Determination of the Surface Predominance of Toxic Elements in Airborne Particles by Ion Microprobe Mass Spectrometry and Auger Electron Spectrometry R. W. Llnton,' Peter Williams, and C. A. Evans, Jr.' School of Chemical Sciences and Materials Research Laboratoty, University of Illinois at Urbana-Champaign, Urbana, Iiiinois 6 180 1
D. F. S. Natusch Department of Chemlstw, Colorado State University, Fort Coliins, Colorado 80523
Surface analytlcaltechnlques lncludlng Ion microprobe mass spectrometry and Auger electron spectrometry have been used to demonstrate elemental surface predominance In coal fly ash partlcles. Substantlatlon and quantltation of the ion microprobe data were achieved by uslng a multltechnique approach including solvent leachlng and bulk multlelemental analysis uslng spark source mass spectrometry. Elemental surface Predominance in coal fly ash Is explained in terms of a volatilization-condensation mechanism. The substantial leachabilities and strongly enhanced surface region concentrations of numerous elements, Including potentlally toxic ones such as Pb, TI, Mn, and Cr, lndlcate that coal fly ash may have a more deleterious enviromntal hrpact than is apparent solely on the basis of conventional bulk analysis.
The surface predominance of potentially toxic elements has been demonstrated recently in coal fly ash particles emitted to the atmosphere ( I ) . Preliminary studies also indicate that elemental surface predominance may be a general phenomenon occurring in airborne particles derived from high temperature combustion operations ( I , 2 ) . These findings are of significance in part because they provide direct confirmation of previous predictions ( 3 , 4 )that surface predominance should occur. The phenomenon is attributed to the condensation of species previously volatilized in the high temperature combustion zone of a particulate emission source (3, 4). The existence of elemental surface predominance is also important in demonstrating that bulk analysis techniques, which give only average concentration values, provide little insight into the actual chemical nature of the particles. For example, results obtained solely on the basis of bulk analysis markedly underestimate the potential environmental impact of airborne particles derived from high temperature combustion operations in that (1) surface regions of the particles have enhanced concentrations of elements, including some which are potentially toxic, and (2) the smallest particles will have much higher bulk (Fg/g) concentrations of such elements because of larger surface area to volume ratios (3). The first factor is of environmental significance because it is the particle surface which comes in contact with, and is extracted by, body fluids upon ingestion or inhalation; which is subjected to aqueous extraction in the natural environment; and which takes part in the catalysis of heterogeneous atmospheric reactions such as the oxidation of SOz to sulfate (5, 6). Enhanced bulk elemental concentrations in the smallest Present address,Department of Chemistry, University of North Carolina, Chapel Hill, N.C. 27514. 1514
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
particles are also highly undesirable from an environmental standpoint because it is the small particles (less than about 5 pm in aerodynamic diameter) which most readily elude particle collection devices (7), which have long atmospheric lifetimes (8),and which penetrate the innermost regions of the lung when inhaled (9). On the basis of the above discussion, it is apparent that there is a need for analytical techniques capable of surface and in-depth elemental characterization of individual particles. Several approaches to obtain this analytical information comprise the subject matter of this paper. Principal methods employed include the surface analytical techniques of secondary ion mass spectrometry (SIMS) and Auger electron spectrometry (AES) as practiced for microanalysis, i.e., ion microprobe mass spectrometry and scanning Auger microsCOPY. This paper also illustrates the benefits of a multitechnique approach, not only by the application of several complementary surface microanalytical techniques, but also by the use of solvent leaching in conjunction with both surface and bulk (spark source mass spectrometry) analyses. Specifically, the combination of leaching, bulk, and surface techniques demonstrates that the observed elemental surface predominance is not the consequence of artifacts, especially those which may result from the use of ion sputtering to obtain depth profiles. It also permits semi-quantitation of elemental concentrations in the surface region. In the case of coal fly ash, the information obtained by this approach provides substantial insight into the physicochemical characteristics of fly ash surfaces and permits assessment of its potential environmental impact. EXPERIMENTAL Sample Selection. The fly ashes studied were emitted by coal fired power generating plants, one utilizing chain grate stoking and the other pulverized coal feed, and both burning midwestern US. bituminous coal. Samples were collected in the stack systems at temperatures in the range of 200-300 "C. Ash from the chain grate stoked plant was studied most extensively because it had generally higher trace element bulk concentrations, facilitating detection by the various analytical techniques. Large particles (45-180 pm physical diameter) were analyzed to facilitate both particle handling and the characterization of individual particles using surface microanalytical techniques. Analytical Procedures and Instrumentation. Solvent Leaching and Bulk Analysis. Solvent leaching of the fly ash was performed using Fisher reagent grade dimethyl sulfoxide (DMSO) and triply distilled water. DMSO was chosen after inorganic species were found in DMSO extracts during studies investigating the extraction of trace organics, while H20 was employed to simulate leaching in the natural environment. Leaching with DMSO was carried out for 48 h in a Soxhlet extractor at a temperature of 40 "C and a pressure of -0.1 mm Hg. A DMSO blank was prepared by leaching of an empty Soxhlet thimble using
identical experimental conditions. Leaching with HzO was performed by placing fly ash in a glass jar containing triplydistilled water for a 12-h period. The jar was also placed in a DISONtegrator Model 320 and sonicated for 1h in a water bath in order to maximize interaction of the solvent with fly ash surfaces. The leachate solution was then filtered twice through Whatman No. 41 filter paper, as was an HzO solution serving as the blank. The aqueous leaching procedure was used primarily to indicate relative elemental leachabilities under the conditions employed, and thus it may not solubilize the entire available fraction of most elements (10). To assess the extent of elemental leachability from fly ash, bulk analyses of particles and solution leachates were performed with an AEI MS-7 spark source mass spectrometer (SSMS)employing photographic detection. Unleached and leached particles were crushed to increase homogeneity, and 50 mg of the crushed fly ash was then mixed with 200 mg of spectroscopic graphite containing 5 mg of MG-1 Spex Spikes. Leachate solutions were prepared for analysis by doping 250 mg of spectroscopic graphite containing 5 mg of the Spex Spikes with 1-2 mL of leachate using 2 mL acetone-free methanol as a wetting agent. Complete evaporation of the solvents was carried out at a low temperature (50 "C). The leachate and particle samples were homogenized by use of a mechanical shaker and subsequently pressed into electrodes. All electrodes were sparked using a 25-ps spark duration and a repetition rate of 300 8. Of the six possible internal standards in the Spex Spikes, 95Mo and 1 5 3 Ewere ~ least subject to natural abundance interferences from the fly ash and thus were chosen as the internal standards. A series of graded photographic exposures was obtained for each sample. Subsequently, ion intensities for the internal standards and elements of interest were determined by densitometry and expressed as Seidel numbers (11). Elemental concentrations in the electrode material were calculated by use of the following equation:
where u, s = analyte and internal standard quantities, respectively; S = Seidel number; m = atomic mass; 4 = isotopic abundance; and C = concentration. The terms containing m are empirical correctionsfor the variation in photoplate sensitivitywith atomic mass. C, in the above equation was also multiplied by the appropriate factor to convert from concentration in the electrode to concentration in the original particle or solution sample. Elemental Depth Profiles. The variation of elemental concentrations as a function of depth for both leached and unleached fly ash particles was determined by ion microprobe mass spectrometry and Auger electron spectrometry combined with ion etching. The procedure for particle mounting involved placing large particles (45-180 pm) in a folded strip of indium foil and hand pressing to imbed them into the foil. This mounting technique has the advantages that indium is electrically conducting, presents few spectral interferences, is soft and malleable allowing imbedding of particles without physical alteration, has a low vapor pressure for ultra high vacuum compatibility and is inexpensive (12). An AEI Model IM-20 ion microprobe was used to obtain elemental depth profiles. Basic features of the instrumentation have been described previously (13). A 25-keV negative oxygen primary beam of 40 nA and 20-rm diameter was rastered rapidly over an area of 100 wm by 100 wm. The rastering procedure was important primarily because it enabled fairly uniform current densities to be maintained over the entire area of the particle being analyzed. Mass spectrometer resolving powers used ranged from 250 to 1800 (10% valley definition) 89 required to resolve molecular ion interferences. Depth profiles were acquired by two methods: (1)continuously monitoring the positive secondary ion intensity of one element vs. time using electrical detection, and (2) use of photographic detection of the positive secondary ions aa dispersed by the double-focusing Mattuch-Herzog analyzer. Results re-
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Figure 1. Ion microprobe depth profiles of the Group A elements (TI, AI, Si) obtained for unleached and leached fly ash samples. All secondary ion intensitieswere normalized to Si (28S~tOT 30Si+)measwed at a depth of 1000 A
-
ported herein were obtained only by electricaldetection since the use of photographic detection was limited by a lack of sensitivity. In order to convert sputtering time to depth, sputtering rates for SiOz were measured under identical primary ion beam conditions as those used for the analysis of fly ash. The time required to sputter a 3000 A SiOzlayer on Si was determined by monitoring the Si intensity and corresponded to a sputtering rate of 4 A (A0.5 A) per second. Elemental depth profiles were also obtained using a Physical Electronics Industries Model 545 Scanning Auger Microprobe. The indium foil containing the imbedded particles was mounted on the standard carrousel at 30" grazing incidence to a 5-keV, 50-wm primary electron beam. Elemental depth profiles were obtained by recording entire Auger spectra after incremental periods of sputtering with 2-keV Art. Sputtering rate calibrations were achieved in an analagous manner to those for the ion microprobe depth profiles. RESULTS AND DISCUSSION Ion Microprobe Analysis. On the basis of its inherently high sensitivity, good depth resolution, and ability to perform microanalysis of samples (including those which are insulators), ion microprobe mass spectrometry was a priori one of the most useful surface analytical techniques for the in-depth characterization of airborne particles. Specific analytical concerns or objectives involved in the application of the ion microprobe to obtain elemental depth profiles of individual fly ash particles were the following: (1) to establish which elements of environmental interest could be characterized, (2) to analyze particles before and after solvent leaching to reduce concerns about possible sputtering artifacts, (3) to use solvent leaching and bulk analysis to substantiate and to quantitate the ion rnicroprobe results, i.e., to estimate surface region elemental concentrations, and (4) to examine general constraints on the analysis such as limits to depth resolution or possible effects due to charging. Experimental results with regard to the above objectives will be summarized in the following paragraphs. Ion microprobe depth profiles for 15 elements in fly ash are shown in Figures 1-6. These elements include all of the major and minor elements in fly ash (greater than about 1%by weight) as well as six trace elements with concentrations below 1000 ppm as determined by bulk analysis. Other elements ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
1515
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Flgure 2. Ion microprobe depth profiles of the Group B elements (Fe, S) obtalned for unleached and leached fly ash samples. All secondary ion intensities were normalized to Si (''Si' or 3oSl+)measured at a depth of NIOOO A 320
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Flgure 4. Ion mlcroprobe depth profiles of the Group C elements (Pb, TI) obtained for unleached and leached fly ash samples. All secondary ion intensities were normalized to Si (**Si+ or %I+) measured at a depth of -1000 A
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Figure 3. Ion microprobe depth profiles of the Group B elements (K, Na, Li) obtained for unleached and leached fly ash samples. All secondaty ion intensitbs were normalized to Si (2sSi+or %i+) measured at a depth of N 1000 A
Flgure 5. Ion microprobe depth profiles of the Group C elements (Cr, Mn, V) obtalned for unleached and leached fly ash samples. All secondary ion intensities were normalized to Si (*'Si+ or 30Si+)measured at a depth of -1000 A
of possible surface predominance and toxicity (3,141 which could not be characterized were Co, As, Ni, Zn, Se, Cd, Hg, and Sb. These elements were present at low bulk concentrations ranging from approximately 10-1000 ppm by weight. Secondary ion mass spectral interferences from species such as molecular ions, hydrocarbons, and isotopes of major elements were common in the fly ash mass spectra and generally required the use of high mass resolution (13, 15) with an accompanying loss in sensitivity. The above eight elements were not of sufficient intensity to be characterized. Elemental identifications were made only when multi-isotopic species exhibited correct isotope ratios or when monoisotopic species could be mass-resolved from interferences and identified by
mass difference measurements. Most of the elements listed above also have intrinsically poor detection limits since they have relatively low positive secondary ion yields (16). The major potential constraints on the depth resolution of the ion microprobe elemental depth profiles of fly ash (Figures 1-6) are variations in the sputtering rate over the area of the particle being analyzed (16),crater edge effects (17,18),escape depths of secondary ions (19), and cascade mixing of subsurface layers resulting from penetration of the primary ion beam into the sample (20,21). For purposes of sputtering rate calibration, an SiOz standard was used to approximate the composition of the glassy aluminosilicatefly ash matrix. The major limitation in applying this calibrated rate to fly ash was
1516
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
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