Laser-induced sound pinging (LISP): A rapid photoacoustic method to determine the speed of sound in microliter fluid volumes

摘要

Laser-induced ultrasound detection is a well-known technique for diverse applications, including medical analysis, materials characterization, industrial quality control, and environmental monitoring. The measurement of the speed of sound in liquids is frequently performed using a single-crystal ultrasonic interferometer, a device which requires a substantial volume of liquid (= 10 mL), hindering its utility in low-volume applications (e.g., biomedical screening). Other specialized techniques, such as the transient grating approach, exist and while some provide additional useful information (e.g., thermal diffusivity and damping parameters), they are costly and complex to implement, frequently requiring a specialist. In response to these instrumental limitations, we introduce a simple, inexpensive, and convenient laser-based approach to directly measure the speed of sound in minute liquid volumes (= 25 mu L) with a high degree of accuracy (percent relative standard deviation in determining sound speed better than +/- 0.15%). This new technique, termed laser-induced sound pinging (LISP) owing to the inspiration drawn from submarine sonar pinging, is demonstrated using an inexpensive tattoo-removal Nd:Ce:YAG laser. In this communication, we validate the concept of LISP and demonstrate its unique capabilities by analyzing a variety of samples on the basis of the speed of sound: the salinity of water, dilutions of bovine milk, and the characterization of representative ionic liquids. Given the rapid nature of measurement, ease of operation, simplicity of analysis (model-free), and microliter sample volume requirements, we anticipate that LISP will progress as a beneficial and practically-relevant analytical tool for answering a broad range of liquid forensic questions, with potential applications in drinking water quality evaluation, food safety and authentication, bioanalysis, and quality-control (e.g., measuring sugar contents in soft drinks). Fieldable and real-time, in-line versions of this instrument can also be envisaged in the near future.