1406 Anal. Chem. 1981, 53, 1406-1410

Simultaneous Determination of Cadmium and Lead in Whole Blood and in Serum by Computerized Potentiometric Stripping Analysis

Daniel Jagner, * Mats Josefson, Stig Westerlund, and Kerstln h e n

Department of Analytical and Marine Chemistry, Chalmers University of Technology and University of Giiteborg, S-4 12 96 Goteborg, Sweden

A simple electroanalytical technlque for the slmultaneous determlnation of cadmlum and lead In whole blood and In serum has been developed. Contrary to voltammetric strlp- plng technlques, the potentlometrlc stripping technlque does not suffer from Interferences from electroactive specles which might be present In the sample. The llmit of detectlon after 2 min of preelectrolysis is equal to 25 nM for both elements and the relative standard devlatlon is better than 4 %. Sample volumes down to 25 I.LL can be handled. Due to the compu- terized reglstratlon of the stripping curve and the background curve a throughput of 12 samples/h is posslble. Thls Is con- slderably more than In hltherto voltammetric stripplng tech- niques. The accuracy of the potenliometrlc stripping tech- nique has been tested by comparison with the results ob- tained by 52 other laboratories on 12 different International standards.

Electroanalytical determination of cadmium and lead in whole blood and in serum by means of either anodic stripping voltammetry or differential pulse anodic stripping voltam- metry has been attempted by several authors (1-10). Since, however, these techniques suffer from interference from all electroactive constituents present in the blood sample, it has been necessary to modify the matrix prior to analysis. The most frequently used technique is wet digestion with mineral acids, but low-temperature ashing has also been attempted (11). The simplest sample pretreatment technique hitherto suggested is the use of the very expensive ion-exchange reagent Metexchange (7). As pointed out by Oehme and Lund (12), this reagent cannot eliminate ghost peaks in the vicinity of the lead stripping peak which may cause erroneous results. Moreover, the blood sample must be diluted to such an extent that cadmium cannot be determined.

Potentiometric stripping analysis (13-16) resembles the anodic stripping techniques in that the trace metal analytes are preconcentrated in a mercury film on a glassy-carbon electrode by means of potentiostatic deposition according to

(1) The stripping (oxidation) step in potentiometric stripping analysis is, however, fundamentally different from that in the anodic stripping techniques in that stripping is achieved chemically by

(2) and the transport rate of oxidant to the electrode surface is kept constant by means of constant rate rotation of the electrode both during preelectrolysis and stripping. By monitoring the potential vs. time behavior of the working electrode during stripping, well-defined stripping plateaus can be registered for each reversibly amalgamated element (13-15). Since no current is drawn through the working electrode during potentiometric stripping, electroactive species present in the sample will not interfere with the stripping plateaus.

M"+ + ne- - M(Hg)

M(Hg) + oxidant - Mn+

Consequently the potentiometric stripping technique is better suited to the analysis of complex samples than are all those techniques which are based on the anodic stripping principle. Hitherto potentiometric stripping analysis has, however, suffered from the drawback that stripping according to re- action 2 occurs very rapidly in samples containing high con- centrations of oxidants. In order to be able to register the stripping plateaus on a normal speed strip-chart recorder, very long potentiostatic deposition times were necessary when the trace element analyte concentration was below 10 pg L-l in the sample. This problem has been overcome by means of the microcomputer system described in detail elsewhere (1 7).

The purpose of this work is to show how computerized potentiometric stripping analysis can be used for the simul- taneous determination of cadmium and lead in whole blood and serum after a very simple sample pretreatment and without deoxygenation of the sample prior to analysis, dis- solved oxygen and mercury(I1) ions being used as oxidants (15).

EXPERIMENTAL SECTION Instrumentation. A Radiometer ISS 820 ion scanning system

(16) was used in combination with a laboratory-constructed microcomputer system using an Intel 8085 8-bit processor (17).

Electrodes and Electrochemical Cell. A Radiometer TTA 80 titration assembly with polyethylene cells of total volume 35 mL was used. The stirring motor of the ?r'A 80 was used to rotate the glassy-carbon working electrode at a constant rate of ap- proximately 2000 rpm. The glassy-carbon electrode was manu- factured by pressure-fitting a 15-mm glassy-carbon rod, with a diameter of 2.1 mm, into a 80 mm Teflon rod with a diameter of 12 mm. The center of the glassy-carbon rod was placed 4 mm from the center of the Teflon rod. A platinum foil was used as counterelectrode and a saturated calomel electrode as reference (Radiometer P 101 and K 4040, respectively).

Chemicals. All chemicals used were of analytical grade with the exception of the mineral acids (Suprapur, Merck). Stock solutions containing 1 g L-l of the metal analytes were prepared in 0.1 M nitric acid. Working standards were prepared every week by dilution with triply distilled water.

Samples. All samples had been haemolyzed with heparin and deep frozen prior to analysis. During thawing the blood samples were homogenized by means of careful shaking.

Mercury Preplating Solution. The solution contained 100 mg L-l of mercury(I1) nitrate in 0.1 M hydrochloric acid solution.

Sample Dilution Solution. This solution contained 75 mg L-' of mercury(I1) nitrate in 0.5 M hydrochloric acid.

Mercury Preplating Procedure. A 20-mL portion of the mercury preplating solution was placed in the electrochemical cell, and rotation of the working electrode was started. The potentiostatic circuitry was adjusted to -0.5 V vs. SCE and preelectrolysis for 1 min was started. After the sample was stripped, which takes less than 0.1 s, the potentiostat was set at -0.6 V and another minute of preelectrolysis was started. This procedure was repeated at -0.7, -0.8, and -0.9 V vs. SCE. At 4 .9 V vs. SCE the 1-min preelectrolysis/stripping cycle was repeated three times making the total preelectrolysis time for mercury preplating equal to 8 min. Once mercury preplated, the electrode could be used for several days, provided that it was carefully rinsed and stored in doubly distilled water. When the electrode is used

0003-2700/81/0353-1406501.25/0 0 1981 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981 1407 __--___---_______ - _ _

Table I. Comparison between the Results Obtained by Potentiometric Stripping Analysis (PSA) for Lead in Whole Blood Samples and the Results Obtained by Four Other Techniques on the Same Intercalibration Samplesa

carbon rod Delves cup dithizone extraction anodic stripping (PSA) no. of coticn, pM t no. of concn, pM + 1 1 0 . of concn, pM + no. of concn,

this work ___ -- -.

coucn, pM labs std dev labs std dev labs std dev labs

0.63 1 3 0.62 + 0.15 21 0.69 i 0.12 5 0.97 t 0.45 0.87 21 0.89 f 0.14 27 0.91 i 0.14 5 0.86 i 0.36 1.45 26 1.42 f 0.16 28 1.53 i 0.26 5 1.19 i 0.17 1 1.55 1.52 22 1.62 + 0.16 29 1.57 t 0.18 4 1.42 i 0.36 1 1.55 2.32 19 2.24 i 0.18 22 2.30 r 0.17 4 2.27 i 0.08 1 2.25 2.51 22 2.46 i 0.22 23 2.54 t 0.22 6 2.52 f 0.30 2.77 20 2.92 _c 0.15 24 3.00 i 0.22 4 3.02 i 0.47 2.94 16 3.05 i 0.23 25 2.98 i 0.27 4 2.81 i 0.21 1 3.29 3.01 23 2.99 ? 0.33 26 2.98 t 0.23 4 2.75 r 0.64 1 2.80 3.33 16 3.43 + 0.20 22 3.37 t 0.33 4 3.58 c 0.52 3.69 20 3.68 r 0.33 22 3.44 i 0.26 5 3.54 t 0.63 4.20 20 3.92 f 0.31 22 3.74 f 0.40 5 3.44 i 0.38 1 3.37

a The samples were diluted 1 : 20 prior to analysis.

-0 4

continuously, mercury preplating once a day is recommended. Occasional cleaning of the mercury-coated electrode with acidified 50% ethanol is recommended, The same mercury preplating solution could be used for a large number of mercury preplatings. Prior to mercury preplating, the glassy-carbon working electrode was polished for 10-20 s with 3 pm diamond paste and rinsed carefully with ethanol.

Analytical Procedure for Cadmium and Lead in Whole Blood. One milliliter of whole blood is pipetted into a 9-mL sample dilution solution and mixed by gentle shaking. The samples should be analyzed within 3 h of mixing. Adjust the preelectrolysis potential to -1.2 V vs. SCE and the preelectrolysis time to 2 min and register the first preelectrolysis/stripping cycle. The working electrode is rotated a t a constant rate during preelectrolysis and stripping. Add one or two standard aliquots of lead and cadmium and repeat the preelectrolysis/stripping cycle after each addition. The length of the stripping plateaus are evaluated as is shown in Figure 1. The concentrations of lead and cadmium are evaluated by means of the normal equations for standard addition.

If only lead is to be determined, the preelectrolysis time can be decreased to 1 min which is the minimum on the instrumental setup used in this investigation. Alternatively, the sample can be diluted to 1:40 without significant loss of precision. If low levels of cadmium are to be determined the preelectrolysis time has to be increased to 4 or 8 min.

Analytical Procedure for Cadmium and Lead in Serum. Four milliliters of serum is pipetted into a 6-mL sample dilution solution and mixed by gentle shaking. The sample is preelec- trolyzed for 4 or 8 min at -1.2 V vs. SCE prior to the registration of the potentiometric stripping curve. The cadmium and lead concentrations are evaluated by means of standard addition.

Analytical Procedure Used in the Preliminary Experi- ments for Copper Determination. One milliliter of whole blood on serum is pipetted into 9 mL of sample dilution solution and mixed by gentle shaking. The preelectrolysis potential is adjusted to -0.9 V vs. SCE and the preelectrolysis time to 2 min. The copper concentration is evaluated by means of standard addition.

RESULTS Potent iometr ic S t r ipp ing Curves in Diluted Whole

Blood Samples. Figure 1 shows potentiometric stripping curves registered after 2 min of preelectrolysis at -1.2 V vs. SCE in a whole blood sample diluted 1:lO. The undiluted sample contained 0.37 pM of lead and 116 nM of cadmium. This corresponds to a normal lead value for unexposed persons and an elevated cadmium value. A stripping curve registered after the addition of a standard aliquot corresponding to 1 p M of lead in the undiluted blood sample is also shown in Figure 1.

AS can be seen from Figure 1, well-shaped potentiometric stripping plateaus can be obtained for both cadmium and lead in the diluted blood samples.

-

la I b I1 %x w

in real time in real time In rea l time

E vs SCE V

I I

1408 ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981

E v s SCE v

- 1 2

-0 8

- 0 4 w in rea l l ime

Commcnccrnent 0 1 l ead slr ipping Plateau I:; J

Figure 2. Potentiometric stripping analysis for cadmium in a whole blood sample containing 120 nM of cadmium ( c w e a), preelectrolysis for 2 min at -1.2 V vs. SCE. Curves b and c have been registered after the addition of 180 and 360 nM of cadmium, respectively.

according to carbon-rod analysis 2.4 pM of lead. Ten repetitive analyses of this sample yielded the result 2.37 pM with a standard deviation of f0.07 pM. Ten repetitive analysis of a sample originating from an unexposed person gave the result 0.37 pM of lead with a standard deviation of h0.02 pM. The corresponding carbon rod value was 0.4 pM. As expected, the potentiometric stripping technique responds to the total concentration of lead in the blood samples, irrespective of its origin.

Most of the preliminary experiments were performed with a stationary electrode and using a three-edged polypropylene rod as stirrer (16). With this setup the relative standard deviation of the lead determination was approximately 12%. The introduction of the rotating electrode decreased the relative standard deviation to 3% in the concentration range 1-4 pM of lead.

Determination of Lead by Means of a Calibration Plot. All lead determinations reported in this work have been performed by means of the standard addition technique. When a large number of samples are to be analyzed, it is sometimes advantageous to use a calibration plot instead of the more time-consuming standard addition technique. In order to be able to do this it is, however, necessary that in- dividual variations in the blood samples do not affect the analytical signal. Moreover, the sensitivity of the working electrode should not change with time and each new mercury preplating should give approximately the same sensitivity. In order to investigate the possibility of using a calibration plot in the lead determinations in whole blood, we plotted the potentiometric stripping signal registered in the sample prior to standard addition against the lead concentration value obtained by the standard addition technique. The results obtained from 23 different blood samples diluted 1:20 or 1 : l O showed a linear relationship between signal and concentration. Obviously a calibration plot procedure could be used instead of a standard addition procedure.

Determination of Cadmium in Whole Blood. Normal concentrations of cadmium in human blood are of the order of magnitude 5-50 nM, Le., considerably less than the lead concentration. Consequently, if normal cadmium values are to be determined, a preelectrolysis of more than 2 min must be used. If, however, only elevated cadmium values are of interest, a preelectrolysis time of 2 min is sufficient. This is illustrated in Figure 2 where a blood sample containing 20 nM of cadmium and spiked with 100 nM of cadmium has been preelectrolyzed for 2 min at -1.2 V vs. SCE. Also shown in Figure 2 are the potentiometric stripping curves registered after two standard additions of cadmium. As can be seen from

E v s SCE V

-0.4 T in r e a l l i m e

i

Figure 3. Potentiometric stripping curve registered after 16 min of preelectrolysis at -1.2 V vs. SCE in a whole blood sample containing 16 n M of cadmium (curve a). Curves b and c have been registered under the same experimental conditions after the standard addition corresponding to 45 and 90 nM of cadmium in the whde blood sample.

Table 11. Potentiometric Stripping Analysis for Cadmium''

preelec- amt of Cd

added, time, (total i std nM min dev), nM

amt of Cd trolysis found

0 25 50

1 OQ 150 200 300 400 500

1 6 i 4 46 t 2 66 i 7 118 t 3 1 5 5 t 3 208 t 6 302 ? 7 407 i 2 5 1 2 t 7

a Each sample was analyzed 4 times,

amt of Cd found

corrected for zero

value, nM

30 50

102 139 192 286 391 496

the figure, well-shaped stripping plateaus are obtained for cadmium in diluted whole blood samples. The detection limit, after 2 min of preelectrolysis, can be estimated from Figure 2 to be approximately 25 nM of cadmium in the undiluted blood sample.

Figure 3 shows the potentiometric stripping curve registered after 16 min of preelectrolysis a t -1.2 V vs. SCE in a blood sample containing 16 nM of cadmium. By comparison with Figure 2, it can be concluded that the length of the cadmium stripping plateau is proportional to the preelectrolysis time in the interval 2-16 min. Consequently, the detectiop limit for cadmium after 16 rnin of preelectrolysis is approximately 4 nM.

The accuracy of the Potentiometric stripping technique for cadmium was tested by analyzing whole blood samples spiked with inorganic cadmium. These samples had previously been used as standards for atomic absorption spectroscopy using the heated graphite technique. The unspiked blood sample contained approximately 16 nM of cadmium. The results are summarized in Table 11. Although each sample was analyzed only four times, it can be concluded that the relative standard deviation in the concentration range lo(t-500 nM is better than 4 % .

Determination of Cadmium and Lead in Serum. Figure 4 shows the potentiometric stripping plateaus registered after 8 min of preelectrolysis at -1.2 V vs. SCE in a serum sample diluted L2.5. Also shown in the figure is the potentiometric stripping curve registered after the addition of a cadmium aliquot corresponding to 18 nM of cadmium in the serum sample.

ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981 1409

E v s SCE V

E v s SCE V

commencement 0 1 copper \\ strippmg plateau

I 12 ms ' in r e a l t ime

Flgure 4. Potentiometric stripping curves registered after 8 min of preelectrolysis at -1.2 V vs. SCE in a serum sample diluted 1:2.5 (curve a). Curve b was registered under the same experimental conditions after a standard addition of cadmium corresponding to 18 nM cadmium in the undiluted serum sample.

As seen from Figure 4, well-shaped stripping plateaus for cadmium and lead can be obtained in serum samples diluted only 1:2.5. Obviously lead could be determined with a shorter preelectrolysis time. The cadmium content in the serum, as evaluated by standard addition, was equal to 3 nM, Le., very close to the reagent blank value of approximately 2 nM. The lead content in serum was evaluated as 0.026 pM after cor- rection for a reagent blank approximately equal to 0.002 pM of lead. Since no reference analytical technique for the de- termination of lead and cadmium in serum was available, the accuracy of the determination could not be checked. The results do, however, indicate the possibility of determining the lead content of serum.

Determination of Copper in Whole Blood. Most of the copper in whole blood is present in the enzyme caeruloplasmin (19). In order to investigate whether or not the potentiometric stripping technique responded to copper in the enzyme, a known aliquot of the enzyme was diluted in the sample di- lution solution. The copper content was determined by means of standard addition after preelectrolysis for 4 min at -0.95 V vs. SCE. The copper recovery was 96 f 5% assuming a molecular weight of 130 000 and eight copper atoms per en- zyme molecule.

Figure 5 shows the potentiometric stripping curve registered in a dilute whole blood sample after preelectrolysis for 2 min at 4 . 9 5 V. The figure also shows the potentiometric stripping curves registered after a standard addition corresponding to 8.0 pM of copper in the undiluted whole blood sample. Ten consecutive determinations of copper in a whole blood sample yielded a mean value of 12.1 pM with standard deviation of 1.8 pM. The unsatisfactorily low precision was due to two samples which, for unknown reasons, differed considerably from the eight others. Obviously more work is necessary to make the determination reliable.

DISCUSSION AND CONCLUSIONS Comparison between Potentiometric Stripping

Analysis and the Anodic Stripping Techniques. The computerized potentiometric stripping technique has a num- ber of advantages compared with the anodic stripping tech- niques, the main advantage being that the sample pretreat- ment procedure is simpler and the risk for interference due to electroactive organic constituents, e.g., pharmaceutical metabolites, in the sample is eliminated. Nor is sample de- oxygenation necessary in potentiometric stripping analysis. Furthermore shorter preelectrolysis times can be used, and the registration of a potentiometric stripping curve is made within a couple of milliseconds. Potentiometric stripping analysis for lead and cadmium in whole blood and serum does not seem to have any disadvantages compared with anodic

11

T In real time

111

Flgure 5. Potentiometric stripping analysis for copper In a whole blood sample. Preelectrolysis for 2 min at -1.2 V vs. SCE (curve I). Curves I1 and 111 have been registered under the same experimental conditions after the standard addition of 8.0 and 16 pM of copper, respectively.

stripping voltammetry or differential pulse anodic stripping voltammetry.

Comparison between Potentiometric Stripping Analysis and the Atomic Absorption Techniques. The main advantage of the potentiometric stripping technique over the three atomic absorption techniques, specified in Table I, is that the potentiometric stripping technique can be used to determine the lead and cadmium concentrations simultane- ously. Moreover, the potentiometric stripping technique can be used for the determination of the low lead concentrations in serum. Other important advantages are that the instru- mentation is small in size, has very low power demand, and does not have any special installation requirements such as ventilation, cooling, or supply of gas. Furthermore, the in- strument is simple to operate in that very few instrumental parameters have to be adjusted prior to analysis. None of the parts of the instrument need to be replaced regularly; the sensor electrode can be used almost indefinitely, provided that it is properly polished and cleaned and that it is mercury precoated at regular intervals.

The time for one whole blood analysis with potentiometric stripping is slightly longer than with, e.g., the carbon-rod technique. Assuming a preelectrolysis time of 2 min and one standard addition, the total time for one analysis will be approximately 5 min. Since, however, the potentiometric stripping analyzer operates semiautomatically, most of this time can be used for preparation of the next sample and evaluation of the results of the previous analysis. Thus a throughput of 12 analyses/h is feasible. If only lead and very high cadmium concentrations are to be determined, the preelectrolysis time can be decreased to 1 min, which would permit 20 analyses/h. If a large number of samples are to be analyzed, it may sometimes be advantageous to use a Cali- bration plot instead of the standard addition technique.

The potentiometric stripping procedure, as it has been reported in this paper, requires more sample than does, e.g., the carbon-rod technique. It is, however, quite feasible to decrease the sample volume from 10 to 5 mL and, probably, even further. If only lead is to be determined, the sample could be diluted down to 1:50 and by increasing the pree- lectrolysis time to, e.g., 8 min, a sample dilution of 1:2QO is feasible. For such an analysis only 25 pL of the blood sample is required.

In the potentiometric stripping technique it is necessary to handle solutions containing ionic mercury. Since only very dilute solutions have to be handled, this is not hazardous for the personnel. Ther mercury waste from the diluted blood

1410 Anal. Chem. 1981, 53, 1410-1417

samples can be estimated to correspond to 1 cm3 of liquid mercury per 50 000 analyses if sample volumes of 5 mL are used. This is negligible compared with other sources of mercury waste in chemical or clinical laboratories.

Since no computerized potentiometric stripping analyzer is commercially available, it is not possible to compare the instrumental cost between this technique and atomic ab- sorption techniques. The computerized potentiometric stripping analyzer is, however, a much less complicated in- strument that is, e.g., a carbon-rod atomizer and it is thus reasonable to assume that its cost would be considerably less than that of atomic absorption instrumentation.

ACKNOWLEDGMENT The authors wish to thank J. Pedersen, Yrkesmedicinskt

Centrum, Goteborg, for helpful advice and for placing the blood reference samples a t their disposal.

Duic, L.; Szechter, S.; Srinivacan, S. J . €!ectroanal. Chem. Interfaclal Electrochem. 1973, 4 1 , 89. Herheuval, X.; Maso, J.-L.; Baudot, P.; Hutin, M.-F.; Burnel, D. Pathol. Blol. 1975, 23, 379. Kisser, W. Arch. Toxicol. 1975, 34, 237. Lopez Fonseca, J. M.; Sanz Pedrero, P.; Bugeiro, M. T. An. Ouim. 1978, 72, 485. Morrell, G.; Giridhar, G. Clin. Chem. 1976, 22, 221. Peter, F.; Reynolds, R. G. Anal. Chem. 1976, 48, 2041. Pinchin, M. J.; Newham, J. Anal. Chim. Acta 1977, 90, 91. Searle, B.; Chan, W., Davidow, 8. Clin. Chem. 1973, 79, 76. Valenta, P.; R k e l , H.; Nurnberg, H. W.; Stoeppler, M. fresenius' Z. Anal. Chem. 1977, 285, 25. Oehme, M.; Lund, W. fresenius' Z. Anal. Chem. 1979, 298. 260. Jagner, D.; Arb, K. Anal. Chim. Acta 1979, 100, 375. Jagner, D. Anal. Chem. 1978, 50, 1924. Jagner, D. Anal. Chem. 1979, 51, 342. Graaback, A. M.; Jensen, 0. J. Ind . ResJDev. 1979, 27, 124. GranQII, A.: Jagner, D.; Josefson, M. Anal. Chem. 1980, 52, 2220. Whitehead, T. P. In "Quality Control in Clinical Chemistry"; Why: New York, London, Sydney, 1977. Sunderman, F. W., Jr. In "Chemical Diagnosis of Disease"; Brown, S. S., Mitchell, F. L., Young, D. S., Eds., Elsevier: Amsterdam, 1970.

RECE~VED for review March 2,1981. Accepted April 27,1981. LITERATURE CITED

(1) Cernlk, A. A. Roc. Anal. Div. Chem. SOC. 1978, 73, 227. (2) copeland. R.; Ostefyoung, R. A,; Skogerboe, R. K. Anal. Chem. 1974,

46, 2093. is gratefully acknowledged. Financial from the Swedish Work Fhironment Fund

Resolution of Overlapped Electrochemical Peaks with the Use of the Kalman Filter

Teri F. Brown'

Department of Civil Engineering, University of washington, Seattle, Washington 98 195, and Materials and Molecular Research Division, Lawrence Berkeley Laboratory, Berkeley, California 94720

Steven D. Brown"

Department of Chemistry, University of California, Berkeley, California Berkeley Laboratory, 1 Cyclotron Road, Berkeley, California 94 720

A major limitation In the use of electrochemical techniques for the quantitative analysis of mixtures is the difficulty of resolving overlapped peaks. This problem is further compli- cated by the low signal-to-noise ratios often encountered in trace analysis and by the use of electrochemical techniques that produce broad, asymmetric waveforms. This paper demonstrates the use of the Kaiman finer for multicomponent analysis of linear sweep voitammograms. Even with the broad, asymmetric linear sweep voltammetry waveform, synthetic data runs show that a peak separation of as little as 2.5 mV is sufficient for peak deconvolutlon In the presence of random noise. Besides separating overlapped peaks, the method also filters the noise from the signal and can be used to separate the capacitive current component from the fara- daic current component. The method is validated further by using the Cd( I I ) / In(I I I ) and Cd( II)/In( I I I) /Pb(II) systems which show peak separations of 40-200 mV. The use of the techniques with two other voltammetric waveforms Is also demonstrated.

Electrochemical techniques show great promise for the analysis of complex mixtures because of their ability to de- termine both concentrations and stability constants. Un- fortunately, the methods are limited by the difficulty of re- solving overlapped peaks. Recent techniques have been able

Present address: D e ar tment of Chemistry, Washington State University, Pul lman, WR 99164.

94720, and Materials and Molecular Research Division, Lawrence

to separate peaks which are similar in height and have half- wave potentials as close as 30-40 mV using either semideri- vative voltammetry (1) or square-wave voltammetry (2) com- bined with modern data analysis techniques. This paper presents a new technique based on the Kalman filter which has the ability to resolve peaks which are much closer together and which is applicable to a wide variety of waveforms.

Three other approaches have been used for the deconvo- lution of overlapped electrochemical peaks. The first approach fits the data to a previously defined functional shape such as the sech2 function used in convolution voltammetry (I) or the empirical functions used by Perone and co-workers in sta- tionary electrode polarography (3) and in square-wave vol- tammetry (2). In this method, the peak shape of each com- ponent is defined by the general functional shape and three to nine parameters. This procedure may be used to reduce the noise in a single component system or to separate peaks and reduce noise in a multicomponent system. Besides the general problem of choosing the correct functional shape, there are three important limitations to the method: (1) the correct number of components must be specified, (2) the peaks must be separated enough to be distinguishable (this is usually expressed as a minimum separation per unit width of the wider peak), and (3) the computational load is heavy, Le., to fit a four-parameter system would require the iterative determi- nation of 12-36 parameters.

The second approach, which is less mathematical in nature, uses a microprocessor-controlled polarograph to subtract a background solution from the multicomponent solution ( 4 ) . Aliquots of all but one of the components are added into the

0003-2700/81/0353-1410$01.25/0 0 1981 American Chemical Society