A low-cost ultrasonic absorption spectrometer for liquids

Ultrasonic absorption spectroscopy can probe intermolecular interactions that inform research into processes for chemical engineering and pharmaceuticals. The only commercial ultrasonic spectrometer costs over one hundred thousand dollars, putting it out of reach for many institutions. We designed an inexpensive ultrasonic absorption spectrometer comprised of off-the-shelf components and parts from a rapid prototyping service. We employed the through-transmission method for measuring absorption, using thirty-one pulse measurements at varying distances. These measurements significantly improved measurement accuracy when compared to both a fixed path technique and the pulse-echo method that rely on only two measurements. We measured pure water to correct for diffraction effects and used repeated water measurements to propagate the uncertainty. To validate the spectrometer, we measured salt solutions, polyvinyl alcohols, and celluloses. We then compared the data to results obtained from a commercial spectrometer and they were well within the margin of error for both devices. Notably from these measurements, we observed a relaxation peak near 1MHz in scandium sulfate, which was only reported once before with the use of a resonance method. The system we present here offers an inexpensive alternative to a commercial ultrasonic absorption spectrometer that is accessible to university researchers and students.