9 Surface Microanalytical Techniques for the Chemical
Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 22, 2018 | https://pubs.acs.org Publication Date: March 13, 1979 | doi: 10.1021/bk-1979-0094.ch009
Characterization of Atmospheric Particulates R. W. LINTON Venable and Kenan Laboratories, University of North Carolina, Chapel Hill, NC 27514
Introduction A prominent example of the recent, large scale mobilization of chemicals in the environment is the introduction of pollutant particles into the atmosphere. Such pollution is the direct consequence of the acquisition and processing of raw materials required to sustain advanced technological societies (1). Many elements now show substantial enrichments over natural background levels in the atmosphere (2-6). Further, nearly all particles produced by anthropogenic sources contain higher specific concentrations (µg/g) of some trace elements than natural windborne particles including crustal dust, volcanic ash, and sea salt aerosol (1, 7-9). It is becoming more evident that evolution often does not provide effective homeostatic mechanisms that permit organisms to tolerate sudden exposures to chemicals that have been previously unavailable because of their low abundance or geochemical stability (10). In the case of atmospheric particles, a major scientific concern, therefore, is to assess the extent to which trace element enrichments may result in deleterious effects on health. The specific long-term environmental effects of increased trace element loading of the atmosphere continue to be difficult to assess. Specific areas of uncertainty requiring further investigation include the following: 1) the mechanisms of particle formation and dispersion in the environment, 2) the chemical transformations and reactivity of the particles in various environmental compartments, 3) the physicochemical characteristics of individual particles, and 4) the specific interactions of the particles with living organisms (11). Conventional studies generally involve the collection of an assemblage of airborne particles followed by determinations of the average or bulk concentrations of pollutant species present (12). However, the results often lack the analytical specificity required to identify particle sources, to determine particle speciation and reactivity, or to assess particle toxicity. An obvious limitation to the use of bulk analysis studies is the direct result of sample heterogeneity. Not only do aerosol samples show wide variability in the physico-chemical characteristics of different particles, but even a single airborne particle may be highly heterogeneous. With regard to the latter, the surface chemical composition of a particle may bear little resemblance to that of its interior (11-14).
0-8412-0480-2/79/47-094-137$05.75/0 © 1979 American Chemical Society Schuetzle; Monitoring Toxic Substances ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
MONITORING TOXIC SUBSTANCES
138
The surface composition of individual airborne particles is of particular importance for the following reasons: 1. A number of potentially toxic trace metal and organic species are highly enriched at the surfaces of many types of environmental particles (11-14).
Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 22, 2018 | https://pubs.acs.org Publication Date: March 13, 1979 | doi: 10.1021/bk-1979-0094.ch009
2. It is the surface of the particle that is directly accessible to extraction by aqueous leaching in the environment or by body fluids following inhalation or ingestion (11, 12, 14). 3. The particle surface may function as a catalytic site for heterogeneous reactions involving the generation or removal of gaseous pollutants [11, 15-17). The physical characteristics of individual particles also are of environmental significance. For example, the smaller particles (diameters on the order of 1 micrometer of less) generally are most important in that they have very long atmospheric residence times [18), are least effectively controlled by pollution control devices [19), are preferentially deposited in the pulmonary regions of the lung [20, 21), and may be most enriched in toxic species on a specific concentration (Mg/g) basis [22-24). The above considerations clearly point to the need for surface microanalytical techniques that allow for the direct determination of the physical and chemical composition of individual particles (11). The purpose of this paper, therefore, is to review modern analytical advances in this area.
Analytical Instrumentation Recent technological innovations have permitted the rapid emergence of an array of spectroscopies used for the determination of the composition and microstructure of the outermost atomic layers of a solid [25, 26). There are three major spectroscopic techniques that combine both microscopic and surface analysis capabilities, and that are beginning to find useful applications in the characterization of environmental microparticles. These are electron excited X-ray emission spectroscopy (electron microprobe, or scanning electron microscope equipped with an X-ray detector) [27, 28), Auger electron spectrometry (scanning Auger microprobe) (29) and secondary ion mass spectrometry (ion microprobe) (30). A l l three techniques have comparable lateral resolutions, i.e. features as small as about 1 micrometer in diameter may be characterized. However, the spectroscopies differ considerably with regard to surface sensitivity and specificity. The electron microprobe (EMP) and scanning Auger microprobe (SAM) respectively monitor the emitted X-rays and Auger electrons that result from electron bombardment-induced core ionization of the sample atom (Figure 1). The excess energy lost by an outer shell electron that fills the core vacancy can be emitted as either an X ray, or imparted to another outer shell electron (the Auger electron) causing its ejection from the atom (Figure 1). The energy of the emitted X-rays and Auger electrons are characteristic of the emitting elements. Since Auger electron emission predominates over the X-ray emission process for low atomic number elements, the Auger technique usually is more sensitive than the E M P for light elements (Li — Na). For heavier elements, the sensitivities of both techniques are roughly comparable and are on the order of 0.1% atomic percent within the analytical volume.
Schuetzle; Monitoring Toxic Substances ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 22, 2018 | https://pubs.acs.org Publication Date: March 13, 1979 | doi: 10.1021/bk-1979-0094.ch009
9.
LINTON
Surface Microanalytical
Techniques
4. PHOTON
I. PRIMARY ELECTRON
Figure 1.
Schematic demonstrating competitive auger electron and x-ray photon emission processes
Schuetzle; Monitoring Toxic Substances ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
139
Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 22, 2018 | https://pubs.acs.org Publication Date: March 13, 1979 | doi: 10.1021/bk-1979-0094.ch009
140
MONITORING TOXIC SUBSTANCES
Core vacancies resulting from electron bombardment are achieved at depths ranging up to micrometers into the sample. The X-rays subsequently produced are able to traverse this depth range and thus the depth resolution of the E M P is poor. Micrometer-sized analysis depths are often much greater than the immediate surface layer normally of interest in the case of environmental particles (11-14). On the other hand, the Auger electrons produced can travel only short distances in the solid without energy loss. Consequently, only the Auger electrons originating within a few atomic layers of the surface (generally
