Scanning Electron Microanalysis and Analytical Challenges of Mapping Elements in Urban Atmospheric Particles

• Published in: Environmental science & technology Vol. 45; no. 17; pp. 7380 - 7386
• Main Authors: Conny, Joseph M, Norris, Gary A
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 01.09.2011
• Subjects: Air Pollutants - analysis; Applied sciences; Atmospheric pollution; Atoms & subatomic particles; Cities; Electron Probe Microanalysis - methods; Environmental Measurements Methods; Environmental Monitoring - methods; Exact sciences and technology; Humans; Mapping; Microscopy, Electron, Scanning - methods; Particle Size; Pollutants physicochemistry study: properties, effects, reactions, transport and distribution; Pollution; Scanning electron microscopy; Spectrometry, X-Ray Emission - methods; United States; Urban areas; X-rays; United States; Atlanta Georgia; Los Angeles California; Seattle Washington; United States > US; Elementary analysis; Scanning electron microscopy; Chemical analysis; Microanalysis; Aerosols; Air pollution; Urban area; Energy-dispersive X-ray analysis
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es2009049

Elemental mapping with energy-dispersive X-ray spectroscopy (EDX) associated with scanning electron microscopy is highly useful for studying internally mixed atmospheric particles. Presented is a study of individual particles from urban airsheds and the analytical challenges in qualitatively determining the composition and origin of heterogeneous urban-air particles from high-resolution elemental maps. Coarse-mode particles were taken from samples collected in three U.S. cities: Atlanta, Los Angeles, and Seattle. Elemental maps distinguished particles with heterogeneously mixed phases from those with homogeneously mixed phases that also contained inclusions or surface adducts. Elemental mapping at low and high beam energies, along with imaging at an oblique angle helped to classify particles by origin. The impact of particle shape on X-ray microanalysis was demonstrated by having the beam enter the particle at ≥52° from normal. Potential misinterpretations of particle composition due to artifacts in the elemental maps were minimized by tilt imaging to reveal particle surface roughness and depth, mapping at low beam energies, noting the position of the EDX detector in the map field, and assessing differences in the mass absorption coefficients of the particle’s major elements to anticipate X-ray self-absorption.