ultrasonic spectroscopy for material analysis - recent advances
Breda O'Driscoll, Cormac Smyth, Arno C. Alting, Ronald W. Visschers, Vitaly Buckin
Spectroscopy Europe, February, pp. 20-25 (2003)
In collaboration with:
Arno C, Alting and Ronald W. Visschers, Wageningen Centre for Food Sciences (WCFS), Wageningen, The Netherlands
The relationships between a materialís properties and acoustical characteristics have been studied for a long time and ultrasonic techniques have been used in non-destructive testing and imaging for decades. Ultrasound as an analytical tool has revolutionised diagnostics in medicine, but the application of ultrasound to materialís analyses has been held back by problems with ultrasonic design, electronics and size of samples handling, complicated measuring procedures and resolution. Recent advances in computing power and digital techniques have made it possible to design a versatile laboratory instrument with applications ranging from ceramics to polymer science to cell biology and emulsions. The high-resolution HR-US family of ultrasonic spectrometers recently launched by Ultrasonic Scientific is an example of this. Most spectroscopists are accustomed to using electromagnetic waves in analysis (UV, VIS, IR, NMR etc.). Ultrasonic spectroscopy is simply spectroscopy employing sound waves. In particular, it uses a high-frequency acoustical wave (similar or higher to those used by dolphins for communication and bats for navigation). The wave probes intermolecular forces in materials. Oscillating compression (and decompression) in the ultrasonic wave causes oscillation of molecular arrangements in the sample, which responds by intermolecular attraction or repulsion. The amplitudes of deformations in the ultrasonic waves employed in analytical ultrasound are extremely small, making ultrasonic analysis a non-destructive technique. Of course an ultrasonic wave, unlike its light counterpart, is able to propagate through opaque samples, in fact through most materials. Another advantage is that it is relatively easy to change the wavelength of an ultrasonic wave: unlike optical techniques where the wave originates in a light source and therefore needs special effort to get a required spectral purity, ultrasonic waves are synthesised electronically. Therefore a typical ultrasonic spectrometer can cover a broad range of wavelengths (10 to 100 times or greater). It could be described as probing the interior of the analysed sample with a set of fingers, which differ in their length by more than an order of magnitude!