Evaluation of computer-controlled scanning electron microscopy applied to an ambient urban aerosol sample
Evaluation of computer-controlled scanning electron microscopy applied to an ambient urban aerosol sample
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DOI：
10.1080/027868201300082085
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发表时间：
2001-01-01
影响因子：
5.2
通讯作者：
Conner, T
中科院分区：
文献类型：
--
作者：
Mamane, Y;Willis, R;Conner, T
对颗粒物（PM）的环境和公众健康影响的关注，激发了人们对能够测量单个气溶胶颗粒的大小和化学成分的分析技术的兴趣。计算机控制的扫描电子显微镜（CCSEM）与能量色散X射线分析（EDX）相结合，可以自动分析粒度，化学和颗粒分类。结合手动扫描电镜和散装分析技术，如X射线荧光，CCSEM可以是一个有价值的工具，用于表征个别环境颗粒物和确定环境PM的来源，本研究的目的是检查几个问题有关的质量和有效性的CCSEM数据。这些包括无人值守的CCSEM多小时运行的稳定性，必须进行分析，以产生代表性的结果，和与CCSEM相关的错误的颗粒的数量。CCSEM被应用于在巴尔的摩，MD收集的24小时环境颗粒样品的分析。粗粒级样品（PM10-2.5）用聚碳酸酯过滤器上的二分采样器收集。在无人值守的8小时运行期间，CCSEM在78个随机选择的视野中共分析了2819个颗粒。存储颗粒直径、纵横比、颗粒位置、20个元素的X射线计数以及每个颗粒及其视野的数字图像。计算数据子集的每个视野的平均颗粒数（N/F）、平均粒径（D-ave）、每个视野的平均质量负载（M-ave）和平均颗粒组成，并与完整数据集的结果进行比较，以评估CCSEM分析随时间的稳定性和获得代表性结果所需的颗粒数。这些比较证明CCSEM在8小时运行中具有优异的稳定性。通过分析约360个颗粒，很好地表征了样品的物理性质（由N/F、D-ave和M-ave表示）。在分析了大约1000个颗粒之后，样品的化学性质（平均元素组成和主要化学类别丰度）收敛到其最终值的百分之几以内。然而，对于许多目的，几百个颗粒可以提供足够的表征。小类丰度的收敛受到统计波动的限制，因为一个类中的粒子数量变得非常小。CCSEM数据的手动审查识别了与CCSEM相关的错误，原因包括遗漏的颗粒、重叠颗粒、对比伪影、尺寸测量错误和不均匀颗粒。大多数错误可以在手动离线查看数据期间得到纠正或消除，或者通过在过滤器上保持适当的颗粒负载来避免。
Concerns about the environmental and public health effects of particulate matter (PM) have stimulated interest in analytical techniques capable of measuring the size and chemical composition of individual aerosol particles. Computer-controlled scanning electron microscopy (CCSEM) coupled with energy-dispersive X-ray analysis (EDX) allows automated analysis of particle size, chemistry, and particle classification. In combination with manual SEM and bulk analytical techniques such as X-ray fluorescence, CCSEM can be a valuable tool for characterizing individual ambient particles and determining sources of ambient PM, The goal of this study was to examine several issues related to the quality and validity of CCSEM data. These included the stability of unattended CCSEM for multihour runs, the number of particles that must be analyzed in order to yield representative results, and errors associated with CCSEM.CCSEM was applied to the analysis of a 24 h ambient particle sample collected in Baltimore, MD. The coarse-fraction sample (PM10-2.5) was collected with a dichotomous sampler on a polycarbonate filter. A total of 2819 particles in 78 randomly selected fields of view were analyzed by CCSEM during an unattended 8 h run. Particle diameter, aspect ratio, particle location, X-ray counts for 20 elements, and digital images of each particle and its field of view were stored. The average number of particles per field (N/F), average particle diameter (D-ave), average mass loading per field (M-ave), and average particle composition were calculated for subsets of the data and compared against results for the full data set in order to assess the stability of the CCSEM analysis over time and the number of particles needed to obtain representative results. These comparisons demonstrated excellent stability of CCSEM over the 8 h run. Physical properties (represented by N/F, D-ave, and M-ave) of the sample were well characterized by analyzing approximately 360 particles. Chemical properties of the sample (average elemental composition and major chemical class abundances) converged to within a few percent of their final values after analyzing about 1000 particles. However, for many purposes several hundred particles may provide adequate characterization. Convergence of minor class abundances was limited by statistical fluctuations as the number of particles populating a class became very small. Manual review of the CCSEM data identified errors associated with CCSEM due to missed particles, overlapping particles, contrast artifacts, sizing errors, and heterogeneous particles. Most errors could be corrected or eliminated during manual off-line review of the data or avoided by maintaining a proper particle loading on the filter.