Anal. Chem. 1988, 60, 2169-2171
Table I. Collection Efficiency for a Triple Band Electrode with a Central Generator Electrode and Two Adjacent Collector Electrodes"
[TBAPF,]/mM
FeCp,
200 100 10 1.0 0.1 0.02
0.83 0.82
cocp,+ 0.80 0.81 0.79 0.81
0.83 0.83 0.83
0.76
Electrode widths are 4.6 pm; separators are 4 wm. Scan rate is 0.01 V s-l. Current measurements were made at the switching potential, 0.8 V for FeCp, and -1.3 V for CoCpz+. Electroactive species are 1.0 mM in CH3CN. a
L."
,
i
3
0
2169
because the current is distributed over the length of the electrode. As indicated in Table I, the collection efficiency for the reduction of CoCp,+ was also examined. Little change is found in the collection efficiencies as a function of ionic strength. Examination of the experimental data shows considerable differences in the amplitude of the individual currents however (Figure 3). As the concentration of supporting electrolyte is lowered to values approaching that of CoCp2+,an increased flux occurs as a result of mass transport by migration in addition to diffusion (7). The current at the generator acting alone, and also when coupled with the two collectors, increases with a decrease in supporting electrolyte concentration. The relative current a t the generator electrode operated with feedback, corrected for changes in the diffusion coefficient (7), agrees well with that predicted for migration in the absence of feedback (Figure 4). The results for both CoCp2+reduction and FeCp, oxidation indicate that mass transport to the generator electrode determines the overall response of the triple band when operated in a collector-generator mode. While this is an empirical result, the predictable nature of the relative generator current makes the triple band electrode an ideal tool for investigations in media with low electrolyte. Registry No. Pt, 7440-06-4; Mylar, 25038-59-9. LITERATURE CITED
Figure 4. Relative current at the central generator electrode with adjacent collectors, as a function of log 7,the ratio of the concentration of the Supporting electrolyte to the concentration of electroactive species. The relative current is defined as the ratio of the measured generator current to the current expected based on diffusion mass transport only. Key: open circles, reduction of CoCp,'; closed circles, oxidation of FeCp,; solid and dashed lines, effect of y on the relative current as predicted by equations in ref 7 for a single-band electrode for one-electron reduction of a monocation and oxidation of a neutral, respectively.
ref 4. It should be noted, however, that the absolute magnitude of the generator and collector currents are not independent of the ionic strength. The electrode currents increase as the electrolyte concentration is decreased due to larger diffusion coefficients at low ionic strength (7). Voltammetry at very low ionic strengths is not distorted by i R drop (11,12)
(1) Wightman, R. M. Anal. Chem. 1981, 53, 1125A, and references therein. (2) Coen, S.;Cope, D. K.; Tallman, D. E. J . Electroanal. Chem. 1986, 275, 29. (3) Deakin, M. R.; Wightman, M. R.; Amatore, C. A. J . Electroansl. Chem. 1986, 215, 49. (4) Bard, A. J.; Crayston, J. A.; Kittlesen, G. P.; Varco Shea, T.; Wrighton, M. S.Anal. Chem. 1968, 5 8 , 2321. (5) Varco Shea, T.; Bard, A. J. Anal. Chem. 1987, 5 9 , 2101. (6) Chidsey, C. E.; Feldman, B. J.; Lundgren, C.; Murray, R. W. Anal. Chem. 1986, 5 8 , 601. (7) Amatore, C. A.; Deakin, M. R.; Wightman, R. M. J . Electroanal. Chem. 1987, 225, 49. (8) Geiger, W. E.; Smith, D. E. J . Nechoanal. Cbem. 1974, 5 0 , 31. (9) Bard, A. J.; Faulkner, L. R. Nectrocbemlcal Methods; Wiley: New York, 1980; pp 566-567. (IO) Sanderson, D. G.; Anderson, L. B. Anal. Chem. 1985, 57, 2388. (11) Bond, A. M.; Henderson, T.L. E.; Thormann, W. J . Phys. Chem. 1986, 9 0 , 2911. (12) Thormann, W.; van den Bosch, P.; Bond, A. M. Anal. Cbem. 1985, 5 7 , 2764.
RECEIVED for review December 9, 1986. Resubmitted December 22, 1987. Accepted June l, 1988. This research was supported by NSF (CHE-85-00529). Partial support was also provided by NATO.
Improved Ultrasonic Extraction Recovery of Benzo[ a Ipyrene from Stack Ash Using High PowerIMass Ratios W. H. Griest* and B. A. Tomkins Analytical Chemistry Division, Oak Ridge National Laboratory, P.O. Box 2008, Building 4500-S, Oak Ridge, Tennessee 37831 -6120
J. R. Caffrey Analytical Chemistry Department, Oak Ridge Gaseous Diffusion Plant, P.O. Box 2003, Oak Ridge, Tennessee 37831
The detailed characterization of particulate organic matter usually requires an initial solvent extraction, which ideally removes the desired analytes quantitatively from the matrix. In practive, however, the efficiency of extraction depends strongly not only upon the nature of the inorganic matrix but also upon the organic compound class in question. As a specific example, aliphatic hydrocarbons may be extracted
almost quantitatively at the nanogram level from coal combustion stack ash by using ultrasonic extraction methods, while the same level of polycyclic aromatic hydrocarbons (PAHs) is recovered incompletely. The recovery of a selected PAH is inversely related to the number of rings present and is frequently less than 25% for multiring carcinogenic species such as benzo[a]pyrene (BaP) (1-3). These difficulties can
0003-2700/8S/0360-2169$01.50/00 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 19, OCTOBER 1, 1988
lead to inaccurate evaluations of the potential health and environmental risks from coal combustion particulate matter. The difficulties in recovering large-ring PAHs from particulate matter has led to the development of many different extraction approaches, including ultrasonic and Soxhlet extraction using conventional (3) or high-boiling solvents ( 4 ) , vacuum sublimation (5),and supercritical fluid extraction (6). A common feature of many studies, however, is the extraction of gram to multigram quantities of electrostatic precipitator (ESP) hopper ash, which is readily available in kilogram quantities. This study was initiated after early experiences in a program for the chemical and biological characterization of coal combustion plume fly ash indicated (7) that only 100-250 mg of ash could be properly collected with great effort from a plume by using the current state of the art in airborne sampling technology. This paper reports the results of extraction studies designed for milligram quantities of ash. The findings indicate that the ultrasonic solvent extraction recovery of large-ring PAHs such as BaP can be significantly improved by simply increasing the power/mass ratio. EXPERIMENTAL SECTION Materials. The stack ash sample used in this study was kindly provided by Dr. Ralph Mitchell of the Battelle Columbus Laboratory (Columbus, OH). It was collected from the post-ESP ductwork aerosol at a 600-MW, pulverized-cod-fired power plant (8). The 7,10-carbon-14-labeledBaP (14C-BaP),29.7 mCi/mmol specific activity, was purchased from the Amersham Corp. (Arlington Heights, IL) and was diluted to an activity of 2.1 X lo6 dpm/mL in toluene. Distilled in glass grade solvents were obtained from Burdick and Jackson Laboratories, Inc. (Muskegon, MI). Instrumentation. All ultrasonic extractions were performed with a 185-W Branson sonifier (Branson Instruments, Inc., Stamford, CT) equipped with a 1.3 cm 0.d. titanium probe tip. Methods. A 0-2.2-pm mass median aerodynamic diameter particle size fraction of the ash was generated by sieving and centrifugation/air elutriation, as described in detail elsewhere (9). Samples of both the bulk ash and this fine particle fraction were slurry-spiked with I4C-BaPat a specific activity of ca. 2 X lo5 dpm/g (0.76 pg of BaP/g of ash) ( I ) . Briefly, dried samples of the ash slurried in methylene chloride were mixed with the radiotracer in toluene, and then the mixture was ultrasonicated for 1 min to disperse the radiotracer uniformly throughout the ash. The solvent was gradually removed under a stream of dry nitrogen accompanied by gentle heating and periodic stirring. The spiked ashes were stored in the dark at room temperature in a dessicator over Drierite. Ultrasonic extractions were conducted by placing a weighed mass of spiked ash in a cylindrical 30-mL fine-porosity sintered glass funnel, adding 10 mL of toluene, and extracting for 2.5 min at various power levels (see below). The extraction solvent was removed by vacuum filtration, and the extraction procedure was repeated. Each experiment was performed in duplicate. Three series of experiments were performed as follows: PowerlMass Ratio Experiments with Mass of Ash Varied. Four sequential extractions at 92.5 W were performed on each of seven different masses of ash (see Figure 1). PowerlMass Ratio Experiments with Power Level Varied. Aliquots of ca. 250 mg of ash were extracted at three different power levels, viz. 74, 85, and 100 W (see text, below). Extended Extraction Study. Seven extractions were performed at 74 W using 149-230 mg of ash (see Table I). Radiotracer extraction recoveries were determined by subjecting the filtered toluene extractions to standard liquid scintillation measurements ( I ) . The temperature of the ash/solvent slurries was estimated by placing a glass thermometer with a 1-cm immersion depth (Eimer & Amend, New York, Model no. 4719) into the extraction funnel immediately after the first ultrasonic extraction was completed. RESULTS AND DISCUSSION
In work of this kind, any conclusions drawn must be tempered with an understanding of the assumptions of the basic
100
I
1
I
I
- L 8
’
l
I
l
I
1
i
80
>
11: W
> 60
W
11:
z
2 + 40
‘0 0
200 “
600I
400
800
4000 I
P O W E R / M A S S R A T I O (W/g)
Figure 1. I4C-BaP extraction recoveries from of the powerlmass ratio.
stack ash as a function
Table I. Extraction Recoveries and Distribution Coefficients for the Ultrasonic Solvent Extraction of l4C-BaPfrom Stack Ash No. 504 and Its 0-2.2-pm Particle Size Fraction
step
trial 1 extraction recovery, 70
Kd’
trial 2 extraction recovery, 70
0-2.2-1m Size Fraction 77.3 56.5
1 2 3 4 5 6 7
9.8 2.7
1.3
1.0 0.7 0.6
total mg ash
12.6 4.4 2.4 2.1
1.6 1.5
72.7 11 3.1 2.1
1.3 0.8 0.6
Kda 39.7 10.1 3.5 2.8 2.0 1.3 1.1
91.5
93.9
166
149 Bulk Ash
1
61.8
2 3 4 5 6 7
11.5 4.0
total mg ash
29.8 7.9
59
13.1
3.2
4.3
2.4 2.1 1.2
2.2 2.1
2.8
1.0 83.9 184
1.1
1.3
1.9 1.4 1.2
33.1 7.3 3.5 2.7 2.1 1.7 1.6
83.6 230
“Kd = (ng of BaP/mL of solvent)/(ng of BaP/mg of ash).
experimental design. Firstly, the compound BaP was selected as a marker not only because it is a known carcinogen and potent health hazard but also because it is very difficult to extract efficiently from coal combustion stack ash. Our previous work (IO), which also described ultrasonic extractions, reported a recovery of only 25%. BaP should model the behavior of other multiring aromatic hydrocarbons that are of health and environmental concern. Secondly, the slurryspiking procedure clearly does not deposit organic species on stack ash in the same manner as vapor-phase sorption and condensation in the stack and plume of a coal-fired power plant. The procedure does, however, deposit them reproducibly in a manner in which the spike concentration is known precisely ( I ) . This situation is contrasted to vapor-phase deposition, in which the manner of compound deposition is much closer to that occurring in the stack and plume of power generating plants, but the exact mass of organic material deposited cannot be determined exactly (1I) unless a special apparatus is used (12). Also, spike levels of tens or hundreds of parts per million (ppm) from vapor-phase deposition are unrealistic ( I O ) . On the balance, we decided to add known
ANALYTICAL CHEMISTRY, VOL. 60, NO. 19, OCTOBER 1, 1988
sub-part-per-millionconcentrations of radiolabeled BaP tracer
to the ash via slurry-spiking, but to recognize that the spiking procedure might not be completely similar to real world samples. Thirdly, the expermental design assumed tacitly that the tracer would bind to the stack ash in the same or similar manner as plume fly ash from a coal-fired power generating plant. The ultrasonic solvent extraction efficiency for recovering large ring system PAHs from stack ash is clearly affected by the ratio of applied ultrasonic power to the mass of ash extracted. Figure 1 shows the recoveries of 14C-BaPfrom ultrasonic extractions performed on bulk unfractionated samples of the ash. Only the mass of ash was varied in these extractions to provide power/mass ratios ranging over a factor of 40. The extraction efficiency, expressed as the cumulative recovery of four sequential extractions at each power/mass ratio, increased from 35% to 74% as the power/mass ratio was increased from 23 to 925 W/g. This observation explains why low extraction recoveries were obtained in previous studies of stack and ESP hopper ash (1,2,10). In one of these studies, 3 g of ash was ultrasonically extracted at a power/mass ratio of 117, yielding a recovery of ca. 25% for 14C-BaP. This datum is in reasonable agreement with Figure 1. Figure 1exhibits nonlinear extraction behavior. The extraction efficiency slowly approaches ca. 75%, starting from 400 W/g, but changes linearly at power/mass ratios below 400. These observations suggest that more than one extraction mechanism and/or type of sorption site is present and that a power/mass ratio of 400 may correspond to a binding energy of BaP to a particular type of sorption site on the ash. If the mass of ash is kept constant, and the power/mass ratio is increased by merely raising the applied power level, the same dependence of recovery upon the power/mass ratio is observed, confirming the observations of the previous experiment. In tests with 250 mg of ash (the maximum currently expected from airborne plume sampling), the recoveries were determined a t applied levels of 74 W (291 W/g), 85 W (340 W/g), and 100 W (400 W/g). The 296 W/g is the minimum power the ultrasonicator can deliver to this system, while the 400 W/g is the maximum power that can be applied to this mass and volume of ash slurry without violent splashing and boiling. The corresponding recoveries of 49, 52, and 65%, respectively, reasonably fit the observations plotted in Figure 1, in which the mass of ash was varied instead of the power. The temperature of the system rose with the increased applied power (32, 51, and 80 “C, respectively). Undoubtedly, the higher extraction temperatures coupled with increased levels of agitation and cavitation improved recoveries at the higher power/mass ratios. For this reason, high-boiling solvents such as toluene would be preferred to volatile solvents such as methylene chloride. Performing the ultrasonic extractions in a heated bath should improve extraction recoveries. An extended number of extractions were performed upon samples of the bulk unfractionated ash and upon its 0-2.2-pm mass median aerodynamic diameter particle size fraction. The latter was generated to model respirable PAH-containing suspended atmospheric particles (13). The extraction recoveries and liquid/solid distribution coefficients, &, for each extraction step are listed in Table I. The extraction recoveries decrease dramatically with each sequential extraction and appear to approach a limiting value. This behavior does not indicate extraction by a simple partition process because the distribution coefficients continually decrease to values characteristic of a particle class subfraction, viz. carbonaceous
2171
particles (9, 14). As a practical matter, Table I shows that four extractions recovered most (but clearly not all) of the radiolabeled BaP. Taken together, this extraction behavior is consistent with at least two different sorption models for the ash. For example, in one model the sorbed 14C-BaPis extracted from a heterogeneous collection of sites with sharply different binding energies. In a second model, the sorbed ‘4C-BaP is extracted from porous particles, in which most of the 14C-BaPis readily available on or near the surface, but some has penetrated deeply into the inner structure. Hence, much more time would be needed to remove the “inner” BaP due to diffusion-limited processes (14). Recent characterizations (9, 14, 15) suggest that both of these possibilities are reasonable and overlapping. Three basic types of stack ash particles, viz. mineral, magnetic, and carbonaceous, have been isolated, and the binding energy of the latter is by far the greatest. Furthermore, carbonaceous particles typically exhibit the porosity that causes the second mechanism to take place. Registry No. Toluene, 108-88-3; BaP, 50-32-8. LITERATURE CITED Griest. W. H.; Yeatts, L. B.; Caton, J. E. Anal. Chem. 1980, 52, 199-201. Harrison, F. L.; Bishop, D. J.; Mellon, B. J. Environ. Sci. Techno/. 1985, 79, 186-193. Junk, 0. A.; Richard, J. J. Anal. Chem. 1988, 5 8 , 962-965. Fitch, W. L.; Everhart, E. T.; Smith, D. H. Anal. Chem. 1978, 50, 2122-2 126. Stenberg, U.; Alsberg, T. E. Anal. Chem. 1981, 53, 2067-2072. Wright, B. W.; Wright, C. W.; Gale, R. W.; Smith, R. D. Anal. Chem. 1987, 5 9 , 38-44. “Blological Effects of Plume Fly Ash,” Technical Coordination Meeting, Stanford Research Institute, Menlo Park, CA, December 12-13, 1985. Mitchell, R. J.; Baytos, W. C. (February 1, 1979), “Collection and Analysis of Fly Ash from Stack Gas Emissions”, report on U.S. EPA Contract 68-03-1371 to W. Pepelco, HERL, US. EPA, Cincinnati, OH. Griest, W. H.; Tomkins, B. A. Envlron. Scl. Techno/. 1986, 2 0 , 29 1-295. Tomkins, 8. A.; Reagan, R. R.; Maskarinec, M. P.; Harmon, S. H.; Griest, W. H.; Caton, J. E. “Analytical Chemistry of Polycyclic Aromatic Compounds Present In CoaCFired Power Plant Fly Ash” in Polynuclear Aromatic Hydrocarbons : Formation, Metabolism, and kasurement ; Cooke, M., Dennis, A. J., Eds.; Batteiie Press: Columbus, OH, 1983; pp 1173-1 187. Miguei, A. H.; Korfmacher, W. A.; Wehry, E. L.; Mamantov. G.; Natusch, D. F. S. Environ. Sci. Techno/. 1979, 73, 1229-1232. Engelbach, R. J.; Garrison, A. A.; Wehry, E. L.; Mamantov, G. Anal. Chem. 1987, 5 9 , 2541-2543. Pierce, R. C.; Katz, M. Envkon. Sci. Techno/. 1975, 9 , 347-353. Soltys, P. A.; Mauney, T.; Natusch, D. F. S.; Schure, M. R. Envlron. Sci. Techno/. 1988, 2 0 , 175-180. Griest, W. H.; Harris, L. A. Fuel 1985, 64, 821-826.
RECEIVED for review March 11,1988. Accepted May 19,1988. This research was sponsored jointly by the Office of Health and Environmental Research, U.S. Department of Energy, and the SRI International under Interagency Agreement SRI Contract No. C-11441 and DOE Project No. ERD-85-499, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. This report was prepared by the Oak Ridge National Laboratory, operated by Martin Marietta Energy Systems, Inc. (Energy Systems), on behalf of the U.S. Department of Energy (DOE), as an account of work sponsored by SRI International (SRI). Neither Energy Systems, DOE, the U.S. Government, nor any person acting on their behalf: (a) makes any warranty or representation, express or implied, with respect to the information contained in this report; or (b) assumes any liabilities with respect to the use of, or damages resulting from, the use of any information contained in the report.
