Solid Phase Photoacoustic Spectroscopy (PAS)
Photoacoustic spectroscopy is well suited and advantageous method for measurement and analysis of solid and semi-solid samples. It is also suitable for powders, fibers, and samples of very small size. The shape of the photoacoustic spectrum is independent of the morphology of the sample.
Signal generation process involves absorption of light in the sample and production of heat followed by propagation of heat-generated thermal waves to the sample surface. Heat is then transferred into the adjacent gas, varying its pressure, which is then measured by a microphone as the photoacoustic signal.
Photoacoustic principle in solid and liquid phase measurements
The sample is sealed in the photoacoustic measurement chamber and irradiated with modulated infrared light through a window. The principle of signal generation is similar for solid- and liquid-phase samples. Periodic heating in the sample is generated when the sample is irradiated with the modulated infrared light. The periodic heat flow to the surrounding gas in the chamber from the sample surface generates expansion and contraction in a thin layer of gas close to the surface. This mechanism is called thermal coupling. Periodic heating of the sample causes also pressure variations to propagate in all directions and a superposition of these acoustic waves at the sample surface generates a surface motion that is coupled to the surrounding gas. This mechanism is called acoustic coupling. The pressure signal that is detected in the gas by a pressure sensor (i.e. microphone) is a combination of these two mechanisms. In a typical solid-phase photoacoustic experiment, thermal coupling is dominant and acoustic coupling can be neglected. The acoustic coupling can be the dominating mechanism for some liquids.
Part of the infrared radiation is reflected from the sample. The amount of reflection depends on the absorption into the sample. Rest of the radiation is absorbed into the sample according to Beer’s law and absorption coefficient to different wavelengths. If the sample is thin, part of the radiation might transmit the sample and hit another surface. In thicker samples the radiation just penetrates deeper into the sample. The temperature rise in a gas layer on the sample surface gets the energy from a certain depth from the sample depending on its thermal diffusivity.
Solid -phase photoacoustic FTIR
Typical photoacoustic Fourier transform infrared (FTIR-PAS) setup for analysis of solid and liquid samples contains an interferometer, a focusing mirror, and a photoacoustic cell. FTIR interferometer consists of a beamsplitter and two mirrors. The infrared beam is split into two beams: one is reflected from a fixed mirror and one from a moving mirror. By combining the two beams each wavelength of the light is modulated with a different modulation frequency. The combined beam is then focused into the solid or liquid sample in the photoacoustic cell. The generated photoacoustic signal can be directly transformed into absorption spectrum.
Depth-varying information of the sample can be obtained by varying the mirror velocity or phase angle of detection. Typically for polymer materials the depth from where the spectra is obtained can be varied from few micrometers to about 100 micrometers.
The FTIR analysis of solid- and liquid-phase samples has a great variety of applications and advantages compared to other techniques. The most important and best-known advantages are
minimal sample preparation required,
suitability for opaque materials,
possibility for depth profiling,
and nondestructive measurement, which means that the sample is not consumed.
The typical applications of solid- and liquid-phase photoacoustic FTIR are the study of
- hydrocarbon fuels,
- clays and minerals,
- wood and paper,
- polymer layers,
- food products,
- biology and biochemistry e.g. proteins, bacteria and fungi,
- medical applications such as human tissue,
- drug characterization and penetration,
- teeth, hair and bacteria,
- and nondestructive measurement of carbonyl compounds, textiles, and catalysts.