Photoacoustic absorption spectra

YCX3 molecules have been chosen as test compounds as in previous works [4-9]. Such compounds, especially Freons and methylhalides are also of interest for ecological problems [54,55]. The IR and photoacoustic absorption spectra measured by Law [26] and Smith, et al [56], in vapor CH3F with accuracy of 0.5 cm 1 were used for estimating bond energy. The RMS error of IR spectrum measured by Duncan at al. [37] of vapor CH3CI was 2.6 cm 1. 

Figure 8.4 The v = 6 C-H overtone spectrum as determined by photoacoustic absorption (Hall, 1984) for increasing size alkynes. Each panel contains the molecular formula and the density of vibrational states per cm 1. The band origin does shift some with size but is roughly at 18450 cm"1 for all the molecules shown (see also Kerstel et al., 1991).

The monochromatic X-ray was obtained by silicon (111) channel cut double crystal using white X-ray (at Beam Line 4A (PF)). The ion chambers were set at the both side of the photoacoustic cell, in order to compare the sp trum of photoacoustic X-ray absorption spectroscopy (PAXAS) with usual absorption spectrum, simultaneously. The chopper at chopping frequency of 10 Hz was t at the up-stream of these detectors. Copper foil (5 pm thick) was used as a sample. 

Fig, 11. Photoacoustic X-ray absorption spectrum and X-ray absorption spectrum for copper foil (5 pm thick). Photoacoustic signal is normalized by ion chamber current. Chopping frequency 10 Hz. Ring current 145-142 mA... 

Japar, S. M., and D. K. Killinger, 1979. Photoacoustic and absorption spectrum of airborne carbon particulate using a tunable dye laser, Chem. Phys. Lett., 66, 207-209. 

Figure 10.18—Photoacoustic detection device. The spectrum is obtained in the photoacoustic units PAS = 100/s//rcf (where 7S is the sample signal intensity and /ref is the reference intensity). The reference consists of activated charcoal. The spectrum resembles an absorption spectrum.

Figure 7. The photoacoustic infrared spectra of (A) a cotton fabric and (B) a Velrez 248-treated cotton fabric by conventional padding (C) Infrared absorption spectrum of the finish (Velrez 248).

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. 

In modern devices the flexible membrane is part of a capacitor with the other plate fixed. The pressure rise causes a corresponding change of the capacitance, which can be converted to an AC voltage (Fig. 4.76b). This sensitive detector, which is essentially a capacitor microphone, is now widely used in photoacoustic spectroscopy (Sect. 6.3) to detect the absorption spectrum of molecular gases by the pressure rise proportional to the absorption coefficient. 

Bennett et al, (55) investigated the hydrolysis reaction of PuOt in 0.1 Af NaC104 solution with laser-induced photoacoustic spectroscopy (LIPAS). The effect of hydrolysis on the absorption spectrum of Pu02 (569 nm 19 Af" cm" ) between 530 and 590 nm was studied in Ae pH range from 3 to 10.5. No hydrolysis was observed below pH 8, and the PuOj concentration was far below the detection limit above pH 10. The data were fitted to the hydrolysis reaction ... 

Figure 17.6. (a) Emission spectra of a 3-mm-thick polycarbonate sheet made by the TIRES technique and by uniformly heating the sample. A blackbody emission spectrum is shown below for comparison, (fo) Emittance spectra of polycarbonate derived from the upper panel spectra compared to an absorption spectrum of polycarbonate recorded by photoacoustic spectrometry. (Reproduced from [7], by permission of the American Chemical Society copyright 1990.)... 

The UV-vis transmittance spectrum shown in Figure 6 contains a Soret band at 406 nm and Q-bands at 510, 538, 580, and 644 nm, the latter being especially characteristic of hemin aggregates as determined by photoacoustic spectroscopy (34) and microspectrophotometry (35). Given the number and energies of iht bands in the absorption spectrum there exist manifold possibilities for resonance Raman enhancement studies. 

A majority of traditional NIR measurements are made on solid materials and these involve reflectance measurements, notably via diffuse reflectance. Likewise, in the mid-IR not all spectral measurements involve the transmission of radiation. Such measurements include internal reflectance (also known as attenuated total reflectance, ATR), external reflectance (front surface, mirror -style or specular reflectance), bulk diffuse reflectance (less common in the mid-IR compared to NIR), and photoacoustic determinations. Photoacoustic detection has been applied to trace-level gas measurements and commercial instruments are available based on this mode of detection. It is important to note that the photoacoustic spectrum is a direct measurement of infrared absorption. While most infrared spectra are either directly or indirectly correlated... 

Despite some small spectral differences, the similarities have been sufficient to confirm the slow step in the electrochemistry of immobilized cobalt porphyrin mediators (113) and to identify the intermediates involved in a tetrathiafulvalene polymer coated electrode (7). A polyxylylviologen -polystyrenesulfonate co-polymer coated electrode, on the other hand, showed no changes in the position of the peaks in the absorption spectra upon immobilization (111). Presumably this indicated an absence of interactions between neighboring viologen moieties. Similar spectral results have been obtained using photoacoustic spectroscopy (PAS). Heptyl viologen adsorbed on Pt exhibited an unshifted spectrum which correlated with the electrochemical results (115). 

Figure 1 - Photoacoustic FTIR spectra of fibers from new cotton cloth (lower Spectrum) and approximately 700 year old cotton (upper spectrum). The aged cotton fiber was sample 3019 (circa A.D. 1250-1300). Both spectra were obtained by averaging 1000 interferometer scans. Note the presence of new absorption bands in the C=0 region in the aged sample.

Pastel and Sausa [110] detected N02 (LOD 400 ppb at 1 atm) with a one-photon absorption photoacoustic process (Fig. 13) by means of A X2B1(0,8,0)-X 2A1(0,0,0). This work, which employed a dye laser operating near 454 nm, was the first report of a high-resolution visible N02 photoacoustic spectrum. They found that low laser intensities favor N02 photoacoustic detection, whereas high laser intensities favor NO detection through REMPI (LOD 160 ppb). Fig. 15 shows the high-resolution photoacoustic spectrum of N02 and the NO REMPI spectrum. 

Huvenne and Lacroix described a mathematical procedure for the correlation of band intensities of the Fourier-transform infrared absorption spectra with those of the corresponding infrared transmission spectra of compounds in KBr discs [22]. The procedure was applied to spectra of flunitrazepam, dipyridamole, and lactose that were obtained through the use of a Nicolet 7199 B FTIR spectrometer with photoacoustic detection. When the photoacoustic spectrum of a plant charcoal was used to correct the spectra for inequalities in the incident light flux before applying the procedure, the correlated band intensities were generally consistent with those obtained using infrared transmission spectra. The procedure may be useful for the direct identification of the drugs. 

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