130
Anal. Chem. 1981, 53, 130-131
n !I 3001.
200
a" 100
4 20
IO
30
O0
Time, min
Figure 4. Loss of analyte by diffusion. Peak area is In arbitrary units for the oxidation of 200 pM dopamine. Conditions are as in Figure 1.
typical sample volume of 10 pL, the detection limit of 1pM represents a total of 10 pmol of analyte. Other Electroactive Materials. Voltammograms of the oxidation of 0.2 mM potassium ferrocyanide and 1 mM dihydronicotinamide adenine dinucleotide (NADH) were obtained. The ferrocyanide produced a well-defined anodic peak with a peak voltage of +0.165 V vs. SCE, similar to that observed at a glassy carbon rotating disk electrode (16). The NADH signal appeared drawn out, with a peak voltage of 0.62 V vs. SCE. This is consistent with the irreversible nature of NADH oxidation at glassy carbon (17,181. The detection limit for NADH would be at least an order of magnitude higher than that of dopamine and ferrocyanide. As was pointed out recently, detection of NADH may be better accomplished by amperometry rather than voltammetry (8). A sample of human cerebrospinal fluid was analyzed by use of a CPBE. The voltammogram resulted in two well-defined peaks, one with a peak potential of -0.03 V vs. SCE and the other with a peak potential of +0.21 V vs. SCE. The peaks probably correspond to the oxidation of ascorbic acid and total catecholamines,respectively, which have been observed in CSF previously (19, 20).
ACKNOWLEDGMENT
,O"*L\ 100 mV Figure 5. Voitammogram of 1 pM dopamine. Conditions are as In
Figure
1.
terval was allowed to lapse before performing the scan. The results, shown in Figure 4,indicate that diffusion from the electrode does not result in a significant decrease in peak area for at least 15 min. Therefore, the measurement step should not be delayed more than 15 min after the sampling.
Concentration Dependence of Electrode Response. Plots of peak area vs. dopamine concentration were linear up to approximately 100 pM, where a slight decrease in slope occurred. It is possible that iR drop contributes to this nonlinearity, although significant peak broadening or shifting was not observed at the higher concentrations. A detection limit of approximately 1 pM dopamine was predicted from the above data and the noise level of the system. Figure 5 shows a voltammogram of 1pM dopamine with use of a CPBE. The peak is well-defined and easily quantifiable. It should be noted that the noise level varies somewhat from electrode to electrode, and the scan in Figure 5 is one of the better determinations. However, a 1 pM concentration would be detectable at all electrodes. With a
Preliminary input from Jeffrey Colquhoun was appreciated. The granular carbon was obtained from Walter J. Blaedel of the University of Wisconsin-Madison.
LITERATURE CITED (1) Adams, R. N.: Kissinger, P. T. Anal. Len. 1076, 9. 783. (2) Cheng, H. Y.; Schenk. J.; Huff, R.; Adam. R. N. J . Electroanel. Chem. 1070, 700, 23. (3) Kessler, M. "Ion and Enzyme Electrodes in Biology and Medicine"; University Park Press: Baltimore, MD, 1976. (4) Kissinger, P. T. Anal. Chem. 1077, 49, 447A. (5) Sternson, A. W.; McCreery, R.; Feinburg, 6.; Adams, R. N. J. Electroanal. Chem. 1073, 46. 313. (6) Sioda, R. E. Elecfrochlm. Acta 1068, 73, 1559. (7) Blaedel, W. J.; Strohl, J. H. Anal. Chem. 1964, 36, 1245. (8) Heinernan, W. R.; Kissinger, P. T. Anal. Chem. 1060, 52, 139R. (9) Hern, J. L.; Strohl, J. H. Anal. Chem. 1078, 50, 1954. (10) Blaedei, W. J.; Boyer. S. L. Anal. Chem. 1073, 45, 258. (11) Sloda, R. E. Electrochlm. Acta 1070, 15, 783. (12) Sioda, R. E. Electrochim. Acta 1072, 34, 411. (13) Alkire, R.; Gracon, B. J. Electrochem. Soc. 1075. 722, 1594. (14) Strohl, A. N.; Curran, D. J. Anal. Chem. 1070, 57, 353. (15) Blaedei, W. J.; Wang, J. Anal. Chem. 1070, 51, 799. (16) Blaedei. W. J.; Engstrom, R. C. Anal. Chem. 1078, 50, 476. (17) Blaedel, W. J.; Jenkins, R. A. Anal. Chem. 1075, 4 7 , 1337. (18) Moiroux, J.; Eking, P. J. Anal. Chem. 1076, 50, 1058. (19) Adams, R. N. Anal. Chem. 1076, 48, 1126A. (20) Lane, R. F.; Hubbard, A. T.: Fukunaga, K.; Blanchard, R. J., Braln Res. 1076, 774, 346.
RECEIVED for review September 2, 1980. Accepted October 27,1980. The authors wish to thank the South Dakota Research Institute and the University of South Dakota Department of Chemistry for financial support.
Determination of Total Arsenic Species by Anodic Stripping Voltammetry S. Win Lee and Jean C. Miranger' Environmental Health Centre, Health and Welfare Department, Tunney's Pasture, Ottawa, Ontario, Canada K1A OL2
Atomic absorption techniques have been applied to the selective determination of As species but usually require more costly instrumentation than electrochemical techniques. The determination of total arsenic at the nanogram levels by
high-speed anodic stripping voltammetry has been reported recently by Davis et al. (1). The method involved two basic steps: first, separation of arsenic from the sample matrix as AsC13 by distillation; second, a rapid anodic stripping volt-
0003-2700/81/0353-0130$01.00/0@ 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
131
Table I. Instrument Precision of ASV Measurements of Arsenic( III)a amt of As, av peak re1 std dev, ng height,b pA % 10
20 50
Y
a
300 500 600 1000
-
tE (volts)
Figure 1. Anodic stripping signals of arsenic(II1)in 7 M HCI: E, -0.15 V vs. Ag/AgCi (saturated NaCI); deposition time = 4 min; sweep rate = 100 mV/s; matrix = mL of 7 M HCI.
ammetric (ASV) analysis at a gold fiim electrode. Reduction of As(V) to As(II1) was required prior to the distillation to obtain total arsenic concentration. The feasibility of using this technique in combination with the charge-transfer analyzer was investigated. This relatively inexpensive multifunctional analyzer was found to be well suited for the rapid screening of arsenic species.
MATERIALS AND METHODS Anodic stripping voltammograms were obtained in the linear scan ASV mode with a charge-transfer analyzer (Environmental Sciences Associates, Inc., Model 3040). This multifunctional instrument consists of eight channela which can be independently processed for parameters such as potential, time, stirring, and mathematical functions. The electrochemical cell has essentially the same features as that previously described by Davis et al. (I) except that the active electrode area was 2 cm2 compared to 3 cm2. ASV measurements were made under the following conditions: deposition time ( T ) 4 min; deposition potential (Ei)of -0.15 V vs. Ag/AgCl (saturated NaC1); sweep rate, 100 mV/s; and supporting electrolyte 7 M HC1. Reduction of As(V) to As(II1) and distillation of AsC13 for 16 min were made by using a 4-unit glass simultaneous distillation apparatus purchased from ESA Inc.
RESULTS AND DISCUSSIONS Detailed studies of the experimental parameters involved in analytical methodology showed similar findings to those reported by Davis and coauthors. However, notable improvements were observed in the physical characteristics of the stripping signals as evidenced by the much sharper peaks, thereby improving the accuracy of measurement (Figure 1). The analyzer also offered excellent instrument precision in ASV determinations between 10 ng and 1pg of As (Table I). The results of analyses of some National Bureau of Standards Reference Materials are shown in Table I1 and compared to those previously reported by Davis et al. Good agreement was
5.5 4.5 2.7 2.2
111
100
w a
22 44 228 609 960 1094 1736
1.0 1.0 1.3
3.2
a Measured in 4.0 mL of 7 M HCI. E , = -0.15 V vs. Ag/ Based on seven repliAgCl (saturated NaCl); T = 4 min. cate measurements.
Table 11. Determination of Total Arsenic in Standard Samples ASV value of As, pg/g (charge transfer analyzer) 10.6
sample orchard leaves (NBS SRM 1571) bovine liver 0.066 (NBS SRM 1577) trace elements 0.076 in water (NBS SRM 1643) arsenic trioxidea 101.8% (NBS SRM 83c) a Expressed as percentage purity.
Davis et al. certified value of value of As, pg/g As, pglg 10.6 10.0 0.057
0.055 0.076 99.99%
obtained with the certified values except for bovine liver. Initially, variations of up to 20% in arsenic signals were encountered from the distillation step. These variable recoveries were the result of variations in nitrogen flow due to size differences in the nitrogen orifices. This problem was minimized by increasing the distillation time from 12 to 16 min. Frequent calibration of the orifices is required to completely eliminate this problem. By use of this modification, the Davis method proved satisfactory when applied to the chargetransfer analyzer. Investigations are currently under way to apply this method to a wide range of environmental samples for the selective determination of As(II1) and As(V) and total As.
LITERATURE CITED (1) Davis, P. H.; Duiude. G. R.; Gimn. R. M.; Matson, W. R.; Anal. C h m . 1978, 50. 137-143.
Zinc, E. W.
RECEIVED for review February 22,1980. Accepted August 11, 1980.
