Fractal morphology, imaging and mass spectrometry of single aerosol particles in flight

Intense, coherent X-ray pulses from a free-electron laser can be used to obtain high-resolution morphology of individual sub-micrometre particles in their native state, while at the same time their composition is analysed by mass spectrometry. Free-electron laser imaging of aerosols Aerosol particles are of importance in fields as diverse as materials engineering, toxicology and climate change, yet it is difficult to analyse the structure and properties of these materials in their native environment. This paper reports an in situ method for imaging individual sub-micrometre-sized particles to nanometre resolution in flight using intense X-ray pulses from the Linac Coherent Light Source free-electron laser. The technique can also simultaneously carry out compositional analysis using time-of-flight mass spectrometry. The morphology of micrometre-size particulate matter is of critical importance in fields ranging from toxicology 1 to climate science 2 , yet these properties are surprisingly difficult to measure in the particles’ native environment. Electron microscopy requires collection of particles on a substrate 3 ; visible light scattering provides insufficient resolution 4 ; and X-ray synchrotron studies have been limited to ensembles of particles 5 . Here we demonstrate an in situ method for imaging individual sub-micrometre particles to nanometre resolution in their native environment, using intense, coherent X-ray pulses from the Linac Coherent Light Source 6 free-electron laser. We introduced individual aerosol particles into the pulsed X-ray beam, which is sufficiently intense that diffraction from individual particles can be measured for morphological analysis. At the same time, ion fragments ejected from the beam were analysed using mass spectrometry, to determine the composition of single aerosol particles. Our results show the extent of internal dilation symmetry of individual soot particles subject to non-equilibrium aggregation, and the surprisingly large variability in their fractal dimensions. More broadly, our methods can be extended to resolve both static and dynamic morphology of general ensembles of disordered particles. Such general morphology has implications in topics such as solvent accessibilities in proteins 7 , vibrational energy transfer by the hydrodynamic interaction of amino acids 8 , and large-scale production of nanoscale structures by flame synthesis 9 .