Photoacoustic IR spectroscopy

Photoacoustic IR spectroscopy is particularly recommended for investigation of opaque samples, i.e., samples showing very low transmittance. This technique is... 

In an interesting article by Sullivan et al. [182], different IR spectroscopic techniques (except photoacoustic IR spectroscopy) were compared (cf. Table 3). 

K.H. Michaelian, Photoacoustic IR Spectroscopy Instrumental, Applications and Data Analysis (WHey/VCH, Weinheim, 2011)... 

Photoacoustic IR spectroscopy has similar advantages to DRIFT spectroscopy in its ability to handle solids with the minimum of preparation. The principle of this technique is that when a modulated beam of IR radiation is absorbed by a sample, temperature oscillations set up thermal waves. If the sample is sealed in a cell and surrounded by gas, then a microphone can pick up the sound waves in the gas and an IR absorption spectrum generated. 

A Mid-IR Absorption Spectrometry 455 17B Mid-IR Reflection Spectrometry 469 17C Photoacoustic IR Spectroscopy 472 17D Near-IR Spectroscopy 473 17E Far-IR Spectroscopy 476 17F IR Emission Spectroscopy 476 17G IR Microscopy 477... 

Photoacoustic IR spectroscopy saw limited application before the advent of FTIR instruments. Now, several manufacturers make photoacoustic accessories for FTIR instruments. In photoacouslic measurements, the sample is placed in a small sample cup within the photoacoustic attachment as illustrated in Figure 17-14. The photoacoustic spectrometer chamber is filled with a high-thermal-conductivily gas such as helium or nitrogen and placed in the FTIR sample compartment. The mirror shown deflects the modulated beam onto the sample. Absorption of the IR beam by the sample can result in nonradiative decay of the excited vibrational states of the sample molecules. This can transfer heat to the surface of the sample and result in the generation of a modulated acoustic wave in the gas inside the chamber. A very sensitive microphone then detects the acoustic wave. 

Principles and Characteristics Infrared spectroscopy is one of the oldest and most established analytical methods in industry. New technical developments, such as IR microscopy, photoacoustic IR spectroscopy and on-line techniques for process analysis are now routinely being used in many laboratories. Furthermore, chemomet-ric data evaluation, which is very frequently used in near-IR spectroscopy, is often advantageous also in the field of mid-IR spectroscopy and strengthens its outstanding position towards both basic and applied research. 

Gas measurement Ion chromatography Photoacoustic IR spectroscopy Thermal optical analysis Liquid chromatography... 

An additional technique that can obtain spatially resolved spectral information is step-scan photoacoustic IR spectroscopy. Spectra can be obtained from different depths of a layered sample quickly and with higher spatial resolution (in some cases less than 1 pm) than the diffraction-limited optics of IR microscopes are capable of obtaining. 

Two FT-IR sampling methods are commonly employed to measure IR spectra of mixture components separated by thin layer chromatography (TLC). Diffuse reflectance and photoacoustic IR spectroscopy are techniques that can be employed when sample materials are opaque. 

There have been a few other experimental set-ups developed for the IR characterization of surfaces. Photoacoustic (PAS), or, more generally, photothemial IR spectroscopy relies on temperature fluctuations caused by irradiating the sample with a modulated monocliromatic beam the acoustic pressure wave created in the gas layer adjacent to the solid by the adsorption of light is measured as a fiinction of photon wavelength... 

Several properties of the filler are important to the compounder (279). Properties that are frequentiy reported by fumed sihca manufacturers include the acidity of the filler, nitrogen adsorption, oil absorption, and particle size distribution (280,281). The adsorption techniques provide a measure of the surface area of the filler, whereas oil absorption is an indication of the stmcture of the filler (282). Measurement of the sdanol concentration is critical, and some techniques that are commonly used in the industry to estimate this parameter are the methyl red absorption and methanol wettabihty (273,274,277) tests. Other techniques include various spectroscopies, such as diffuse reflectance infrared spectroscopy (drift), inverse gas chromatography (igc), photoacoustic ir, nmr, Raman, and surface forces apparatus (277,283—290). 

ATR FT-IR spectroscopy has also been employed to monitor the solid-phase synthesis of substituted benzopyranoisoxazoles [180]. Finally, Huber et al. [181] have also reported that this technique is particularly suitable for the characterization of supported molecules in combinatorial chemistry, as well as for the identification of side products and for Photoacoustic (PA) FT-IR. 

Within the IR spectroscopy arena, the most frequently used techniques are transmission-absorption, diffuse reflectance, ATR, specular reflectance, and photoacoustic spectroscopy. A typical in situ IR system is shown in Fig. 7. Choosing appropriate probe molecules is important because it will influence the obtained characteristics of the probed solid and the observed structure-activity relationship. Thus, the probe molecules cover a range from the very common to the very rare, in order to elucidate the effect of different surfaces to very specific compounds e.g. heavy water and deuter-ated acetonitrile, CDsCN). The design of the IR cell is extremely important and chosen to suit the purposes of each particular study. For catalytic reactions, the exposure of catalytic metals must be eliminated in cell construction, otherwise the observed effect of the catalyst may not be accurate. 

Although the applications of photoacoustic FT-IR spectroscopy have shown several pi i] ir features in various sampling situations, the... 

Figure 3.6-4 Schematic diagram for a few techniques in nonlinear (coherent) Raman spectroscopy (CSRS Coherent Stokes Raman Spectroscopy SRGS Stimulated Raman Gain Spectroscopy IRS Inverse Raman Spectroscopy (= SRLS Stimulated Raman Loss Spectroscopy) CARS Coherent anti-Stokes Raman Spectroscopy PARS Photoacoustic Raman Spectroscopy).

Near-IR spectroscopy proved valuable for the analysis of pharmaceutical powders in a 1981 paper by Becconsall et al. [73]. Near-IR and UV photoacoustic spectroscopy were used for determination of propranolol (PR)/magnesium carbonate mixtures. Spectra were collected from 1300 to 2600 nm with carbon black as the reference. An aromatic C-H combination band at 2200 nm and an overtone band at 1720 nm were used to quantify PR. In this case, the UV data were nonlinear, while the NIR method provided a linear calibration. 

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