Photoacoustic

Analysis with photoacoustic detection is the oiJy method that involves the direct measurement of the absorption of the infrared radiation. The technique involves the conversion of the infrared energy into thermal energy (within the sample), followed by thermal energy transfer from the sample 

It is always advisable to adhere closely to the manufacturer s instructions, especially in regard to the use of a dry purge gas. Failure to do this will result in data dominated by the spectrum of water vapor. Photoacoustic measurements are based on thermal processes, which are intrinsically slow, and in order to maximize sensitivity, slow interferometer scan rates are recommended for best performance. Note that the sample size is normally limited to a few millimeters. Always docmnent the purge gas that is used and the scan speed of the instrument because these impact the intensity and quality of the final spectrum. 

In concept, the smaller the sample size, the lower the amount of material required (a logical conclusion) however, the trade is optical thoughput. Traditional instruments are limited by the image size of the infrared beam at the sampling point, which is typically between 6 and 8 mm in diameter. 

An improved matching between sample size and beam size can be achieved by the use of a beam condenser accessory, providing beam size reductions on the order of 4 1 or 6 1 times. With such accessories it is possible to handle physical sample sizes in the range of 0.5-0.25 mm in diameter. For sample sizes smaller than this, it is often necessary to use an infrared microscope. 

When working with any of the microsampling methods, cleanliness is of utmost importance. Traces of impurities picked up during sample preparation can have higher concentrations, and therefore greater impact on the final spectrum, than the sample itself This attention to cleanliness must extend to the purity of any solvent used during sample preparation and also to preparation materials, such as KBr, if micropellets are to be prepared. 

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PAS Photoacoustic spectros- Modulated incident infrared Vibrational states of surface... 

The temi action spectroscopy refers to those teclmiques that do not directly measure die absorption, but rather the consequence of photoabsorption. That is, there is some measurable change associated with the absorption process. There are several well known examples, such as photoionization spectroscopy [47], multi-photon ionization spectroscopy [48], photoacoustic spectroscopy [49], photoelectron spectroscopy [, 51], vibrational predissociation spectroscopy [ ] and optothemial spectroscopy [53, M]. These teclmiques have all been applied to vibrational spectroscopy, but only the last one will be discussed here. 

Crippa P R, Vecii A and Viappiani C 1994 Time-resolved photoacoustic-spectroscopy—new developments of an old idea J. Photochem. Photobiol. B-Biol. 24 3-15... 

Barrett J J and Berry M J 1979 Photoacoustic Raman spectroscopy (PARS) using cw laser sources Appl. Phys. Lett. 34 144-6... 

Siebert D R, West G A and Barrett J J 1980 Gaseous trace analysis using pulsed photoacoustic Raman spectroscopy Appl. Opt. 19 53-60... 

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... 

INFRARED TECHNOLOGY AND RAMAN SPECTROSCOPY - INFRARED TECHNOLOGY] (Vol 14) Photoacoustic spectroscopy... 

Photoacoustic spectrometry (pas) differs from the other methods in that the detector is a microphone. This makes pas wavelength independent. 

Only slightly less accurate ( 0.3—0.5%) and more versatile in scale are other titration techniques. Plutonium maybe oxidized in aqueous solution to PuO " 2 using AgO, and then reduced to Pu" " by a known excess of Fe", which is back-titrated with Ce" ". Pu" " may be titrated complexometricaHy with EDTA and a colorimetric indicator such as Arsenazo(I), even in the presence of a large excess of UO " 2- Solution spectrophotometry (Figs. 4 and 5) can be utilized if the plutonium oxidation state is known or controlled. The spectrophotometric method is very sensitive if a colored complex such as Arsenazo(III) is used. Analytically usehil absorption maxima and molar absorption coefficients ( s) are given in Table 10. Laser photoacoustic spectroscopy has been developed for both elemental analysis and speciation (oxidation state) at concentrations of lO " — 10 M (118). Chemical extraction can also be used to enhance this technique. 

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). 

A closely related technique useful for localized gas concentrations and leaks is photoacoustic detection and ranging (padar) (90). A laser pulse tuned to an absorption line generates an acoustic signal that is detected by a paraboHc microphone. A range resolution of 1 cm out to 100 m is feasible. 

A. Rosencwaig, Photoacoustics and Photoacoustic Spectroscopy, WRey-lnteiscience, New Yoik, 1980. 

P. Hess andJ. Pelzl, eds.. Photoacoustic andPhotothermalPhenomena, Springer-Vedag, Berlin, 1987. 

PHOTOACOUSTIC-FTIR DETERMINATION OF MANCOZEB IN PESTICIDE FORMULATIONS... 

Mancozeb is a dithiocarbamate pesticide with a very low solubility in organic and inorganic solvent. In this work we have developed a solvent free, accurate and fast photoacoustic FTIR-based methodology for Mancozeb determination in commercial fungicides. The proposed procedure was based on the direct measurement of the solid samples in the middle infrared region using a photoacoustic detector. A multivariate calibration approach based on the use of partial least squares (PLS) was employed to determine the pesticide content in commercially available formulations. 

Kim, J. 1., Stumpe, R., and Klenze, R. Laser-induced Photoacoustic Spectroscopy for the Speciation ofTransuranic Elements in Natural Aquatic Systems. 157,129-180 (1990). 

Poston, P. E., and Harris, J. M., Maximum Likelihood Quantitative Estimates for Peaks Application to Photoacoustic Spectroscopy, Anal. Chem. 59, 1987, 1620-1626. 

Berlis A, Lutsep H, Barnwell S, Norbash A, Wechsler L, Jungreis CA, Woolfenden A, Redekop G, Hartmann M, Schumacher M. Mechanical thrombolysis in acute ischemic stroke with endovascular photoacoustic recanalization. Stroke 2004 35 1112-1116. 

Toyoda T, Tsuboya 1, Shen Q (2005) Effect of rutile-type content on nanostructured anatase-type Ti02 electrode sensitized with CdSe quantum dots characterized with photoacoustic and photoelectrochemical current spectroscopies. Mater Sci Eng C 25 853-857... 

The scanning transmission electron microscope (STEM) was used to directly observe nm size crystallites of supported platinum, palladium and first row transition metals. The objective of these studies was to determine the uniformity of size and mass of these crystallites and when feasible structural features. STEM analysis and temperature programmed desorption (TPD) of hydrogen Indicate that the 2 nm platinum crystallites supported on alumina are uniform In size and mass while platinum crystallites 3 to 4 nm in size vary by a factor of three-fold In mass. Analysis by STEM of platinum-palladium dn alumina established the segregation of platinum and palladium for the majority of crystallites analyzed even after exposure to elevated temperatures. Direct observation of nickel, cobalt, or iron crystallites on alumina was very difficult, however, the use of direct elemental analysis of 4-6 nm areas and real time Imaging capabilities of up to 20 Mx enabled direct analyses of these transition metals to be made. Additional analyses by TPD of hydrogen and photoacoustic spectroscopy (PAS) were made to support the STEM observations. 

The application of infrared photoacoustic spectroscopy to characterize silica and alumina samples is reported. High quality infrared photoacoustic spectra illuminate structural changes between different forms of silica and alumina, as well as permit adsorbate structure to be probed. Adsorption studies on aerosil suggest adsorbed species shield the electric fields due to particle-particle interactions and induce changes in the vibrational spectra of the adsorbates as well as in the bulk phonon band. It is shown that different forms of aluminum oxides and hydroxides could be distinguished by the infrared spectra. 

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