Optofluidic Force Induction meets Raman Spectroscopy and Inductively Coupled Plasma – Mass Spectrometry: A new hyphenated technique for comprehensive and complementary characterisations of single particles

Nanoparticles are produced at accelerating rates, are increasingly integrated in scientific and industrial applications, and are widely discharged into the environment. Analytical techniques are required to characterise parameters such as particle number concentrations, mass and size distributions, molecular and elemental compositions as well as particle stability. This is not only relevant to investigate their utility for various industrial or medical applications and for controlling the manufacturing processes, but also to assess toxicity and environmental fate. Different analytical strategies aim to characterise certain facets of particles but are difficult to combine to provide a more comprehensive picture. In this work, we suggest the on-line hyphenation of optofluidic force induction (OF2i) with Raman spectroscopy and inductively coupled plasma-time-of-flight-mass spectrometry (ICP-ToF-MS) to harness their complementary technology-specific advantages and to promote comprehensive particle characterisations. We optically trapped individual particles on a weakly focussed vortex laser beam by aligning a microfluidic flow antiparallelly to the laser propagation direction. The position of particles in this optical trap depended on the hydrodynamic diameter and therefore enabled size calibration. Additionally, laser light scattered on particles was analysed in a single particle (SP) Raman spectroscopy set-up for the identification of particulate species and phases. Finally, particles were characterised regarding elemental composition and their distributions in mass and size using SP ICP-ToF-MS. In a proof of concept, we analysed polystyrene-based plastic and TiO2 particles, and demonstrated the opportunities provided through OF2i-Raman-SP ICP-ToF-MS as new hyphenated technique.