Photoacoustic Detection

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. 

Concrete applications of micro reactors for chemical analysis, albeit so far not a core application, have been described [5]. Among other uses in chemical analysis, micro devices for gas chromatography, infrared spectroscopy, and photoacoustic detection are mentioned. 

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... 

For solids that do not lend themselves to the above procedures (such as strongly absorbing, non-crushable or insoluble solids certain elusive molecules), photoacoustic detection is a possibility if an FTIR is available. This is a very expensive, seldom used accessory that includes a detector placed directly into the compartment with the sample (see Fig. 10.18). 

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.

The sample is submitted to radiation modulated by the interferometric device. Temperature variations within the sample are translated into pressure variations within the cell. A microphone is used to detect the sound (this is called photoacoustic detection). The acoustic interferogram obtained from the pressure waves is converted into a classical spectrum. 

Gas-phase photoacoustic detection of propane in N2 was attempted on a Si chip. In this method, a modulated light beam is incident on the sample. If the wavelength of the modulated light couples to an energy transition in a gas, the gas absorbs the modulated light resulting in periodic gas expansions and contractions, which are manifested as an acoustic wave. This wave can be detected by a microphone. In contrast to conventional absorption spectroscopy, the sensitivity of photoacoustic spectroscopy scales inversely with dimension, and hence this method is favored in the microscale. This is because photoacoustic spectroscopy is a differential technique in which the absorption is measured as the intensity per unit surface area [796],... 

Infrared spectroscopy has been widely applied when studying the silica surface.1 Figure 3.3 shows typical FTIR (Fourier Transform Infrared) spectra with photoacoustic detection (cfr. appendix A) of Kieselgel 60, treated for 17 h at (a) 373 K (b) 673 K and (c) 973 K. The Y-scales of the three spectra are not comparable. 

Fourier Transform Infrared Spectroscopy with Photoacoustic detection... 

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. 

A good practice is to monitor the workspace inside the fume cupboard with a continuous handheld or tabletop chemical agent detector, if such a device is available. There are several models commercially available based on ion mobility, flame photometric, enzymatic, or photoacoustic detection (9). 

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. 

Vibrational spectroscopy is a very versatile and, chemically, well-resolved technique for the characterization of carbon-oxygen functional groups. The immense absorption problems of earlier experiments seems to be overcome in present times with modem FT-IR, DRIFTS or photoacoustic detection instruments. 

Photoacoustic detection measures the pressure variation caused by the heating of a gas in the vicinity of the sample. If the sample is enclosed in a photoacoustic cell equipped with a microphone, the modulated signal can be interpreted as a function of temperature, pressure, and the quantity of the sample. 

Japanese workers have published a paper regarding the study of photoelectro-chemical reactions at a n-type polycrystalline zinc oxide electrode using photoacoustic detection [125], They monitored in situ photoelectrochemical reactions at semiconductor electrodes using photoacoustic techniques. 

The measurement of very small absorption coefficients (down to lO-5 cm-1) of optical materials has been carried out by laser calorimetry. In this method, the temperature difference between a sample illuminated with a laser beam and a reference sample is measured and converted into an absorption coefficient at the laser energy by calibration [13]. Photoacoustic spectroscopy, where the thermal elastic waves generated in a gas-filled cell by the radiation absorbed by the sample are detected by a microphone, has also been performed at LHeT [34]. Photoacoustic detection using a laser source allows the detection of very small absorption coefficients [14]. Photoacoustic spectroscopy is also used at smaller absorption sensitivity with commercial FTSs for the study of powdered or opaque samples. Calorimetric absorption spectroscopy (CAS) has also been used at LHeT and at mK temperatures in measurement using a tunable monochromatic source. In this method, the temperature rise of the sample due to the non-radiative relaxation of the excited state after photon absorption by a specific transition is measured by a thermometer in good thermal contact with the sample [34,36]. 

Photoacoustic Detection of Natural Circular Dichroism in Crystalline Transition Metal Complexes... 

The photoacoustic detection of natural circular dichroism has been demonstrated successfully. Preliminary results at several fixed wavelengths on a limited number of samples show qualitative agreement with corresponding transmission data. In spite of the small signal levels involved, reproducibility is good. It appears that powders suffer scattering depolarization effects, which also occur with diffuse transmission measurements. 

Photoacoustic spectroscopy is often the most appropriate form of optical spectroscopy when material absorption is weak. ISLS spectroscopy offers the advantages of photoacoustic detection in a geometry that is compatible with the requirements of a DAC. The ISLS approach enables the experimenter to tune in material modes for observation, by adjustment of the excitation pulse width and wavelength. Control of the probe wavelength permits... 

---