AATN = f ( k , t )
and the normalized absorbance of any species j that is the product of first-electron transfer reaction or is involved in subsequent chemical reaction is defined as:
(89)
For every value of k t , there is a corresponding value of rlAT.''. After the working curves (&IT" us. log k t ) are established, it becomes a simple matter to determine kinetic or nonkinetic parameters. Obviously, J ( k , t ) can be expressed in closed form if exact solutions exist.
ACKNOWLEDGMENT We would like to thank S. W. Feldberg for his many helpful discussions and advice. To T. Kuwana and his associates, we are indebted for programs as yet unpublished. From simulation working curves, i.e., A s N and Ajlv us. log k t and Equations 87 and 88, one can calculate AsO(X) - As(X,t) and A,(X,t) a t time t. With the aid of Equations 67 and 68, one can obtain a function f ( k , t ) in the form of Equation 74 and its corresponding numerical values.
Received for review March 19, 1973. Accepted July 12, 1973. This investigation was supported in part by National Science Foundation Grant GP-28051 and the Office of Naval Research.
Selected Trace Metal Determination of Spot Tape Samples by Anodic Stripping Voltammetry Kathryn E. MacLeod and Robert E. Lee, Jr. Quality Assurance and Environmental Monitoring Laboratory, U.S. Environmental Protection Agency, National Environmental Research Center, Research Triangle Park, N.C. 2771 7
As part of a program to determine the contribution of trace metals in fuels to ambient air levels, anodic stripping voltammetric analysis was applied to two-hour samples collected with an AIS1 spot tape sampler. The sampled spots were analyzed for lead, cadmium, and copper after ashing at low temperature and extracting with acid. The quantity of metal present in the samples ranged from 7 to 350 ng of cadmium, from 80 ng to 2.4 pg of lead, and from 6 ng to 1 pg of copper. The maximum relative standard deviation for the method was less than 12% for all three metals with the averages of 4.5% for Cd, 5.9% for lead, and 3.7% for copper. Analytical sensitivities were sufficient to characterize diurnal variations of these metals in samples collected in Chicago and Washington, D.C.
Anodic stripping voltammetry is used routinely in our laboratory for the determination of trace concentrations (ppb level) of metals in fuels, biological samples, and other environmental materials. As part of a program to determine trace metals in motor vehicle fuels, we examined the feasibility of applying ASV analysis to ambient air samples collected adjacent to highways where suspended particulate matter is composed primarily of fuel combustion products. Samples were collected with AISI tape samplers, which are commonly used in many air pollution programs to determine the soiling index (coefficient of haze) of suspended particulate matter. The analytical procedures described in this report permit the determination of the soiling index and of ambient concentrations of lead, cadmium, and copper in two-hour samples. This method can easily be extended to measure other ASV-responsive metals including Zn, Bi, Ag, T1, and Sb.
The determination of particulate metals suspended in air is usually limited to a 24-hour sampling period using a hi-volume air sampler ( I ) . Continuous methods for trace metal determinations are currently in the research stage and are not available for routine measurements. A major problem in successfully reducing the sampling time to derive information on diurnal trace metal patterns has been the insensitivity of available analytical methods. Among several trace metal analytical techniques that have recently become available, anodic stripping voltammetry (ASV) offers the required sensitivity and the capability for multielement analysis (2).
EXPERIMENTAL
Robson and K. E. Foster, Amer. Ind. Hyg. Ass., J., 24, 404 (1962). (2) W. R. Matson, R . M . Griffin. and G . B. Schreiber, "Trace Substances in Environmental Health-IV," University of Missouri, 1971, p 396.
Apparatus. Samples were collected with AISI tape samplers (3) located a t EPA's Continuous Air Monitoring Project stations ( 4 ) in Chicago and Washington, D.C. The AISI tape sampler determines the soiling property of suspended particulate matter by means of a n absorbance measurement. Ambient air is drawn through a circular portion 1 inch in diameter of a continuous strip of Whatman No. 4 filter paper 2 inches wide. The sampling interval used in this study was 2 hours, after which a n automatic mechanism advanced the tape to expose a clean portion of the filter. The optical density of the sampled portion of the tape is measured by reference to a clean unsampled portion of the tape used a s a zero reference. Results are generally reported in units of coefficient of haze (Coh), which is defined as the quantity of
(1) C. D.
2380
(3) ASTM, "ASTM Standards on Methods of Atmospheric Sampling and Analysis," 1962, p 497. (4) G. A . Jutze and E. C. Tabor, J. Air Poliut. Confr. Ass., 13, 278
(1963).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
light-scattering solids deposited on a standard filter tape that produces a n absorbance of 0.01 unit when measured by whitelight transmittance where the blank filter is zero absorbance. The sampling rate in this study was 20 scfh. All analyses were made using Environmental Science Associates (ESA) 4-cell multiple anodic stripping unit, MASA 2014, previously described by Matson et ai. ( 5 ) with a 4-cell extension unit, PMI 1014s. The unit was set with a positive sweep rate of 60 mV/sec, auto sweep hold “on” a t 1070 mV, the initial potential a t -1100 mV, and the plating potential of the eight cells a t -1100 mV. The current range was set a t 0.5 mA full scale with a recorder speed of 10 sec/in. An ESA reagent cleaning system 2014P was used for the buffer solution. The reduction potential of the cleaning system was kept at -1400 mV. A Tracerlab LTA 600 low temperature asher was used to ash the filters used in the analyses. Reagents. The buffer used was a 0.1 M sodium chloride-sodium acetate solution made with ACS reagent grade chemicals. The buffer solution was cleaned and stored in the reagent cleaning system described above. The water used was either double distilled in a n all-glass still or de-ionized using a Continental Water Systems Model 360 De-ionizer. G. F. Smith Company vacuum-distilled, lead-free perchloric acid, 70%. and Calvert Chemical Company redistilled nitric acid were used to make up a 1 : l (v/v) perchloric-nitric acid solution. All glassware was precleaned by soaking for 48 hours in a 10% by volume perchloric acid solution in water. The standards were purchased from Fisher Scientific Company and diluted as required in our laboratory. Procedure. A circle 34-inch in diameter representing 56.24% of the total exposed area was cut from the center of each AIS1 tape sampler spot using a nickel-burnished steel cork borer. T h e %inch cut-out was used to avoid the possibility of including portions of blank paper along with the spot. A study on the weight of the cut-out us. that of the whole spot shows that of five samples the percent weight varied from 60 to 53 with an average of 5770, indicating a reasonably uniform distribution of particulate matter. Each cut-out was placed in a borosilicate glass sample boat and ashed 30 minutes in the low temperature asher at 200 watts with an oxygen flow of 80 cm3/min. At the end of the 30 minutes, the filter was completely ashed, leaving only a light residue. Carefully, 0.1 ml of the 1:1 perchloric-nitric acid solution was added to the sample in the boat. The dissolved sample was then transferred quantitatively with water to a 10-ml volumetric flask. These slightly acidic solutions were found to be stable over a period of two weeks; however. they were analyzed within two days of preparation. In the analysis procedure, it was necessary to hold all parameters constant to assure reliability of the results. Parameters such as plating time, stirring time and rate, stripping rate, p H of solution, and thickness of mercury coating on the electrodes affected the data output. Plating and stirring times were kept to within 5 seconds while the stripping rate was set on the instrument and held constant throughout. The pH was controlled by adding identical amounts of the buffer and acid solution each time. The mercury coating, the only parameter that could not be controlled completely, was regulated by recoating the electrodes overnight with a dilute mercury solution; slight changes that may have occurred in the coating were compensated for by running standards on each cell several times a day. A 5-ml aliquot of buffer was pipetted into each of the cells which were then placed on the cell holders. At equal intervals (30 seconds was found to be convenient), the cells were switched to the plating mode and the nitrogen stream was turned on. Fifteen seconds before the end of the plating time, the nitrogen stream was turned off. With the cell turned to “strip,” the sweep rate was turned from “reset” to “on” and the cell was stripped. This was done to each cell in turn. A 1.0-ml aliquot of sample was then added to the buffer in each cell and the plating and stripping sequences were repeated. At the end of the stripping sequence, each of the cells was again stripped to ensure that all the metals had been removed from the electrode. When standards were run, they were added to the sample in the cell; thus, the resulting recorder trace was buffer plus sample plus standard and any matrix effects were taken into account.
(5) W. R. Matson, D. K . Roe, and D. E. Carritt, Anal. Chern.. 37, 1598 (1965).
2000
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1200 2000 0400 1200 2000 0400 I200 2000 0400 1200 2000 0400 1200 ?OOO 0400 1200 2000 0400 1200 2000 0400 11251 11-26 I l l 2 7 1 1 1 2 8 1129 1130 121 1122
I
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SAMPLING PERIOD h o w
Concentration of Pb, Cd, and Cu in 2-hour AIS1 filter tape samples collected in Chicago from 11-25-68 through 12-2Figure 1.
68
The peak heights of the recorder trace were proportional t o the quantity of each metal present in the solution. The actual millivolt to nanogram ratio was configuration-dependent as described above and varied for each cell.
RESULTS Repeatability Studies. Repeatability studies were made on each of the eight cells of the ASV instrument using a standard solution containing 100 ng each of lead, cadmium, and copper per milliliter of solution. The same procedure used for actual samples was followed i . e . , a buffer solution was plated and stripped each time before 1 ml of the standard solution containing the lead, cadmium, and copper was added to the cell. A mimimum of four replicate determinations was made on each ASV cell for each metal. The per cent relative standard deviation ranged from 1.1 to 10.7 with an average of 4.5 for Cd; 1.6 to 11.9 with an average of 5.9 for Pb; and 0.9 to 6.3 with an average of 3.7 for Cu. These studies indicated that a maximum relative standard deviation of less than 12% could be attributed to instrumental variability and operation error (pipetting, mixing time, e t c . ) for all three metals.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
2381
,
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WASHINGTON 0
1300
-
Table I. Comparison of Trace Metal Concentrations of 24-Hour Samples in Chicago and Washington, D.C.
Metal 1100-
900
Cadmium Chicago
-
Washington, D.C.
Lead Chicago
-
700
Washington, D.C. Copper Chicago
j.( 3w
This study, N m 3
0.00130.0155 0.0200.031 0.3591.oo 0.4480.538 0.00080.2809
1967 NASN average quarterly, ~ g l (7) m ~
0.00-0.02
0.00-0.00
1.0-1.4 0.6-1.3
0.06-0.12
1968 NASN 4 m 3 (8)
0.0043 0.0016 0.0000 0.0000
(11/19) (12/6) (11/19) (12/6)
1.30 (11/19) 0.49 (12/6) 0.40 (11/19) 0.73 (12/6) 0.10 (11/19) 0.03 (12/6)
IL 0 w
8
IO0
200
P I t :I I
I
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I
II
SAMPLING PERIOD, hours
Concentration of Pb and Cd in 2-hour AIS1 filter tape samples collected in Washington, D.C., from 11-30-68 through
concentration of Pb, Cd, and Cu varied from filter tape to filter tape and, in some cases, equaled the sampled values. In six blank disks cut from the Chicago filter tape, concentrations expressed as nanograms per disk ranged from 44 to 116 with an average of 80 for Cd; 430 to 870 with an average of 623 for Pb, and 130 to 270 with an average of 183 for Cu. In the Washington, D.C., blank disks, Cd ranged from 44 to 70 ng with an average of 55 ng; the P b range of 180 to 330 ng with an average of 248 ng was significantly lower than that from the Chicago tape. The blank Cu concentrations in the Washington, D.C. filter tape were too variable to be of use; consequently, Cu data are not reported. In Chicago, the measured metal concentrations were lower in samples collected during periods of heavy rainfall (0400 to 2300 hours on November 28 and 1200 to 1800 hours on December 1) than in samples collected over similar periods during dry conditions.
DISCUSSION
Figure 2. 12-1-68
Recovery Studies. Although it has been shown that Pb, Cd, and Cu are recovered quantitatively from airborne particulate matter collected on paper filters ( 6 ) , specific recovery studies were made with the AISI tape sampler spots to make sure there was no loss during the ashing process. Four filter blanks were cut out and impregnated with 100 ng each of lead, cadmium, and copper. These were then carried through the entire analytical scheme of ashing, acid extraction of the residue, dilution with de-ionized water, and analysis by ASV as described above. The per cent recovery ranged from 73 to 93 with an average of 84 for Cd, 81 to 133 with an average of 102 for P b , and 87 to 108 with an average of 96 for Cu, indicating that the procedure used was reliable. Field Data. AISI tape samples collected in Chicago from November 25 through December 2, 1968, and in Washington, D.C. from November 30 through December 1, 1968, were analyzed as described. Results for the metal determinations are shown in Figures 1 and 2. Discontinuous sections in the plots represent lost samples. Each sample filter disk was corrected by an average blank value derived from the same tape. The background (6) T. Y . Kometanic, J. L. Boone, 8 . Nathanson, S. Siebenberg, and M. Magyar, Environ. Sci. Techno/., 6, 617 (1972).
2382
The metal concentration ranges measured with the AISI tape samplers and ASV analysis were compared with data reported by the National Air Surveillance Network (NASN) using 24-hour hi-volume samplers and emission spectrographic analysis. Table I indicates that in Chicago, the Cd, Pb, and Cu concentrations determined in this study compare reasonably well with the 1967 NASN quarterly average data and with available daily NASN measurements made in 1968. In Washington, D.C., our data for Pb were in agreement with the NASN data; however, our data for Cd indicate higher concentrations than reported by the NASN. This may reflect the relative insensitivity of emission spectrographic analysis for Cd. Figure 3 presents a composite of P b concentrations on a 2-hour basis for the week of sampling in Chicago. A clear bimodal pattern is exhibited; peak concentrations occur from 0800 to 1000 hours and from 1600 to 1800 hours during the hours of heaviest traffic. During a period of heavy rainfall on November 28, 1968, the lead concentrations were markedly reduced although the bimodal pattern remained. Similar plots for Cd and Cu (Figure 4) were inconclusive in associating a clear relationship with traffic. The predominant source of measured P b was gasoline combustion; however, the sources of Cd and Cu cannot be (7) U.S. Environmental Protection Agency, "Air Quality Data for 1968." APTD Report No. 0978. Research Triangle Park, N.C., 1972. (8) G. Akland, U.S. EPA, Research Triangle Park, N.C., personal communication. 1973.
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 14, DECEMBER 1973
1 1 1 1 1 1 1 1 1 1 1
1200
l
l
l
l
l
l
l
l
l
l
h
l
900 AVERAGE Pb CONCENTRATION 111.2568 IO 12-2-681 800
700
-
600
-
B i 0
2 c= + L
w
L u
8
500-
400
-
300
-
200
-
100
-
SAMPLING PERIOD. houri
Average Cd and Cu concentrations at 2-hour sampling periods in Chicago from 11-25-68 through 12-2-68 Figure 4.
0000 0200 0400 0600 0800 1000 1200 1400 1600 1800 2000 2200 2400 SAMPLING PERIOD, hours
Average lead concentration at 2-hour sampling peri11-25-68 t h r o u g h 12-2-68 and during rainfall on 1128-68 in Chicago
Figure 3. ods from
as easily defined. Studies in our laboratory have shown that gasoline contains as much as 0.05 ppm of Cd and 5 ppm of Cu (9). These levels appear to be too low to be a dominating influence on ambient levels near highways although some contribution from gasoline combustion is likely. The collection efficiency for the paper tape filters is lower than that for the glass-fiber filters used in the NASN ( 6 ) . Consequently, it is possible that an indeterminate portion of the aerosol sampled with the AISI tape sampler passed through the Whatman No. 4 filter. A comparison of our data with the NASN data in Table I suggests that a relatively small amount, if any, of Cd and Cu was lost since the agreement is quite high. This conclusion is supported by a recent study (10) showing that the aerodynamic particle size on a mass basis is significantly greater than 1 pm in diameter for Cu (50% or greater) and Cd (72% or greater). On the other hand, the aerodynamic size of Pb associated particles is considera(9) R. E. Lee, Jr.. and D. J. von Lehmden "Trace Metal Pollution in the Environment,"J. Air Pollut. Confr. A s s . , in press. (10) R. E. Lee. Jr., S . S . Goranson, R. E. Enrione, and G. B. Morgan, Environ. Sci. Techno/., 6, 1025 (1972).
bly less than 1 pm in diameter (60% or more). It is, therefore, likely that a portion of the P b aerosol is not retained by the AISI tape sampler. A more serious consideration in assessing the quality of data derived with the sampling technique that we have described is the high background level of metals in the paper filter. As reported above, considerable variation in the background metal content of paper filter tapes was observed. In some cases, a relatively high blank value can swamp the measured .sample value. This may have occured with Cu in the Washington, D.C. samples. For more precise measurements, it is recommended that the paper filter tape be washed with distilled water before use or that a cleaner tape medium such as a membrane filter tape be used. Untreated paper tape should be analyzed to determine the background metal concentration before use. The results of these studies show that information of suspended particulate matter can be derived from AISI tape samples. The sensitivity of the ASV analysis should also permit the determination of metals on short-term high volume air samples or other sampling devices which can collect airborne suspended particulate matter. The ASV method can be applied to studies to assess emissions from specific sources such as P b from fuel combustion as described above, or may be applied to ambient air studies in the vicinity of smelters and incinerators, and research studies such as monitoring metal aerosols in animal exposure chambers. Received for review April 30, 1973. Accepted June 2 2 , 1973. Mention of commercial products does not constitute endorsement by the U.S. Environmental Protection AgenCY.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
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