Ultrasonic spectroscopy is generally described as the field of study concerned with the extraction of information from an ultrasonic wave after interaction with a material. Ultrasonic methods for nondestructive testing of materials have been shown to have widespread applications. Specimens can be investigated by means of various ultrasonic measurements such as sound wave velocity, ultrasonic attenuation, acoustic reflection, acoustic emission, and ultrasonic interferometry. Ultrasonic sensors for noninvasive measurements in the chemical and process industries have been reviewed by Asher. Recently, an experimental microchip device that uses acoustic waves to sense the types and amounts of gases in the air by detecting resonant frequency shifts has been developed. In this paper a new ultrasonic resonance technique for the identification of microliters of liquids is described. This technique is similar in principle to stationary wave methods that are used extensively for measurements of propagation constants (velocity) and elastic properties (absorption) of solids. It allows for the rapid comparison between liquids which exhibit different acoustic properties. On the basis of preliminary results, the potential and limitations of the technique are evaluated.
Edward P. C. Lai, Becky L. Chan, and Susan Chen, "Ultrasonic Resonance Spectroscopic Analysis of Microliters of Liquids," Appl. Spectrosc. 42, 526-529 (1988)