Gas-phase analysis of individual nanoparticles
Knowing the identity and origin of atmospheric particles impacts the health of people and the planet. The detection of neutral particles is particularly challenging as many techniques rely on ionisation that can damage fragile samples. Blister-based laser-induced forward transfer (LIFT) is a relatively new femtosecond laser ablation technique that is mild enough to accommodate such samples. A material is transferred to a metal substrate and irradiated from behind by a short laser pulse, causing a blister to form. Rapid movement of the metal substrate facilitates efficient material transfer and effective isolation. Scientists improved on a matrix-free blister-based LIFT technique, producing an apparatus for gas-phase analysis of small particles with EU support of the project DECIMA (Detection and characterization of individual micro- and nanoparticles). In particular, DECIMA developed a new LIFT technique to introduce nanoparticles into high vacuum for ionisation and analysis. Particle transfer efficiency as high as 90 % has been realised and the ejected particles form a very narrow particle beam for well-controlled placement. The average particle velocity is about 50 m/s, slow enough to allow investigation of particle composition and morphology with depth resolution. Scientists integrated the LIFT technology with time-of-flight mass spectrometry (TOFMS), of both positive and negative ions. Researchers fully characterised the operation of the new apparatus using atomic force microscopy and theoretical studies to optimise laser characteristics and prevent sample materials damage. The technology is simple to realise and compatible with any sample, including carbon nanostructures, aerosol particles and biological objects. It was successfully used to transfer fullerene C60 molecules from a metal-coated substrate into the extraction region for TOFMS. Further, it effectively isolates the transferred materials from other ablation materials and minimises laser heating so that the particles are delivered to the analytical instrumentation unaltered. DECIMA progress will be important in getting the nanoparticle detection and characterisation set-up closer to proof-of-principle in the laboratory and eventual prototype development. Commercialisation of the integrated LIFT-TOFMS apparatus could have significant positive impact on health, environmental safety, and understanding of weather patterns and global climate change.
Keywords
Nanoparticles, detection, laser-induced forward transfer, laser ablation, TOFMS