Anal. Chem. 1087, 59, 163-167 (IO) Weber, S. G.; Purdy, W. C. Anal. Chlm. Acta 1978, 100, 531. (11) Morgan, D. M.; Weber, S. 0. Anal. Chem. 1984, 5 6 , 2560. (12) Anderson, J. L.; Maldoveanu, S. J . Electroenel. Chem. Interfaclel Electrochem. 1984,‘ 179. 107. (13) Karger, E.; Martin, M.; Gulochon, G. Anel. Chem. 1974, 46, 1640. (14) Snyder, L. R. Gas Chromatography, 1970; Stock, R., Ed.; The Institute of Petroleum: London, 1971. (15) Carreira. L. A. UGA Scienffflc Appllcat/ons Package; Department of Chemistry, University of Georgia: Athens, GA, 1965. (16) Chen, J. C.; Weber, S. G. Anal. Chem. 1983. 5 5 . 127. (17) Anderson, J. L.; Whiten, K. K.; Brewster,J. D.; Ou, T. Y.; Nonidez, W. K. Anal. Chem. 1985, 57, 1366. (18) Anderson. J. L.; Welsshaar. D. E.; Tallman, D. E. Anal. Chem. 1981, 5 3 . 906. (19) Foley, J. P.;Dorsey, J. 0. Anal. Chem. 1983, 5 5 , 730. (20) Snyder, L. R.; Kirkland,J. J. Introduction to Modem LlquM Chromatography, 2nd 4.;Wlley: New York, 1979; p 235. (21) Reece, C. 2.; Scott, R. P. W. J . Chromatogr. Sci. 1980, 18, 479.
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RECEIVED for review June 27,1986. Accepted September 3, 1986. This project was funded in part by the Office of Research and Development, U.S. Environmental Protection Agency, under Grant No. R-808084-02. Additional funding was provided by the University of Georgia. The Environmental Protection Agency does not necessarily endorse any commercial products used in this study. The conclusions represent the views of the authors and do not necessarily represent the opinions, policies, or recommendations of the Environmental Protection Agency.
Analysis of Eastern Coal Fly Ash Using Separated Sampling and Excitation Atomic Emission Spectrometry Paul M. Beckwith* and Richard L. Mullinm The Detroit Edison Company, 6100 West Warren Avenue, Detroit, Michigan 48210
David M. Coleman* Department of Chemistry, Wayne State University, Detroit, Michigan 48202
New methodology Is described, using separated sampllng and excitation atomic emission spectrometry, for the direct d e termination of elemental constituency of eastern coal Hy ash. Thls technique uses a controlled-waveformhighvoltage spark to “sample” coal fly ash mixed in a graphite pellet. No dissolution or other form of sample preparation is required. The resultant vapor is subsequently entrained into an inductively coupled plasma and observed spectroscopically. A twopoint calibration is employed: British Chemical Standards Firebrick (BCS 315) and a pure graphite pellet are used as the high and low standards, respectively. Resuits for eight elements (Fe, Ai, Ca, Mg, TI, Na, K, and Si), reported as oxides, show typical accuracy and precision of f5% relative standard deviation. Results for a variety of calculation methods, with and without internal standards and/or constituent normaiizatlon, are presented. Problems of volatilization in hlgh sodium ash (e.g., western fly ash) and possible solutions are discussed. Application to a variety of similar matrix samples is discussed.
The spectrographic analysis of coal fly ash for elemental composition has important ramifications in the electrical power industry. Such elemental determinations can be used to predict or explain problems encountered by operators of pulverized coal-fired boilers. The ash, depending upon its composition, can have profound effects on the operation of large scale utility boilers. For example, ashes with high iron content (>20% calculated as FezO3)can cause the collection of liquidized ash (slagging) on boiler radiant surfaces. Ashes with high sodium content (>3%) can cause the collection of dry ash on boiler convective surfaces and resultant corrosion of those surfaces. In addition, most pulverized coal-fired boilers use electrostatic precipitators to collect the ash as it leaves the boiler. The efficiency of these precipitators can 0003-2700/87/0359-0163$01.50/0
be significantly altered by the coal fly ash’s elemental composition. Further, possible uses of coal fly ash, e.g., as a pozzolana in concrete, require that an accurate elemental composition be established. Analysis of these materials by usual methodology involves time-consuming and error-prone dissolution. Analysis by atomic absorption spectrometry, or inductively coupled plasma (ICP) emission spectrometry, may take 5-10 h to complete, in large measure due to sample preparation steps. These techniques are generally targeted for the determination of trace elements. Caruso et al. (1) recently examined trace constituents in National Bdreau of Standards (NBS) Coal Fly Ash with direct insertion (milligram sample sizes) of the powder into a high-power (ca. 2 kW) inductively coupled plasma discharge. Wheeler and Jacobus (2) reported on the use of energy-dispersiveX-ray fluorescence to determine major and minor constituents of coal and of coal fly ash. For many years the Detroit Edison laboratory has used a classical dc arc emission method in which 0.050 g of sample is added to 1.000 g of spectrographic grade graphite. A fixed quantity of germanium (0.050 g) is added as an internal standard. The powders are carefully mixed and then placed in a hollow carbon cup cathode for arcing. While analytical results have generally been acceptable the difficulties of photographic detection, resultant time-consuming densitometry, interferences due to volatilization, and instability of the dc arc make this method less viable than it once was. The methodology and theory of separating the sampling process from the excitation process in atomic emission spectrometry have been studied extensively by one of the authors (e.g., ref 3 and 4). Watters, at NBS (5), reported one of the first instruments wherein a high-voltage spark discharge was used to sample an electrode surface; the resultant aerosol was carried through Tygon tubing and introduced into a conventional inductively coupled plasma discharge. Significant improvements in detection limits and concentration linearity were noted. Detection limits for the analysis of 1-g steel 0 1986 American Chemical Society
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Table 1. Spectral Lines element
wavelength, nm
element
wavelength, nm
Ca 11
M EI samples, dissolved in 100 mL, were comparable to ICP d t a Beaty et al. (6) refined this technique and subsequently introduced a commercial instrument based on their concepts in 1982. Principles of this SSEA instrument have been published (7).Research reported here usen a Jarrell-hh (now Thermo Jarrell-Ash) SSEA instrument. In this paper a method is discussed wherein a small quantity of eastem coal fly ash is included in a premed graphite pellet, which is sampled by a high-voltage spark. The resultant material is transported to and excited by a n ICP discharge. Intensities are correlated to analytical composition. A dir e r e a d i n g spectrometer allows a large number of elements to be determined simultaneously. Analytical figures of merit. comparative techniques, and problems associated with high sodium samples are discussed. EXPERIMENTAL SECTION Instrument. The instrument is a Jarrell-Ash MK 111SSEA spectrometer system with a 2400 linea/mm parabolic gratii. The entrance slit was set at 25 pm. All exit slits were 50 pm except for the Na and K channels, which were 100 pm. The mrresponding Jarrell-Ash ICP generator was operated with 1.1-kW forward power and lesa than 10-W refleeted power. The plasma was viewed 15 mm above the load mil. A standard quartz torch was used. Argon flow rates of 20,0.65. and 0.5 L/min were used for the outer, carrier, and intermediate flows, respectively. The controlledwaveform spark source was adjusted to give the current waveforms recorded in Figure 1. Argon support gas (1.0 L/min) was used. The focal curve exit slits of the direct-reading spectrometer are all 50 pm with the exception of the Na and K channels, which are 100 pm. The spectrometer is configured for vacuum operation. The spectral lines used are shown in Table 1. Since this instrument is used in several different modes for study of diverse sample types, the focal curve contains 48 exit slits and photomultipliers. Only exit slits corresponding to elements of interest are interrogated during these studies. The instrument contains an external *N + 1" airpath monochromator for single-channel investigation of emission lines that are not preconfigured on the spectrometer focal curve. Standards and Samples. The preparation of standards and unknown samples was identical. For most work BCS 315 (British Chemical Standards Firebrick) was employed as the high-concentration standard. Spectral grade graphite (Ultra Carbon, 325 mesh) was used directly, without further grinding, as a diluent in making of pellets. Pellets (0.5in. diameter) were prepared by mixing 20.0% sample and 80.0% graphite, by weight, to give a pellet that weighs approximately 1.0 g. A pure graphite pellet was used for the low-concentration standard. No further grinding of samples was required. AU samplea and reference materials have particle diameters less than 40 pm. A detailed study of particle size effects is in progress. The pellet components were mixed for 10 min in a Spex 8ooo Mixer/Mill (Spex Industries, Metuchen, NJ) and then presaed into a pellet in a stainless-steeldie at 2oooO psi. Since the spark only minimally penetrates the pellet, standards or samples may be reburned by abrading the surface of the pellet against clean tissue paper. It is essential that all standards, samples, and unknown powders be briefly dried at 110 "C and then stored in a desiaator. Without drying it was often difficult to prepare homogeneous pellets of -me samples. Precision and accuracy suffered dramatically in such cases. The sample pellet is positioned in a split holder fabricated from a 1.0-in.-diameter X 1.0-in. brass cylinder. This cylinder is machined to hold the pellet flush with the spark stand. Maintenance of a fixed interelectrode gap spacing (2.5 nun) is essential for good accuracy and precision. Details of the pellet sample holder will
4 Breaks 2 Breaks Flgua 1. Effect of spark parameters on the sampling of NBS coal fly ash mixed In graphite pellets. The pellets are 0.50 in. diameter. F a waveforms A and C the ordinate = 0.5 msldilsiOn (dlv) and the abscissa = 20 Aldlv. For waveforms B and D the wdinate = 20 psldii. For the waveform D inset the cfdlnate = 10 wldiv and the abscissa = 10 Aldlv. Current measurements were made with a Pearson Electronics (Pal0 Aiio. CA) Type 110 Rogowskl coil current transformer.
be provided upon request. Several *burns" are achieved from each pellet surface by careful positioning of the pellet relative to the counter electrode. The polflow tubing between the spark stand and the ICP discharge is periodically cleaned with "pipe cleaners" and pressurized argon. Memory effects, with samples of similar matrix, are minimal. Spark sampling conditions used for these studies are shown in Figure 1. We routinely monitor the specific nature of the current waveform as an important diagnostic tool. Changes in the peak current, Coulombic integral, or the presence of current reversals manifest themselves in terms of decreased analytical accuracy and/or precision. Such errors are subtle and hard to define without knowledge of the current waveform. Initial work at 'two breaks/half-cycle" resulted in poor accuracy and precision. These sparks, with peak currents of 90 A and durations of 120 ps, contributed to rather violent sampling. An excess of large particulate formation, as a consequence of this sampling condition. was noted. Quantitative results were not possible. Results reported here were obtained with the higher repetition rate, with lower peak current sparks indicated in insets C and D. As shown, sampling in the case of two breaks is spatially diffuse and irreproducible. In the four-break situation much *gentler" sampling conditions are noted with increased positional stability with respect to the sampled eledrode surface and the generation of finer particulates. RESULTS A N D DISCUSSION An important consideration for fly ash samples imbedded in graphitic matrices is volatilization or distillation characteristica of the enmple bum. This is shown in Figure 2 for four elements in NBS SRM 1633a Fly Ash. These data indicate that a "preburn" period of approximately 30 s is required before steady-state ablation of the sample is achieved. Following this preburn a spectroscopic integration period of 40 s is taken. I t is interesting to note that distillation characteristics of this sampling process are not unlike those encountered in classic dc arc spectroehemical analysis. All intensities are normalized to 100% in Figure 2. Low-Sodium Materials. Results for the analysis of NBS SRM 1633a and BCS (British Chemical Standards) 309 (Sillimanite), using a variety of calculation methods, are shown in Table 11. The instrument was calibrated vs. BCS 315 for both determinations. Three replicate determinations were averaged for both standardization and analysis of the
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
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Table 111. Comparison of Calculation Methods
std
compd
actual”
Ib
11‘
IIId
NBS 1633e
A1203 CaO Fe203 MgO KZO SiOz NazO Ti02 A1203 CaO
28.0 1.64 14.1 0.79 2.38 51.5 0.26 1.39 61.1 0.22 1.51 0.17 0.46 34.1 0.34 1.92
28.6 2.10 14.1 0.88 0.0 52.3 0.52 1.54 59.8 0.22 1.61 0.0 0.60 34.8 0.71 2.32
31.3 2.19 15.6 1.17 0.0 58.6 0.74 1.59 69.7 0.26 1.89 0.0 0.51 41.8 0.94 3.37
28.2 2.02 13.9 0.80 2.06 51.3 0.29 1.45 60.4 0.23 1.48 0.16 0.53 34.2 0.39 2.59
BCS 309 0
20
40
60
80
100
120
Figure 2. Volatlllzatlon characteristics (time plot) of NBS 1633a Fly Ash in graphite pellets. This Is typical for eastern fly ash samples: (A) Na, ( 0 )AI, (0)Ca, and (0)Si. The Intensity of each element shown is normalized to 100 % .
Table 11. Analysis of Selected Standards
std BCS 309
compd
actualn
foundb
ES
Fe2OB
1.51 61.1 0.22 0.17 1.92 0.34 0.46 34.1 14.1 28.0 1.64 0.79 1.39 0.26 2.38 51.5 0.53 0.77 1.81 0.05 0.19 0.05 0.09 96.2 3.01 42.4 0.34 0.57 1.23 0.13 0.52 51.2
1.53 f 0.08 59.7 f 0.10 0.24 f 0.004 0.17 & 0.004 2.40 f 0.28 0.39 f 0.005 0.48 f 0.01 35.1 f 0.32 14.1 f 0.9 25.6 f 0.3 1.96 f 0.05 0.74 f 0.01 1.47 f 0.02 0.36 f 0.004 2.43 f 0.03 52.2 f 0.5 0.57 1.26 2.65
