Square wave voltammetry - a rapid and reliable determination method of Zn, Cd, Pb, Cu, Ni and Co in biological and environmental samples

51

J. Elecfrwnul. Gem.. 214 (1986) 51-64 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

SQUARE WAVE VOLTAMMETRY - A RAF’ID AND RELIABLE DETERMINATION METHOD OF Zn, Cd, Pb, Cu, Ni AND Co IN BIOLOGICAL AND ENVIRONMENTAL SAMPLES

P. OSTAPCZUK,

P. VALENTA

and H.W. N~RNB~RG

Insrzruie of Applied Physrcal Chemistv, D-B 70 Jiilich (E R.G.) (Received

*

Nucfeur Research Center (KFA), Jiihch, P.O. Box 1913,

19th May 1986)

ABSTRACT

The application of square wave voltammetry (SWV) for the routine determination of six elements has been examined and optimized. Square wave voltammetry is more sensitive for the determination of Zn, Ni and Co in comparison with differential puke am&c stripping voltammetry (DPASV). For Cd and Pb, no significant improvement in sensitivity was observed. At Cu concentrations < 1 pg I- ‘, only DPASV can lead to satisfactory results. The great advantage of SWV is the shortening of the determination time for Zn, Cd, Pb and Cu, which is only half the time needed by DPASV and similar to the time needed for the determination of one element by ET AAS.

The principal obstacle in efforts to increase the sensitivity of the polarographic method of analysis is the problem of separation of the charging current and the faradaic current. Several instrumental variations have been developed to improve the discrimination between these two forms of current. One of these variations, differential pulse polarography or voltammetry, has gained wide acceptance as an analytical method for trace concentrations of several metals [l-3]. Another approach to the charging current problem is square wave voltammetry (SWV), first reported by Barker and Jenkins in 1952 at the dropping mercury electrode [4,5], although its first applications were limited by the lack of good commercial instrumentation at reasonable cost. However, recent developments in theory and instrumentation in a number of laboratories, coupled with analytical applications, indi-

* Dedicated

to the memory of Professor H.W. Nbrnberg.

Fig. 1. Potential-time

wave-form in SWV.

cate that square wave polarography is becoming an electrochemical method with a wide range of application [6-lo]. Modem square wave voltammetry is a complex but powerful technique that requires the power and flexibility of the microcomputer for its development and a modem low-cost microprocessor for its commercial implementation. The potential-time wave-form applied in square wave voltammetry (Fig. 1) consists of a series of staircase steps of the same width as the square wave period. On each staircase step, a symmetrical square wave is superimposed. In the first half of each period, the square wave pulse is applied in the same direction as that of the staircase step and is referred to as the forward pulse. The current measured during this pulse is referred to as the forward current, i,. Correspondingly, during the second half period the square-wave pulse is applied in the opposite direction to that of the staircase step and the negative current is termed the reverse current, i,. The usual signal output in the modem version of SWV [ll] is the difference between the mean forward current, i,, and the mean reverse current, i,, during the measuring time and is referred to as the measured current, Imeas. The parameters of SWV are the square wave frequency, f, the step height of the staircase, E_,, and the height of the square wave pulse. E,,. The aim of this work is to examine whether square wave voltammetry is advantageous, or even the method of choice, for simultaneous determination of several heavy metals in biological and environmental samples.

EXPERIMENTAL

Instrumentation A polarographic analyzer system consisting of a model 384 B console, DMP-40 digital plotter, model 303A SDME and model 305 stirrer was purchased from

53

EG&G, Princeton, N.J., USA. The 384 B version is a microcomputer based system which offers eight different electrochemical techniques, an automated evaluation of the voltammetric curve and printing of the analysis result. Chemicals Ferchloric, nitric and hydrochloric acids and ammonia solution were Merck “Suprapur”. The supporting electrolyte for deter~nation of Zn, Cd, Pb and Cu was 0.02 1M perchloric acid and for Ni and Co 0.05 1M ammonia buffer (NH, + HCl) with 1 X 10F4 M d~methyl~yo~me (reagent grade) (121. The water used for making the solutions was supplied from the reagent-grade ion exchange system Mini-Q 15 (Millipore, Bedford, Mass., USA). Sample pretreatment The universally applicable wet digestion procedure of a mixture of nitric and perchloric acids used for various biological and environmental materials has already been described elsewhere [13]. Procedure A series of optimization studies was made. Table 1 shows the operating parameters for the determination of all six elements. The parameters described in the table as variables are discussed in detail below.

TABLE 1 Operating parameters for SWV determination of some metals with polarographic analyzer model 384 B EG&G. For all experiments, the M (medium) drop size was used (A = 0.015 cm2 ) Parameters

Initial potential/V Final ~otentiaI/V Purge time/s a Condition time/s Condition potential/V Equilibration time/s Deposition time/s Pulse height Frequency Scan increment Forced linear fit Blank substraction

Elements Zn

Cd, I%, Cu

Ni, Co

-1.2 -0.8 240 (30) 0 0 5 30 var. var. VtW.

-0.7 0.2 240 (30 s) 120 -1.0 5 120 var. var. var.

-0.7 - 1.25 240 (30) 0 0 5 30 var. var. var.

yes yes/no

yes yes/no

yes yes

a A purge time of 30 s was used after standard addition.

The square wave amplitude, ES,,, can be changed between 1 and 250 mV, and its effect on peak height and background current as well as on the resolution of adjacent peaks was studied. The square wave frequency was varied between 1 and 120 Hz and the staircase step height between 1 and 10 mV. As a result of these studies, an optimal set of parameters for routine determination of the investigated elements has been found. RESULTS AND DISCUSSIONS

The results of the optimization of the square wave amplitude, Em, are shown in Fig. 2. The peak current increases linearly with increasing amplitude to a value of about 30 mV. For higher ~p~tudes, a m~mum peak current for Zn, Cd, Pb and Cu is attained. For Ni and Co, where the determination is based on a different electrochemical process, the peak current levels off at higher square wave amplitudes. The observed current is, however, significantly higher than that for other elements at the same bulk concentration and the same deposition time. For Zn, Cd, Pb and Cu, which are soluble in mercury, the oxidation current is controlled by the diffusion of these metals into a mercury drop. For Ni and Co, which are reduced as complexes adsorbed on the mercury surface, the rate determining step of the reduction current is the electron transfer and the process is thus similar to oxidation of a thin layer of metals [14]. Figure 2b shows voltammograms for Cd and Pb ion solutions obtained with square wave amplitudes, E,,, of 1,20 and 80 mV, respectively. The improvement in resolution of Cd and Pb peaks, if smaller square wave amplitudes are used, is obvious. Also the baseline becomes smoother. Larger square wave amplitudes provide greater sensitivity at the expense of poorer resolution of overlapping peaks. For amplitudes higher than 50 mV, peak deformation was observed. The peak current increases nearly linearly with increasing square wave frequency, f (Fig. 3). Here also the peak currents for Ni and Co, respectively, increase more steeply with increasing frequency than for other elements. In contrast to the amplitude, the square wave frequency has no influence on the peak shape and the resolution of overlapping peaks. Figure 4 represents voltammograms of a solution of Ni and Co ions, obtained with staircase steps, ES,, of 1,4 and 10 mV, respectively. When smaller staircase steps are used, the resolution of both peaks is very good. Larger staircase steps provide a significant increase in peak current at the expense of poorer resolution of overlapping peaks. For staircase steps higher than 5 mV, awkward peak deformation was observed. No significant influence of staircase step on background current was noted. The results of this optimization study show that the set of square wave amplitude of 20 mV, frequency of 100 Hz and staircase step of 2 mV is best suited for routine determination of these metals. The last two parameters determine the optimum scan rate of 200 mV s-l. Direct comparison of square wave voltammetry with differential pulse polarography may be misleading. Most commercial differential pulse voltammetric instruments sample the current approximately 50 ms after the application of

55 (a) Ni

I

E

‘*’

I 4.61

= SW

E

100

50 ,(b)

6W

*

1 mV

4.1

4.1

-

6.’

I

’

-0.6

4.4

4.2

*

‘*’

I

8

a.6

I4

-0.2 c-

Fig. 2. (a) Influence of the square wave amplitude, E,. on the peak current, I. cMc: 5 ng ml-‘;. deposition time, t,,: 60 s. (b) Influence of the square wave amplitude, E,, on the peak resolution. cc-: 6 ng ml-‘; cpb: 10 ng ml-‘, deposition time, r,,: 60 s: f: 100 Hz: Es_: 20 mV.

a single pulse to each drop. To compare square wave voltammetry over a similar time interval would mean using a 10 Hz square wave frequency. In that case using a 50/n mV pulse amplitude for both techniques and a 10 Hz square wave, the ratio of the peak square wave current to that of the differential pulse would be only 1.3 : 1,

Zn Cd

Fig. 3. Influence of SW% frequency, f, on the peak current. cMc: 5 ng ml-‘; deposition time: 60 s: E SW: 20 mV; E_: 2 mV.

E

step

=

1

mv

E

step = 4 nlv

E

step =

10 mV

Ni

4

4 E-

lico

3. u < H m$?2.

3 E-

a <

w2 "0

i

-0.8

-1.0

-1.2

0.

~ -0.B

-1.0 E/V

1.

-1.2

E.

~ ,0. E

-1.0

-1.2

E/V

Fig. 4. Influence af staircase step height on the peak resolution. cMC: 5 ng ml-‘; E_,,: 20 mV.

I~: 60 s: f: 100 Hz:

corresponding to a 30% increase in sensitivity [ll]. However, for routine determination, higher square wave frequency is normally used, leading to larger peak heights. Figure 5 represents voltammograms obtained with square wave and differential pulse voltammetry, respectively, for Zn, Cd, Pb and Cu in a rain water sample. For SWV and DPASV, the optimal parameters normally used for routine determination of these metals were chosen (Table l), whereas the deposition time for both methods was the same. For the Zn determination, square wave voltammetry is clearly the more sensitive method. The SWV peak current of Zn is about 5 times larger than

WV

zn

i

E/V

I

DPASV

CU

E/V

Fig. 5. Determination see Table 1).

of Zn, Cd, Pb and Cu in rain water sample by SWV and DPASV (for parameters

58 TABLE 2 Influence of pH on the SWV and DPASV Zn peak currents. Deposition time: td = 30 s, cZn = 20 ng ml-’ PH

4.69 4.0 3.0 2.0 1.69 1.40 1.0 0.69 0.4

Peak current/nA

Peak potential/V

swv

DPASV

SWV

DPASV

42.10 42.00 42.40 42.60 43.08 43.20 42.80 38.18 20.35

6.20 6.18 6.46 1.74 8.21 8.92 9.36 8.97 3.40

-0.944 -0.940 - 0.936 -0.940 - 0.948 - 0.956 - 0.968 - 0.976 - 0.980

- 0.948 - 0.944 -0.946 - 0.966 - 0.974 - 0.984 - 0.998 - 1.008 -1.000

that of DPASV, whereas the hydrogen reduction current is only 3 times higher using SWV than using DPASV. Moreover, with the parameters used, the DPASV zinc peak appears at a 30 mV more negative potential than that of SWV. Consequently, the DPASV zinc peak is not so well separated from the hydrogen reduction current. This interference appears only in solutions having low pH values. For the determination of Zn concentrations lower than 10 pg l-‘, the use of salts or buffer solution of pH > 3 is not recommended because of the contamination risks. In the concentration range mentioned, the best supporting electrolyte is a 0.01 A4 acid solution (HCl, HClO, or HNO,, respectively, Merck, Suprapur). Table 2 represents the influence of pH on the SWV and DPASV zinc peaks, respectively. The highest SWV peak is obtained at pH 1.4. Analogously, the peak current increase was observed using SWV for Cd and Pb, whereas the ratio between the peak current and the baseline current is similar for both methods. The Cu peak obtained by SWV is badly shaped in contrast to that obtained by DPASV. Like that of Zn the SWV peak potential of Cu is shifted by about 30 mV towards positive potentials with respect to that of DPASV. If the analyzed solution contains anions which enhance mercury oxidation and do not change the Cu peak potential, the determination of Cu by SWV is difficult (Fig. 5). Figure 6 shows Cu determination in a human bone sample with a bulk Cu concentration of about 3 ng ml- ‘. By square wave voltammetry with a normally used frequency of 100 Hz, only a little wave superimposed on a steeply increased mercury oxidation current was observed. If the SW frequency is changed to 25 Hz, a very well shaped copper peak results with a height similar to that obtained by DPASV. The decrease in frequency leads to a significant decrease in the Hg oxidation current and thus to better resolution of the Cu peak. Thus for low Cu bulk concentration, the diminution of the SW frequency, or the use of DPASV, are necessary to cope with the masking of the Cu peak by the Hg oxidation response.

59

(C)

(Bf

(A)

2.

2.

8.

t

DPASV 6-C

F =

100 Hz

l.Ss \ -t.la"9

KS-

Fig. 6. Copper determination fd: 120 s.

cu

in human bone sample by SWV (a, c) and DPASV (b). cc-: 3.6 ng ml-‘:

Another problem appears in the Cu determination in solutions containing an incompletely destroyed organic matrix. Figure 7 represents a voltammogram of Cu determination in a human brain sample after pressurized digestion with nitric acid 1151. The later digestion method is used preferentially for unique samples of individual persons in which several metals have to be determined. The peaks obtained by SWV and DPASV indicate the presence of two different copper forms in the solution. The peak at -0.016 V corresponds to a Cu(1) complex and the second peak at 0.072 V to CufII) compounds. In DPASV, the Cu(I) peak is only

DPPSV

(8)

s.m-

*N

I -e.2

0.0 E/V

Fig. 7. Copper determination in human brain sample after pressurized digestion with HNO,. electrolyte: 0.02 M HCIO, (A, B) and 0.1 M HCI (c). cc”: 3.9 ng ml-‘; td: 120 s.

Supporting

60

poorly shaped. The determination of total copper concentration in this solution is thus impossible. Changing the supporting electrolyte to 0.1 M hydrochloric acid eliminates these difficulties and a well shaped peak of Cu(1) results in both voltammetric methods. Analogously, the use of hydrochloric acid as a supporting electrolyte enables the direct determination of copper in urine samples 1161. Our investigation has shown that, in the latter case, DFASV gives better results for the direct determination of Cu in urine than SWV, if the concentrations of Cu lie in the range for non-exposed persons (lower than 10 ng ml-‘). The use of computer controlled equipment for a flexible choice of parameters makes it possible to eliminate the background current. The procedure of background current correction is very simple and is described in detail in the instrument manual. The blank correction leads, in some cases, to a sig~fi~nt increase in the determination sensitivity. Thus, for the determination of Ni and Co in body fluids, the background correction helps to eliminate the Ni signal corresponding to Ni contamination of the supporting electrolyte and to increase the sensitivity of Co determination [l?], as shown in Fig. 8. Under conditions described for SWV in Table 1, Ni can be determined with about at 40 fold increase in sensitivity depending on the efficiency of the pretreatment - compared to the differential pulse mode. For Co, the sensitivity increase is about 30 fold. The background current is a function of the capillary properties and the capillary age. New capillaries have very good electrochemical properties and their use supports a low, constant baseline. Afterwards, depending on the water quality and the kind of solution analysed, the capillary properties worsen and the background current is higher. In SWV, irregularities of the baseline are often observed in the Pb potential range (Fig. 5). In this case, the Pb determination performed without blank

tm

AOPV

swv HI

2 ho

0 _>.-I.1

”

-13.8 _;l.O

”

Fig. 8. Comparison between SWV and ADPV Ni and Co determination respectively. cNI: 0.5 ng ml-‘; cc,: 0.2 ng ml-‘.

61

correction is imprecise. Unfortunately, sometimes the baseline is changed after the addition of the sample solution and then the background correction can lead to analytical errors. So, for the determination of Cd and Pb at concentrations lower than 0.1 pg l-‘, the use of a new capillary is recommended. For routine determination at higher bulk concentrations, the background correction is very useful as it produces mainly the same improvement as a change of capillary. For biological and environmental samples, complete digestion of the organic material before voltammetric determination is usually necessary. In ultratrace analysis and particularly in the determination of volatile elements, the pressurized decomposition was found to be advantageous in comparison with the frequently open digestion procedure most used [18]. Nitric acid is used as the main oxidant in decomposition vessels made from PTFE. With this decomposition method, only partial ashing is achieved, even for relatively simple materials that could otherwise be mineralized quantitatively by wet digestion at higher temperatures. The incomplete ashing is usually sufficient for subsequent ET AAS determination, but not always for voltammetry. Figure 9a represents Cd and Pb voltammograms obtained in a solution after pressurized digestion of a human liver sample. Presence of NO, in the solution renders the determination of Cd and Pb with DPASV impossible. The great virtue of square wave voltammetry is the rejection of all currents that correspond to slow electrode processes. In voltammetric trace analysis, oxygen reduction often produces an unacceptably large and unstable background current. However, with SWV the oxygen reduction current is almost completely rejected [19]. Similar elimination of

(b)

(a)

&C-

ADPV

Fig. 9. Determination of Cd and Pb by DPASV and SWV (a) and Ni and Co by ADPV and SWV (b) in human brain sample after pressurized digestion.

3

33.3 42.9 144

0.112 0.048 0.868

ET AAS

HNO,

0.192 0.634

swv

ICdl/mg ks- ’

comparison

swv 0.180 0.051 0.904

pg kg-’

189 (n = 5) 186 (?I = 6)

swv DPASV

2.5 2.5

rsd/ w

’

11.8 (n = 5) 11.9 (n = 6)

Lead/ mg kg5.6 4.0

%

rsd/

’

23.1 (n = 5) 22.6 (n = 6)

Copper/ mg kg-

5.1 6.4

w

rsd/

3.52 1.51 60.0

3.64 1.49 61.5

swv

98.5 (n = 4) 95.0 (n = 5)

Nickel/ mg kg-’

3.63 1.54 60.1

sws

3.2 2.6

rsd/ w

8.67 7.82 15.7

8.15 6.87 17.8

swv

14.4 (n = 4) 14.4 (n = 5)

Cobalt/ mg kg-’

ET AAS

HNO,

ET AAS

ICul/mg kg-’

HNO, HN&/ HClO,

fW/mg kg-’

SWV and ET AAS

HNO, / HClO,

between

of Cd, Pb. Cu, Ni and Co in a soil sample by SWV and DPASV

Cadmium/

determination

Determination method

4

Routine

35.8 39.9 125

swv

swv

39.6 45.7 144

ET AAS

’ HN%’ HClO,

kg-

HN03

fznl/mg

of Zn, Cd, Pb and Cu in grass samples,

TABLE

Stolberg Yerseke, NL Dortmund

Grass

Determination

TABLE

7.4 7.5

rsd/ 0

9.21 7.68 17.1

swv

HNQ’ HClO,

63

the NO, reduction current can be achieved with SWV as in the same solution, the determination of Cd and Pb with the latter method is possible if the concentrations of the determined elements are > 1 ng ml- ‘. For lower concentrations, pretreatment with perchloric acid is necessary. Also for Ni, and especially for Co, determination in the solution after pressurized digestion, square wave voltammetry permits greater precision of determination than the differential pulse mode (Fig. 9b). SWV determination of Zn, due to its relatively high concentration in environmental samples and the enhanced sensitivity of SWV, is preferable in all cases. The problems with Cu determination have already been described above (Fig. 6). Table 3 summarizes the results of heavy metal determination in various grass samples after pressurized digestion with HNO, using ET AAS or SWV as the determination method. The concentrations of these metals found by SWV agree well with the respective AAS values. In one case of lower Cd concentration, the determination was not possible. If wet digestion with a mixture of per&lo& and nitric acids was used, the Cd determination proceeded without difficulty, A great advantage of the voltammetry in comparison to AAS is the wide bulk concentration range in which the determination of metals soluble in mercury is possible. Long deposition times make the determination of some ng 1-l by DPASV or SWV possible. For concentrations in the ppm, or even higher, range, changing to the direct method (for example differential pulse polarography) is recommended. Table 4 represents results of a simultaneous determination of six elements in a soil sample with SWV and DPASV after wet digestion with a HNO,/HClO, mixture. For both methods, the standard deviation was 5% or lower for all metals, with the exception of Co, and no significant differences in the reproducibility between SWV and DPASV were noted. An earlier disadvantage of voltammetry was the longer time needed for a single analysis and hence the relatively high manpower required. SWV offers a great advantage in that an experiment is performed much faster than using the normal and differential pulse techniques. Whereas the latter techniques typically run at scan rates of 1 to 10 mV s-l, SWV enables the use of scan rates up to 1 V s-l, or more, allowing the determination time to be cut substantially. The comparison between

TABLE 5 Time comparison for the determination of heavy metals by voltammetry and AAS (the time was calculated for two standard additions) Determination method

Number of determined elements

Analysis time

SWV(v=2OOmVs-‘) DPV(~=5mvs-‘) ET AAS Flame AAS

4 4 1 1

20 40 20 20

min min min s

the determination time for SWV and DPASV, respectively, in Table 5 shows that the time needed for the simultaneous determination of 4 elements, i.e. Zn, Cd, Pb and Cu, by SWV is half that by DPASV and similar to the time needed for the determination of 1 element by ET AAS. CONCLUSIONS

It has been demonstrated that square wave voltammetry is a very sensitive and rapid analytical technique. From our investigation, the result is that SWV is more sensitive for the determination of Zn, Ni, Co and also Se than the differential pulse mode. It can be expected that SWV will be very sensitive due to its high scan rate in all cases where the reacting species is accumulated by adsorption on the electrode surface. This mode includes elements forming insoluble compounds with mercury (Cl-, SCN-, Se, Te, etc.) or with other elements (Cu,As,, Cu,Se, CuzIz, etc.) on the drop surface as well as compounds adsorbed on the mercury surface (Ni, Co, Al). For Cd and Pb determination, SWV gives no important improvement in determination sensitivity in comparison to DPASV. At Cu concentrations < 1 pg l-‘, only DPASV can lead to satisfactory results. The short analysis time in SWV makes this technique very attractive for routine determination of the described elements in biological and environmental samples. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

19

H.W. Niimberg, Sci. Tot. Environ., 12 (1979) 35. H.W. Nbrnberg, Electrochim. Acta, 22 (1977) 935. H.W. Nbrnberg, Anal. Chim. Acta, 164 (1984) 1. G.C. Barker and J.L. Jenkins, Analyst, 77 (1952) 685. G.C. Barker and J.L. Jenkins, Anal. Chim. Acta, 18 (1958) 118. MS. Krause and L. Ramaley, Anal. Chem., 41 (1969) 1365. E.B. Buchman and D.D. Soleta, Talanta, 30 (1983) 459. Z. Stojek and J. Osteryoung, Anal. Chem., 53 (1981) 847. J.E. Anderson and A.M. Bon, Anal. Chem., 55 (1983) 1934. J.A. Turner, J.H. Christle, M. Vukovic and R.A. Osteryoung, Anal. Chem., 49 (1977) 1904. J.G. Osteryoung and R.A. Osteryoung, Anal. Chem., 57 (1985) 101A. B. Pihlar, P. Valenta and H.W. Nhmberg, Fresenius Z. Anal. Chem., 307 (1981) 337. P. Ostapnuk, M. Goedde, M. Stoeppler and H.W. Ntimberg, Fresenius Z. Anal. Chem.. 317 (1984) 252. Kh.Z. Brainina, Stripping Voltammetry in Chemical Analysis, Keter Publishing House, Jerusalem, 1974. M. Stoeppler, P. Valenta and H.W. Nllmberg, Fresenius Z. Anal. Chem., 297 (1979) 22. J.P. Franke and R.A. de Zeeuw, Pharm. Weekbl., 111 (1976) 725. P. Ostapczuk, P. Valenta, M. Stoeppler and H.W. NUmberg, in S.S. Brown and J. Savory (Eds.), Chemical Toxicology and Clinical Chemistry of Metals, Academic Press, New York, 1984, pp. 61-64. M. Stoeppler and H.W. Nhmberg, Technique and Instrumentation in Analytical Chemistry, Vol. 4, Part B: Hazardous Metals in A. Vercruysse (Ed.), Human Toxicology, Elsevier, Amsterdam, 1984, p. 104. M. Wojciechowski, W. Go and J. Osteryoung, Anal. Chem., 57 (1985) 155.