Observation of particles manipulated by ultrasound in close proximity to a cone-shaped infrared spectroscopy probe.

The presented investigations aimed to enhance surface sensitive infrared spectroscopy for chemical analysis by ultrasonic particle manipulation. The combination of these techniques has the potential for new measurement concepts for use in the chemical analysis of suspensions. Local increases of particle concentration brought about by ultrasound could facilitate measurements of molecular-specific infrared spectra of the suspending phase and particles independently. By changing the frequency of an ultrasonic standing wave around 2 MHz it was possible to control the position of particles in respect to the optically sensitive region of the infrared spectroscope. Results obtained with a set-up that enabled us to explore the application of an ultrasonic standing wave to push suspended particles at or into mum distances of the sensing element of an in-line fiber optic probe and subsequently retract them from there are presented. Light micrographs suggested, that the task was successfully accomplished with polystyrene beads suspended in methanol, aggregates were manipulated to and from the cut surface of the truncated, cone-shaped fibre probe tip by changes of the ultrasonic frequency between 1.85 and 1.87 MHz. Feasibility was confirmed by infrared absorption spectra recorded when PTFE particles suspended in tetrahydrofuran were used.