Photoacoustic detectors

Rgure 8.5 Schematic diagram of signal wavelength IR detector. 

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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. 

Phosphorescence, 191, 223 Photoacoustic detector, 177 Photoelectric effect, 238 Photoelectron, 238 Photovoltaic, 176 Plasam, 274 Plate, 10 Polarogram, 361 Polarographic wave, 361 Polyethylene glycols, 32 Polysiloxanes, 31 POPOP, 333 PPO, 333 Precession, 129 Precision, 386 Pulse polarography, 364 Pulsed NMR, 155 Pyroelectric, 175... 

Some radiation detectors, i.e., photoemissive detectors (vacuum phototubes or photomultipliers) or semiconductor detectors (photodiodes or phototransistors) directly produce an electrical signal by quantum effects. Their output is strongly dependent on the wavelength of the detected radiation. Thermal detectors, i.e., thermocouples and thermopiles, bolometers, pyroelectric detectors, or pneumatic and photoacoustic detectors record a temperature increase through radiation and convert this into an electrical signal. This is proportional to the flux of the absorbed radiant power, independent of the wavelength. 

Figure 4 Photoacoustic spectrum of native aminoethyl TentaGel S beads (lower curve) and of TentaGel S beads coupled via a trityl linker with Fmoc-protected tryptophan (upper curve). The carbonyl bands of the ester (at 1750 cm1), of the carbamate (at 1723 citT1), and of the amide (at 1660 cm-1) are clearly seen in the overlay plot. The spectra were obtained with 50 scans at 8 cm-1 resolution by means of a Bruker IFS 55 spectrophotometer equipped with a MTEC photoacoustic detector.

Testing procedure In general, standard methods were used, with some improvements to obtain better resolution. A photoacoustic detector was used to obtain spectra of fumed silica. A carbon black background was used. In studies of adhesion of coatings on metal substrates, a gold coated background was used as the reference. 4 Diffuse respectra, and kaolin. 2 A... 

Evaporation of VOCs from 10 cleaning agents with different properties has been measured (Vejrup and Wolkoff, 1994). The nonpolar VOCs gave rise to peak emissions immediately after the application of the products. Cleaning agents have also been investigated with a photoacoustic detector (PAD) in combination with a FLEC (Vejrup and Wolkoff, 1995). 

Photoacoustic detectors for tise with FTIR spectrometers have been developed and are presently available for a number of commercial instruments. Since PA-FTIR represents a very recent area of interest, the published literature is, therefore, rather limited (26-34). The photoacoustic and diffuse reflectance methods are complimentary and both have been shown to be applicable to coal analysis (, , , 31, ). 

Fig. 9. High temperature photoacoustic detector system (Al-T Al-B heated Al block, Cuv cuvette, Tef. teflon swagetok, Q quartz rod, M front siuface mirror, SS stainless steel housing and Pb lead disc)...

An intracavity photoacoustic detector was also described by Leslie and Trusty In this work absorption coefficients of CH4 were measured in the 2500-2800 cm region, applying a DF laser for excitation of CH4. The weakest CH4 absorption studied was about 10 times weaker than the strong CH4 absorption at 3,39 pm. The intracavity cell was not intended to be resonant in this case, however, by using a resonance found at 310 Hz, a substantially better performance was obtained. The smallest absorption measured with this setup was 1 10" cm with about 10 1 S/Nat a 1 sec time constant. 

Photoacoustic-FTIR has been done, using the FTS 7000 series machine from Digilab, equipped with a photoacoustic detector model 300 from MTEC. Helium was used as purge gas and the software Win-IR-Pro was used for data analysis. 

FIGURE 52.13 Effect of a 2-h anaerobic treatment (nitrogen flow) on the pattern of acetaldehyde and ethanol emissions from rice seedlings (14 days old) measured simultaneously with PTR-MS and CO-based laser photoacoustic detector. The plants were placed under 0%-O2 conditions at time t = 0.25 h. At t = 2.35 h, the rice plants were returned to a flow of air (reprinted from Reference [23]). 

The simplest application of a piezoelectric transducer is its use as a strain indicator, since the strains applied to a piezoelectric element can be quantified simply by measuring the electrical field which is created across the boundaries of that element. In the case of photoacoustic detectors, pressure pulses, created by the absorption of light from a chopped beam, deform a thin plate and are transformed into a series of voltage pulses. In such applications the electrical signals created are either dc or low-frequency, so that the design of the device and interface electronics is straightforward. 

Coarse or hard powders are not well served by either the compressed pellet or mull technique, mainly because of difficulties associated with grinding. In such situations, the best approaches require the use of an accessory, such as a diffuse reflectance or photoacoustic detector. Both diffuse reflectance and photoacoustic methods [99,100] may be applied to most forms of powdered solids. As a rule, photoacoustic measurements, which are the only form of true absorption measurement, are not significantly influenced by sample morphology. An alternative procedure for powders is ATR, especially a horizontal accessory, preferably equipped with a pressure applicator. Note that the use of pressure is recommended to ensure intimate contact between the sample and the IRE (internal reflectance element) surface. Normally, the sample must conform to the surface of the IRE, and because the strength of the IRE is typically limited, the procedure is recommended only for soft powders. However, with the introduction of diamond-based ATR accessories [101-103], it is possible to handle most types of powdered material. 

To overcome the problems discussed previously it is necessary to use an FT-IR instrument with very good S/N performance and good stability at lower optical path difference velocities. Perkin Elmer 1800 series or 1700 series (particularly models 1720/1760) FT-IR spectrometers coupled to an MTEC model 100 PA photoacoustic detector adequately meet these requirements. 

Fig. 1.20. Schematic diagram of a photoacoustic detector. After Perkins [56]. Reprinted with permission from W.D. Perkins, in Practical Sampling Techniques for Infrared Analysis (PB. Coleman, ed.), CRC Press, Boca Raton (1993). Copyright CRC Press, Boca Raton, Florida.

The IR beam, suitably modulated (by a chopper for dispersive IR instruments or by the Michelson interferometer in the FT-IR instruments) is directed on to the sample. If the sample absorbs the Incident radiation at any particular wavelength, the absorbed radiation is converted to heat by nort-radiat1ve processes. This heat is transferred to the surrounding gas by thermal diffusion. The gas expands and contracts at the modulation frequency resulting in a pressure wave within the sealed cell. This pressure wave is detected by the microphone and the signal from the microphone then becomes the output of the photoacoustic detector that is processed by the IR instrument detector electronics. 

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