.Y.Patil Institute of Pharmaceutical Science & Research, Pharmaceutical Chemistry Department, SantTukaram Nagar,
Pimpri, Pune- 411018, E.mai.-sohanchitlange@rediffmail.com
Mobile No.-09922904305, Phone No. 020-27420261, Fax. No. 020-27420261

Mr. Amol A. Kulkarni
Dr. D.Y.Patil Institute of Pharmaceutical Science & Research,
Pharmaceutical Chemistry Department, Sant Tukaram Nagar, Pimpri, Pune- 411018
E.mai.-amolkulkarni89@rediffmail.com
Mobile No.-09881377037, Phone No. 020-27420261, Fax. No. 020-27420261

Ms Kiran Bagri (B.Pharm)
Dr. D.Y.Patil Institute of Pharmaceutical Science & Research, Pharmaceutical Chemistry Department,
SantTukaram Nagar, Pimpri, Pune- 411018, E.mai.- kiran_sky84@yahoo.com
Mobile No.-09890087720, Phone No. 020-27420261, Fax. No. 020-27420261