Spectrometry, ultrasonic

General ultrasonic spectrometry relies on direct measurements of the physical changes caused by the US-sample interaction, namely velocity changes and attenuation of the radiation. The different modes of this technique arise from factors such as (1) the way US is applied e.g. as a single frequency, broad-band pulses, scanning frequency), after which modes are named (2) the way US impinges on the sample (normal, parallel, oblique), after which the waves produced in the material (longitudinal, shear, oblique) are named (3) the way the experimental data provided are used viz. amplitude or phase spectra) or processed viz. frequency or time domain). 

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Quality attributes of food emulsions, such as appearance, stability, and rheology, are strongly influenced by the size of the droplets that they contain (Friberg and Larsson, 1997 McClements, 1999). For example, the creaming stability of an emulsion decreases as droplet size increases. Analytical techniques that provide quantitative information about droplet size are therefore required to aid in the development and production of high-quality emulsion-based food products. A variety of analytical techniques have been developed to measure droplet size, e.g., laser diffraction, electrical pulse counting, sedimentation techniques, and ultrasonic spectrometry (McClements, 1999). These techniques are used for fundamental research, product development, and quality assurance. This unit focuses on the two most commonly used techniques in the food industry, laser diffraction and electrical pulse counting. 

The major disadvantage of the laser diffraction and electrical pulse counting techniques is that they are only directly applicable to dilute emulsions or emulsions that can be diluted without disturbing the particle size distribution. However, many food emulsions are not dilute and cannot be diluted, either because dilution alters the particle size distribution or because the original sample is partially solid. For concentrated systems it is belter to use particle-sizing instruments based on alternative technologies, such as ultrasonic spectrometry or NMR (Dickinson and McClements, 1996). 

Determining the droplet size distribution of an emulsion by ultrasonic spectrometry involves two steps. First, the ultrasonic velocity and (or) attenuation coefficient of the emulsion is measured as a function of the frequency — preferably over as wide a range as possible. Second, the experimental measurements are compared with theoretical predictions of the ultrasonic properties of the emulsion, and the droplet size distribution providing the best fit between theory and experiment is determined. 

Most areas of interest for determinations of particie size have been covered by ultrasonic spectrometry, and are discussed briefly beiow. 

The specific, particle sizing method chosen depends on the type of. size information needed and the chemical and physical properties of the sample. In addition to the three techniques discussed here, molecular sieving, electrical conductance, microscopy, capillary hydrodynamic chromatography, light obscuration counting, field-flow fractionation, Doppler anemometry, and ultrasonic spectrometry-are commonly applied. Huch of the particle sizing methods has its advantages and drawbacks for particular samples and analyses. 

U. Riebel, F. Loffler, The fundamentals of particle size analysis by means of ultrasonic spectrometry. Part. Part. Syst. Charact. 6(1 ), 135-143 (1989). doi 10.1002/ppsc. 19890060124... 

F. Babick, F. Hinze, M. Stintz, S. Ripperger, Ultrasonic spectrometry for particle size analysis in dense submicron suspensions. Part. Part. Syst. Charact 15(5), 230-236 (1998). doi 10.1002/ (SICI)152W117(199810)15 5<230 AID-PPSC230>3.0.CO 2-D... 

Ultrasonic Spectrometry On-line Particle Size s Analysis at Extremely High Particle Concentrations... 

Ultrasonic spectrometry is a new method of particle size analysis based on ultrasonic extinction measurements at multiple frequencies. Due to some inherent features of the measuring principle, which will be discussed in the following, measurements by ultrasonic spectrometry can be extended to particle concentrations as high as 20 % by volume and more. 

For particle size analyses, ultrasonic extinction is measured for a number of different frequencies consecutively. The whole procedure of measurement, including frequency selection, extinction measurement and mathematical evaluation is automated. One measurement of particle size distribution and particle concentration takes about 2 to 5 minutes, depending on the resolution and the reproducibility required. Some cumulative area density distributions measured by ultrasonic spectrometry, compared to the results obtained from a microscopic evaluation, are shown in Fig. 3. Broad and narrow distributions are clearly distinguished, owing to the high resolution and reliability of the method. 

Fig. 3 Some particle size distributions measured by ultrasonic spectrometry, compared to results of microscopic evaluation.

Fig. 9 The influence of particle volume concentration Cv on the particle concentration as determined by ultrasonic spectrometry, whereby Lambert-Beer s law was used.

Thanks to a low sensitivity to disturbances from multiple scattering, particle size measurements by ultrasonic spectrometry can be extended to extremely high particle concentrations. An evaluation of the measurements based on Lambert-Beer s law can provide results of sufficient quality for particle concentrations up to about 20 % by volume. An extension of ultrasonic measurements to even higher concentrations will require a more thorough understanding of steric interactions, including steric interactions between particles of different size. 

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