Photoacoustic Spectroscopy PAS

Instrumentation. The light of a Xe-lamp is passed through a monochromator and a mechanical chopper. Part of the monochromatic light is directed towards a carbon black sample fitted with a microphone that serves as a reference sample. 

Upon reduction of DHV a deeply colored film of the salt DHVBr is formed on the electrode surface. This becomes clearly evident in the photoacoustic spectrum, when the electrode potential is switched from an initial value positive to the reduction potential to a value negative of the reduction potential. The recorded spectrum was found to be in good agreement with spectra of HVBr films previously recorded. For further details, see [125]. 

---

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 photoacoustic effect was first discovered by Alexander Graham Bell in the early 1880s [18], but it was not applied to Fourier transform infrared (FTIR) spectroscopy until a century later [19,20], Significant advantages of FTIR photoacoustic spectroscopy (PAS) include the following (1) Spectra may be... 

The technique employed is IR-FT photothermal beam deflection spectroscopy (PBDS). It is an off-shoot of photoacoustic spectroscopy (PAS) [1] and is based on the "mirage detection of the photothermal effect invented by Boccara et al. [2] and shown to result in a spectroscopic technique of remarkable versatility and utility. Some applications of "mirage spectroscopy," mainly in the visible, and theoretical treatments, have been described [3 6]. The method has now been developed for use in the IR. The spectrometer and techniques are described in detail elsewhere [7], but it will be useful to give a brief outline of the principles. 

Time-resolved Photoacoustic Spectroscopy. In photoacoustic spectroscopy (PAS) the heat evolved by the absorption of light in the sample is transformed into sound waves which are detected by a microphone. In steady-state spectroscopy the light is continuous, but it is also possible to use a pulsed laser and to observe the change in the intensity of the sound signal with time. In this respect time-resolved PAS is somewhat similar to thermal lensing, but both techniques have different limitations and advantages. 

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). 

Time-resolved photoacoustic spectroscopy [41] The steady-state technique of photoacoustic spectroscopy (PAS) is widely used for the measurement of absorp-... 

Photoacoustic spectroscopy (PAS) is revolutionary in the way that it is one of the few spectroscopic techniques that is not based on the direct or indirect measurement of electro-magnetic radiation. It is grounded on the ancient observation of Graham Bell, that the exposure of different solid and liquid substances to a rapidly interrupted beam of light results in the emission of acoustic energy at the same frequency as that at which the incident radiation was modulated. 

In addition to the IR, Raman and LIBS methods previously discussed, a number of other laser-based methods for explosives detection have been developed over the years. The following section briefly describes the ultraviolet and visible (UV/vis) absorption spectra of EM and discusses the techniques of laser desorption (LD), PF with detection through resonance-enhanced multiphoton ionization (REMPI) or laser-induced fluorescence (LIF), photoacoustic spectroscopy (PAS), variations on the light ranging and detecting (LIDAR) method, and photoluminescence. Table 2 summarizes the LODs of several explosive-related compounds (ERC) and EM obtained by the techniques described in this section. 

Infrared microscopy, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, and photoacoustic spectroscopy (PAS) techniques may be suitable for some types of sample but the use of... 

The properties of photochromic molecules such as DHI 7 and related compounds are dependent not only on structure but to a large extent also on their environment. Clear differences are to be expected if solution or solid phases of photochromic molecules are compared, i.e. (1) containing supramolecular anchor groups, (2) in liquid crystalline phases, or (3) polymer-containing photochromic systems. It is necessary to use the appropriate spectral methods for the detection of different properties. In addition to normal methods, photoacoustic spectroscopy (PAS), polarized light, and others may be applied. 

The feasibility of thermic and calorimetric detection of the absorbed radiation has been mentioned in the context of grazing-incidence experiments. This is quite close to the class of photothermal techniques with which a number of different detection schemes is employed (Coufal, 1986). Out of these, photoacoustic spectroscopy (PAS) is frequently used in infrared spectroscopy (Graham et al., 1985 Urban et al., 1990 McClelland, 1992) while inspite of its potential, thermal beam deflection has not yet found as many applications as in other spectral ranges, possibly due to the lower availability of suitable lasers (Low and Morterra, 1985). 

Photoacoustic spectroscopy (PAS) is a nondestructive analytical technique in which light entering the photoacoustic cell passes through undetected if the sample is nonabsorbing, but heats up and expands the gas in the cell if the light is absorbed. This expansion makes an audible sound whenever absorption occurs and is detected by a microphone. The SNR may increase with the sample surface area. PAS determinations were carried out for hfac chelates of Sc, Y and the rare earth Ce, Pr, Nd, Eu and Er. ... 

Since photoacoustic spectroscopy (PAS) was first reported in 1975, the method was gradually improved, and now, nanograms to picograms of substances can be successfully detected and quantified on TLC plates. When the method was first developed, the separated spot had to be scratched from the plate and introduced into the PAS cell for the measurements but now, the PAS cell can be fixed directly onto the layer surface. Other instruments can be laser-based densitometers with signal enhancement at a resonant frequency of the cell. The detection limit is very low in this case, and the linearity of the quantitative determination is over 3 orders of magnitude. Moreover,... 

Complementing other established techniques such as ATR, photoacoustic spectroscopy (PAS) (2) is a quick, nondestructive method for analyzing vari-... 

The above statements of the nature of the photoacoustic effect are drawn primarily from the conclusions of the theory developed by Rosencwaig and Gersho (R-G) (9), based on a onedimensional thermal piston model. Since this is the starting point for extending the theory of photoacoustic spectroscopy (PAS) to include photoacoustic circular dichroism (PACD), a brief survey of the salient factors and important parameters of that treatment follows. 

Photoacoustic Spectroscopy.—Photoacoustic spectroscopy (PAS) and its applications have been recently reviewed. A single-beam i.r. PAS spectrometer has been constructed for the range 800—4000 cm using a broad-band carbon rod spectral source in preference to a laser. A double-beam in-time PAS instrument has been described, in which a single microphone was used to monitor both the... 

The detectors used in gas chromatography for the detection of individual VOCs can also provide information about a mixture without a separation step. These VOC detectors include, for example, the flame-ionization detector (FID) and the photo-ionization detector (PID). Other direct-reading instruments have also been used for determining VOCs, such as photoacoustic spectroscopy (PAS) (see Chapter 1.6). Other types of sensors (e.g. electronic noses ) may become important in the future. 

Photoacoustic spectroscopy (PAS) is based on the transfer of modulated infrared radiation to a mechanical vibration. Gaseous, liquid or solid samples can be measured by using PAS, with the... 

For the investigation of the kinetics and thermodynamics of intermediate phases of rare earth oxides and photochemical reactions of the surface of rare earth oxides [52], it is reported that photoacoustic spectroscopy (PAS) technique is effective enough. Some information on the lattice structure of powder grains can be attained from the spectra. Raman and resonance Raman spectroscopy were also investigated on rare earth oxides of EU2O3, Dy203, and Tm203 [53]. 

---