Photoacoustic spectroscopy measures the effect of electromagnetic radiation on an analyte sample through acoustic detection. The absorbed electromagnetic radiation creates heat in the sample, causing thermal expansion and ultimately creating a pressure wave or sound that is detected in the form of an acoustic wave using a piezoelectric detector or microphone.
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The Working Principle of Photoacoustic Spectroscopy
The principle of photoacoustic spectroscopy is based on the photoacoustic effect. When a sample is irradiated with light, the electrons are excited either vibrationally or electronically and jump to a higher energy level.
As the electrons return to the ground state, excess energy is released as heat, resulting in thermal expansion. This expansion creates localized pressure waves, which are measured in the form of acoustic waves. The detected acoustic waves are plotted in the spectrum against different wavelengths of irradiated light.
While this mechanism holds true in the case of gasses, the mechanism in solids follows RG theory, where the source of acoustic waves results from repeated heat flow from the condensed-phase sample to the surrounding gas and subsequent acoustic wave propagation through the gas column to a detector with a microphone.
Photoacoustic Spectroscopy on Solids, Liquids, and Gasses
The primary advantage of applying photoacoustic spectroscopy to solids is that no sample preparation is required. Moreover, because photoacoustic signals depend on absorbed light, this technique holds true, even for powders.
In liquid samples, five mechanisms determine the excitation of acoustic waves: dielectric breakdown, thermo-elasticity, vaporization, electrostriction, and radiation pressure. Photoacoustic spectroscopic studies of the liquid samples are based on thermoelastic effects. An acoustic wave is generated by the thermal expansion of the laser-heated volume.
Numerous photoacoustic studies have been performed in gaseous media, most of which are concerned with trace gas monitoring. In addition to photoacoustic spectroscopy, other spectroscopic schemes for photoacoustic detection have been applied, including vibrational overtone spectroscopy, Doppler-free spectroscopy, and photoacoustic Raman spectroscopy.
Signal Generation and Detection in Photoacoustic Spectroscopy
Acoustic waves are generated via direct or indirect photoacoustic generation. In the first case, heat is generated by pulsed or modulated irradiation inside a liquid, gas, or solid sample, and detection occurs either at the interface or inside the sample.
In the latter (indirect) case, heat is generated through modulated illumination inside a liquid or solid sample and is transmitted to an interface. The generated acoustic waves in both the direct and indirect cases are detected using either an electric microphone or a condenser.
However, the application of microphones is hindered in the case of condensed matter owing to their restricted bandwidth. Therefore, piezoelectric transducers are used to detect ultrasonic pulses in such cases.
Depth Profiling Using Photoacoustic Spectroscopy
The depth profiling of various materials using photoacoustic spectroscopy has been investigated both theoretically and experimentally. In this measurement, the photoacoustic signal is primarily influenced by the heat produced in the area between the lighted surface and the subsurface, with an inner distance of the thermal diffusion length.
By comparing photoacoustic spectral data with various thermal diffusion lengths, it is possible to provide qualitative information about the depth profile of the sample. This technique is helpful for estimating the depth profile of samples, such as those made up of a few uniform layers of defined thickness parallel to the irradiated surface of the material.
However, owing to the intricate thermal diffusion mechanism, depth profiling cannot be applied to real-world materials where the optical absorption coefficient depends directly on the depth from the surface.
Fourier Transform Infrared (FTIR) Photoacoustic Spectroscopy
FTIR spectroscopy has been used as a reliable and low-cost technique to characterize a broad range of materials. However, the inefficiency of this technique in measuring the absorption of opaque and dark samples and the interference of sample particle size have made it incompatible with many waste materials.
In this regard, integrating photoacoustic spectroscopy with FTIR helped overcome the limitations of conventional FTIR caused by reflection and scattering issues.
Applications of Photoacoustic Spectroscopy
Important applications of photoacoustic spectroscopy include the following:
- Photoacoustic spectroscopy is highly sensitive and accurate, making it ideal for gas detector applications. In addition to detecting toxic gasses, photoacoustic spectroscopy can be used to detect atmospheric gas levels.
- Each material has a unique spectrum. Hence, materials with multiple components in unknown samples can be identified by matching them to the material-specific spectrum.
- FTIR photoacoustic spectroscopy is used for the in situ evaluation of samples to quantify chemical functional groups and hence identify chemical substances without crushing them into powder.
- Photoacoustic spectroscopy has been used for the depth profiling of ligands and antitumor agents.
- This technique has been applied to investigate nanosized magnetic particles in drug delivery systems.
Advantages of Photoacoustic Spectroscopy
Following are the major advantages of using photoacoustic spectroscopy:
- Photoacoustic spectroscopy requires minimal or no sample preparation.
- This technique can be used for depth-profiling experiments in opaque and scattered samples.
- It can be used to measure optically neutral samples.
- The measurement sensitivity increases with increasing light source intensity.
In summary, photoacoustic spectroscopy is based on the absorption of electromagnetic radiation, followed by the detection of pressure fluctuations in the form of acoustic waves. This technique has a broad range of applications, including the tracing of gasses with high sensitivity, the possibility of depth profiling even in opaque samples, the determination of absorption spectra in powdered and solid samples, and minimal to no influence of light scattering compared to traditional methods.
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References and Further Reading
Patel, P., Hardik, M., Patel, P. (2013). A Review on Photoacoustic Spectroscopy. International Journal of Pharmaceutical Erudition. http://www.pharmaerudition.org/ContentPaper/2013/3(1),%2041-56%20final%20may13.pdf
Bekiaris, G., Bruun, S., Peltre, C., Houot, S., Jensen, L. S. (2015). FTIR–PAS: A powerful tool for characterizing the chemical composition and predicting the labile C fraction of various organic waste products. Waste management. https://doi.org/10.1016/j.wasman.2015.02.029
Sigrist, M, W. (1989). Laser Photoacoustic Spectroscopy. Europhysics News. https://www.europhysicsnews.org/articles/epn/pdf/1989/11/epn19892011p167.pdf
Quan, K. (2023). Photoacoustic Spectroscopy. [Online]. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules