X-ray fluorescence mapping and micro-XANES spectroscopic characterization of exhaust particulates emitted from auto engines burning MMT-Added gasoline

The elemental distribution and compositional homogeneity in auto exhaust pa rticulates emitted from methylcyclopentadienyl manganese tricarbonyl-(MMT-) added gasoline engines have been investigated using a newly installed synch rotron X-ray microprobe. Two representative groups of exhaust particulate m atter, as defined in a recent bulk X-ray absorption fine structure (XAFS) s pectroscopic study at the Mn K-edge, were studied. The micro-X-ray absorpti on near-edge structure (XANES) spectra indicate a relatively homogeneous di stribution of phases within a given particulate sample, down to a spatial e xtent of 40 mum (the resolution of microprobe). The micro-XANES also enable d analysis of several areas which displayed compositions different from the bulk sample, supporting the general theory describing manganese species fo rmation in the exhaust. The ability to evaluate small regions also enabled direct verification of manganese sulfate from the S XANES despite the vast excess of sulfur present in other forms. The presence of a chloride compoun d, introduced through the sample dilution air and engine intake air, was al so revealed. The study demonstrates the value of the combined X-ray microfl uorescence with excitation by polychromatic radiation for elemental mapping and micro-XANES spectroscopy for chemical speciation in the study of dilut e environmental materials containing low-Z constituents such as Cl, S, and P.