Photoacoustic sampling depth

Depth profiling of a solid sample may be performed by varying the interferometer moving-mirror velocity (modulated IR radiation). By increasing the mirror velocity, the sampling depth varies, and surface studies may be performed. Limitations do exist, but the technique has proven to be quite effective for solid samples [21]. In addition, unlike diffuse reflectance sampling techniques, particle size has a minimal effect upon the photoacoustic measurement. 

Gardalla and Grobe compared attenuated total reflectance and photoacoustic sampling for surface analysis of polymer mixtures by Fourier transform infrared spectroscopy. They show that analysis by attenuated total reflectance is more suitable for smooth surfaces and is faster. Photoacoustic methods have shallower sampling depths than attenuated total reflectance but the latter technique is applicable over a range that is more controllable. 

The infrared absorption coefficient and thermal wave decay coefficients, a(v) and flj, respectively, determine the magnitude of the photoacoustic signal. The term ot( exp —[a( +fls]x in the expression for temperature oscillation leads to a linear PA signal dependence on infrared absorption when a( thermal wave decay length, L, although it is sometimes referred to as the sampling depth, penetration depth, or thermal diflusion depth. The sample layer extending a distance L beneath the surface contributes... 

Two approaches can be used to measure the depth profile of samples such as polymer laminates with a step-scan interferometer. The first makes use of the fact that the sampling depth varies as/, whereis the PM frequency, as discussed above. In this case, several spectra must be measured with different PM frequencies. The second approach makes use of the fact that the phase of the photoacoustic signal varies with the distance from the surface from which the thermal wave originates. 

Photoacoustic method can be applied any types of shapes and states of the sample and also to non-destructive depth-profiling. Thus this finding expands the methodology of EXAFS much wider than before. High temperature super conductor, biological samples and other varieties can be studied. 

The photoacoustic effect was first discovered by Alexander Graham Bell in the early 1880s (27), but not applied to FTIR spectroscopy until a century later (28,29). Significant advantages of FTIR photoacoustic spectroscopy (PAS) include (a) spectra may be acquired on opaque materials (commonly found in pharmaceutical formulations), (b) minimal sample preparation is necessary, and (c) depth profiling is possible. 

PAS has been found to be an ideal technique for the study of surfaces and adsorbed species. In studies of optically thin samples, pulsed laser excitation has been shown to enable sensitivities of 10 to be obtained. The degree of chemical modiflcation of silica gel surfaces has been monitored by PAS. A linear relationship between the photoacoustic signal and the amount of carbon or nitrogen adsorbed on the surface was found. PAS has also been used to study the photoinduced transient formed when eosine Y was adsorbed onto ZnO powder, and the thin oxide layers (<4nm thick) on a copper electrode. An investigation of layered samples has shown that PAS may be capable of depth discrimination. ... 

Photoacoustic FITR (PA-FTIR) is based on measuring the heat associated wifli absrsption of IR lacfiation. The heat generated is released to an inert gas above the sample. The sample can be analysed in the form of a film, and the depth from whidi a signal is obtained can be varied [62]. Evanson et al. [59,60] u PA-FTlR and ATR-PTIR to study die molecular interactions between surfactants and poIy(ethyl acrylate(EA)-methacrylic acid(MAA)) latex and the surface oirichment with nonionic and anioiic surfactants on films formed from die latex. 

In PAS, the time-resolved photoacoustic response to sinusoidally modulated incident source radiation has a phase delay which depends on the depth in the sample... 

Vibrational spectroscopy represents two physically different, yet complementary spectroscopic techniques IR and Raman spectroscopy. Although both methods have been utilised for many years, recent advances in electronics, computer technologies and sampling made Fourier transform infrared (FTIR) and Raman (FT-Raman) one of the most powerful and versatile analytical tools. Enhanced sensitivity and surface selectivity allows non-invasive, no-vacuum molecular level analysis of surface and interfaces. Emphasis is placed on recent advances in attenuated total reflectance (ATR), step-scan photoacoustic (SS-PA), Fourier transform infrared (FTIR) and FT-Raman microscopies, as utilised to the analysis of polymeric surfaces and interfaces. A combination of these probes allows detection of molecular level changes responsible for macroscopic changes in three dimensions from various depths. 7 refs. 

The most common infrared sampling techniques used to examine paint samples are attenuated total reflectance and photoacoustic spectroscopies. Liquid paints, dried films and paint chips may all be investigated in this way. Depth profiling can be useful when examining paint, as the surface properties will vary importantly from the bulk properties. In addition, most paint films contain two or more layers with different compositions, and so reflectance techniques are necessary for the characterization of individual layers. 

When analyzing a sample using PAS spectroscopy, the depth beneath the surface from which a photoacoustic signal originates is dependent upon the interaction between the sample s optical absorption length, and thermal diffusion length, /ij. 

In order to determine the composition and structure of a biomaterial surface different methods which provide varying degrees of information are commonly used (Fig. 6). Surface-sensitive infrared spectroscopy suppHes the characteristic absorption bands of functional groups with an informational depth of 0.1-10 pm by measurement in attenuated total reflectance (IR-ATR). In the case of samples with rough surfaces photoacoustic spectroscopy (PAS), which allows an informational depth of approximately 20 pm, can be used [72]. The achieved informational depths are usually larger than the thickness of the modified interface, so that the spectra include information on the bulk composition as well. As a consequence, surface-sensitive infrared spectroscopy is often not sensitive enough for the characterization of the modified surfaces. 

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