Acoustic spectrometer

SPIN FLIP RAMAN LASER OPTO-ACOUSTIC SPECTROMETER... 

Note that US-based detector manufacturers are usuaiiy reiuctant to provide technicai information and schemes of their instruments (e.g. types of components, assembiy performance). By exception. Dr. Dukhin has kindiy aiiowed part of the description of the DT100 acoustic spectrometer [100] to be inciuded in this section. 

Dispersion Technology DT100 acoustic spectrometer. This instrument is a modern example of commerciai equipment which, in its basic design, foiiows the transmission tone-burst variabie gap technique pioneered by Andreae [101-103]. Figure 9.16A shows a photograph of a typicai unit. 

Figure 9.16. (A) The Dispersion Technology DT100 acoustic spectrometer. (B) Perspective and top views of the acoustic sensor. (Reproduced with permission of Elsevier, Ref [100].)...

Acoustics is not only a particle sizing teehnique, but also provides information about the microstmcture of the dis-peresed system. The acoustic spectrometer can be considered as a microrheometer. In acoustics, stresses are applied in the same way as with regular rheometers, but over very short distances on the micrometer scale. In this way, the microstmcture of the dispersed system can be sensed. Currently, this feature of acoustics is only beginning to be exploited, but it is certainly very promising. 

Acoustic spectroscopy of dispersed systems operates with frequencies above 1 MHz and usually up to 100 MHz. The size of the bubbles must be well below 10 nm in order to affect the complete frequency range of the acoustic spectrometer. 

Bubbles can only affect the low-frequency part of the acoustic spectra (below 10 MHz). The frequency range 10-100 MHz is available for particle characterization even in the bubbly liquids. Acoustic spectrometers can both sense bubbles and characterize particle size. We can confirm this conclusion with thousands of measurements performed with hundreds of different systems. Sensitivity to bubbles, in fact, is an important advantage of acoustics over electroacoustics. The presence of bubbles may affect the properties of the solid dispersed phase. For instance, bubbles can be centers of aggregation, which makes them an important stability factor. 

Currently, there are three acoustic spectrometers on the market Ultrasizer from Malvern, Opus of Sympatec, and... 

The variable-gap technique is an essential feature of the acoustic spectrometer. This makes it possible to cover a wide dynamic range of possible attenuations. For instance, pure water is almost transparent to ultra sound at a low frequencies (below 10 MHz). The attenuation of water reaches only 5 dB/cm at 50 MHz. Therefore, the attenuation of water should be measured at large gaps, as little information is obtained from small gaps. 

We have also tested the accuracy and precision of the D-1200 Acoustic Spectrometer using Standard Dow Latex with an expected median particle size of 0.083 pm. The results are shown in Table 2. 

In this section we will describe another non-optical technique for the detection of optical absorption opto-acoustic spectroscopy. The principle of a laser-based opto-acoustic spectrometer (spectrophone) is given in Fig.9.12. A molecular sample at relatively high pressure is contained in a closed volume and is irradiated with a chopped cw laser beam tuned to resonance. The excited molucules are mainly de-excited radiationlessly at the prevailing pressure. The excitation energy is transferred to translational energy... 

Acousto-optic Filters. The newest type of spectrometer to become commercially available is the acousto-optic tunable filter (AOTF). An AOTF is a sohd-state, electronically tunable bandpass filter based on the diffraction of optical waves by acoustic waves in an optically anisotropic crystal. 

Transmission spectroscopy offers two significant advantages over photoacoustic spectroscopy of powders. First, transmission spectroscopy is not susceotible to external acoustic disturbances. Commercial spectrometers must be modified for vibrational isolation in order to obtain good photoacoustic spectra. Secondly, transmission spectroscopy can use solid state detectors with very fast response times, whereas photoacoustic spectroscopy is much slower, with spectra taking a few minutes to collect as compared to a few seconds for transmission spectra when both are taken with an FTIR. 

The interferometer assembly has been modified to provide acoustic isolation from both building vibrations and airborne noise (7). These improvements have greatly enhanced the signal to noise. The spectra presented here show a signal to noise ratio in excess of 500 for silica samples and in excess of 100 for the alumina samples the differences are due to different sample porosities. No smoothing of the spectra has been performed, and all spectra reported are direct reproductions of the plotter output from the spectrometer. 

Procedure Set up an acoustic reactor in a light-proof cabinet with a photomultiplier (PM) tube positioned facing the cell as shown in Fig. 15.3a and b. Fill the cell with distilled water and close the cabinet. A potential should now be applied to the PM tube, the output (spectrally integrated) of which is produced on an oscilloscope (note that the ultrasound cell can easily be placed inside a commercial spectrometer in order to record the emission spectrum). Switch on the ultrasound and you should observe on the oscilloscope a change in voltage, directly proportional to the intensity of sonoluminescence emission. The following experiments can be performed to explore the different types of light emission and some of the factors that influence these emission processes. 

Tsukahara, Y., Ohira, K Saito, M and Briggs, G. A. D. (1989b). Evaluation of polymer coatings by ultrasonic spectroscopy. In Acoustical imaging, Vol. 17 (ed. H. Shimizu, N. Chubachi, and J. Kushibiki), pp. 257-64. Plenum Press, New York. [214] Tsukahara, Y., Ohira, K., and Nakaso, N. (1990). An ultrasonic micro-spectrometer for the evaluation of elastic properties with microscopic resolution. IEEE 1990 Ultrasonics Symposium, pp. 925-30 [149]... 

Backward LP (Fig. 5.21) is usually applied to repair the first few points of an FID, distorted by some spectrometer perturbation or a mis-set acquisition parameter, e.g. incorrect receiver gain. Backward LP is also used to reconstruct an FID back to t=0 in those cases where the start of data acquisition has been delayed, e.g. to exclude unwanted spectrometer noise such as the signals from acoustic ringing, and the first few data points are missing. In this case backward LP cancels or at least suppresses the corresponding spectral artefacts such as baseline roll etc. 

Photoacoustic Spectroscopy.7 When modulated IR radiation is absorbed by a sample, the substance heats and cools in response to modulated IR energy impinging on it. This thermal hysteresis is converted into pressure waves that can be communicated to surrounding gases and detected by acoustic detectors (essentially a sensitive microphone in the enclosed sample chamber). In such measurements, the acoustic detector replaces the IR detector of the spectrometer. 

On-line particle sizing by ultrasonic (acoustic attenuation) spectroscopy was developed for use during batch crystallization processes.14 Crystallization of the alpha polymorph of (l) -glutamic acid from aqueous solution was monitored by continuously pumping the crystallizing solution through an on-line ultrasonic spectrometer. The method enabled measurement of the crystal size distribution and solid concentration throughout the... 

An application of ultrasound that is becoming increasingly popular in the food industry is the determination of creaming and sedimentation profiles in emulsions and suspensions (Basaran et al., 1998). Acoustic techniques can also assess nondestructively the texture of aerated food products such as crackers and wafers. Air cells, which are critical to consumer appreciation of baked product quality, are readily probed due to their inherent compressibility (Elmehdi et al., 2003). Kulmyrzaev et al. (2000) developed an ultrasonic reflectance spectrometer to relate ultrasonic reflectance spectra to bubble characteristics of aerated foods. Experiments were carried out using foams with different bubble concentration and the results showed that ultrasonic reflectance spectrometry is sensitive to changes in bubble size and concentration of aerated foods. 

---