You are here:
EVALUATION OF COMPUTER-CONTROLLED SCANNING ELECTRON MICROSCOPY APPLIED TO AN AMBIENT URBAN AEROSOL SAMPLE
Citation:
Willis, R. D., Y. Mamane, AND T L. Conner. EVALUATION OF COMPUTER-CONTROLLED SCANNING ELECTRON MICROSCOPY APPLIED TO AN AMBIENT URBAN AEROSOL SAMPLE. Presented at PM 2000 AWMA Conference, Charleston, SC, January 24-28, 2000.
Impact/Purpose:
The NERL SEM/EDX facility was established to provide a state-of-the-art resource for individual particle analysis. Specific goals of the laboratory are as follows: 1) Provide chemical, morphological, and size characterization of particles in support of ambient air characterization studies, source apportionment, exposure and health studies, and other particulate matter research.; 2) Develop analytical capabilities to support research objectives; 3) Communicate results to clients and the scientific community.
Description:
Recent interest in monitoring and speciation of particulate matter has led to increased application of scanning electron microscopy (SEM) coupled with energy-dispersive x-ray analysis (EDX) to individual particle analysis. SEM/EDX provides information on the size, shape, composition, heterogeneity, and possible sources of ambient particles. Such analyses are often costly and time-intensive. Computer-controlled scanning electron microscopy (CCSEM) allows automated analysis of particle size and elemental content of suitably prepared samples. Despite widespread use of CCSEM, data quality and accuracy are rarely discussed. In this paper we examine issues affecting the quality and validity of CCSEM data. Specifically, we assess the stability of unattended CCSEM over several hours, evaluate the sensitivity of CCSEM results to initial instrument settings, confirm the accuracy of particle volume and mass estimates by CCSEM, and determine the number of particles that must be analyzed in order to yield representative results.
CCSEM was applied to an urban aerosol sample collected on a polycarbonate filter with a dichotomous sampler. More than 2800 particles from 78 randomly selected fields were analyzed over an 8-hour period. Particle diameters and aspect ratios, X-ray counts of 20 elements, and images of each particle and its field of view were stored. Results show that a minimum of several hundred particles is needed to ensure representative results, while little additional information is gained by analyzing more than 1000 particles. The study demonstrated the stability of CCSEM over many hours. We conclude that CCSEM is a powerful tool for the characterization of individual atmospheric particles and a valuable complement to bulk analysis techniques.
This work has been funded wholly or in part by the United States Environmental Protection Agency under contract #68-D5-0049 to ManTech Environmental Technology, Inc. and under purchase order #9D-0710-NTTX for Dr. Yaacov Mamane. It has been subjected to Agency review and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.