Advanced Microscopy for Trace and Bulk Particle Characterization

Summary

Advanced microscopy and spectroscopy techniques are used in this project to study the chemical and physical properties of particles contained in complex matrices. It evaluates individual particles as well as particle populations.

Description

Figure 1.  Elemental signatures for major particle classes by weight fraction of ternary end-members. A) silicate. B) Fe oxide. C) vehicle-related (Fe-Ba-S-Cu-Si-Sb-Zr). And D) CaMg carbonate.

The goal of this project is to help develop innovative metrologies and measurement protocols for micro/nano-scale analysis of particles.

In this project, state-of-the-art optical and electron-based microscopy are used to conduct research for quantitative and qualitative analysis of chemical compounds, complex mixtures, and bulk and micro/nanoscale particles. Overall, the methods include particle sample collection and preparation, various forms of light microscopy including optical fluorescence, focused ion beam analysis (FIB), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy, instrument automation, image acquisition and analysis, data mining, tomography and three-dimensional (3-D) reconstruction of individual particles.

Bulk Particle Characterization

SEM automated X-ray particle analysis is currently being used to study the particle population of urban dust particle samples collected on polycarbonate filters near the Enid A. Haupt Garden gateposts and heavily trafficked Independence Ave in Washington, DC, USA.  The study aims to evaluate airborne dust as a potential Mn source at a location where rock varnish is actively forming [Vicenzi et al., 2016]. Samples are analyzed on a high-performance SEM equipped with four 30 mm² silicon drift detectors (SDD) mounted at » 90° azimuthal increments. Typically, a beam energy of 20 keV is used and 1 nA probe current as measured in a Faraday cup and remeasured before and after automated analysis. The automated particle analysis is performed using the SEMantics extension to NIST DTSA-II [Ritchie and Filip, 2011]. The spectra obtained from the four SDD are combined into a single spectrum for quantification using NIST Graf [Lindstrom and Ritchie, 2014]. A novel algorithm called Diluvian Clustering [Ritchie, 2015] is then used to cluster the data obtained for about 40,000 particles.  Subsequently, the data is reprocessed using a manually developed rule set based on the insight produced by the Diluvian Clustering and insight into the types of materials likely to be present in urban environmental dust.

Our preliminary data show low, but detectable levels of Mn. Figure 1 shows representative elemental signatures of four major particle classes: silicate, Fe oxide, vehicle-related and CaMg carbonate. Efforts are underway to estimate the mass of Mn transported by atmospheric dust deposition for particles under 10 micrometers in diameter. To our understanding, this is the first study to examine the linkage between individual particle analysis and active rock varnish formation.

Individual Particle Characterization

FIB tomography is used to create 3-D reconstructions of irregularly shaped dust particles and further study their light scattering properties. This study combines advanced microscopy techniques and optical property modeling to understand how morphology and composition of individual atmospheric particles affect their optical properties. The way in which particles scatter and absorb light has implications for aerosol optical remote sensing and aerosol radiative forcing applications. Furthermore, their optical properties depend highly on the diverse chemical composition, structures, sizes, and shapes of the particles. However, understanding light scattering and absorption by non-spherical particles can be very challenging. Dust particles were analyzed using focused ion-beam scanning electron microscopy (FIB-SEM) and energy-dispersive x-ray spectroscopy (EDX) to obtain images and elemental composition of individual particles and reconstruct their 3-D configurations using sophisticated visualization software. The FIB-SEM-EDX analysis has the main advantage of imagining hidden imbedded phases and voids that are commonly present in heterogeneously-mixed particles but which are difficult to detect using conventional SEM [Conny, 2013]. The 3-D reconstructions are then used in a discrete dipole approximation method (DDA) [Draine and Flatau, 1994] to determine their scattering properties for a range of shapes, sizes, and retractive indices. Scattering properties are obtained using actual shapes of the particles, as well as volume-equivalent geometrical shapes like spheres, cubes and tetrahedrons. Q-space analysis is used to interpret the angular distribution of the scattered light obtained for each particle. Q-space analysis has been recently used to distinguish scattering by particles of different shapes, and it involves plotting the scattered intensity versus the scattering wave vector on a log-log scale [Sorensen et al., 2017].

Currently, Q-space comparisons between actual and geometric shapes are underway with the objective of determining which geometric shape best represents the angular distribution and magnitude of the scattered light. Current work also focuses on the effects of the imaginary part of the refractive index on the light scattering of the particles. Preliminary data show that the agreement between models (actual vs. geometric shapes) varies substantially with particle size with better agreements observed for smaller particle sizes. Lastly, the shape and size of particles were found to cause more variability in the light scattering data than differences in chemical composition.

References:

Conny, J.M. (2013). Environ. Sci. Technol. 47, 8575-8581.

Draine, B.T. and Flatau, P.J. (1994). J. Opt. Soc. Am. A. 11, 1491-1499.Lindstrom, A.P. and Ritchie, N.W. (2014). Microsc. Microanal. Proc. 20(S3), 748.

Ritchie, N.W.M. and Filip, V. (2011). Microsc. Microanal. Proc. 17(S2), 896.

Ritchie, N.W.M. (2015). Microsc. Microanal. 21(5), 1173-1183.

Sorensen, C. M. et al. (2017). Atmosphere 8, 68.

Vicenzi, E.P. et al. (2016). Herit. Sci. 4, 26.

Related Publication

Ortiz-Montalvo, D.L., Vicenzi, E.P., Ritchie, N.W., Grissom, C.A., Livingston, R.A., Weldon-Yochim, Z., Conny, J.M. and Wight, S.A. (2018). Accepted for publication in Microsc. Microanal. Proc.

Conny, J.M., and D. L. Ortiz‐Montalvo (2017). Effect of heterogeneity and shape on optical properties of urban dust based on three‐dimensional modeling of individual particles, J. Geophys. Res. Atmos., 122, 9816–9842, doi:10.1002/2017JD026488.

Major Accomplishments

To our knowledge, first study to use sophisticated SEM automated X-ray particle analysis to examine the linkage between individual particle analysis of dust and active rock varnish formation.

The use of FIB tomography 3-D reconstructions of irregularly shaped dust particles together with optical property modeling and the novel Q-space analysis creates an innovative and powerful setting to study aerosol optical properties in a unique way.

Understanding that size and shape of particles can have a larger effect on their light scattering behavior than differences in chemical composition.

Outreach: Handheld models of microscopic particles were 3-D printed using our reconstructed 3-D models of irregularly shaped dust particles. The 3-D printed particles have been used in various outreach activities for demonstration purposes and to teach students and the audience about cutting-edge technology at NIST.