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Comparison of detector systems in oxidative stripping potentiometry
Laboratory automation and information management
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Laboratory
Automation
and Information
Management
33 (1998) 207-215
Comparison of detector systems in oxidative stripping potentiometry Christopher W.K. Chow
*,
David E. Davey, Dennis E. Mulcahy
Analysis and Sensors Group, School of Chemical Technology, Received
University of South Australia, The Levels, South Australia 5095, Australia
15 January
1998; accepted
13
June 1998
Abstract
For oxidative stripping potentiometry (OSP) measurement, the achievement of reliable results is highly dependent upon the transport of oxidant to the electrode surface and, as a consequence, careful hydrodynamic control is crucial for reproducibility. Two types of detector system, batch and flow, with three different electrode systems, a standard voltammetric magnetic stirrer cell assembly, an voltammetric cell with optically controlled stirrer and a thin layer flow cell with peristaltic pump were compared. In the sensitivity studies, the static stripping procedure enhanced the sensitivity of the measurement compared to forced convection stripping. The use of a flow cell and peristaltic pump provided all the flexibility needed for OSP measurement. The sensitivity can be improved using the combination of fast deposition and slow stripping flow rates. From the viewpoint of reproducibility of the signals, excellent results were obtained for the use of the flow cell with peristaltic pump. 0 1998 Elsevier Science B.V. All rights reserved. Keywo&c
Oxidative
stripping
potentiometry;
Different detector systems: Sensitivity;
1. Introduction
Oxidative stripping potentiometry (OSP) is a relatively new technique for trace metal analysis particularly when seen against the long history of other electrochemical methods [ 1,2]. Since the first report of this technique in 1976 [3], numerous applications to a variety of analyses have appeared in the literature. The technique is based on aspects of the already well established anodic stripping voltammetry (ASV). OSP is, however, rapidly coming to be a preferred technique for trace metal analysis and it requires
* Corresponding author. Australian Water Quality Centre, Private Mail Bag, Salisbury, South Australia, 5108, Australia. Fax: + 61-8-825-90228; E-mail: [email protected]
Reproducibility;
Copper analysis
simpler equipment [4-61. Apart from the excellent detection limits and sensitivities, stripping potentiometry also has the advantages of multi-element capability, allowing the determination of several elements in the same sample and the option of speciation by proper selection of the experimental conditions [7]. OSP, makes use of the same preconcentration step as ASV but follows it with a different stripping step. The amalgamated metals are brought back from the surface of the working electrode into solution by a chemical oxidation. Alternative oxidants include dissolved oxygen, Hg*+, MnOCr O*-, Fe3’ and Ce4+. All metals amalgamatld’ at tie ‘working electrode will be re-oxidised at a rate dependent upon the rate of transport of the oxidant to the working elec-
0925.5281/98/$ - see front matter 0 1998 Elsevier Science B.V. All rights reserved. PII: Sl381-141X(98)00006-9
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trode. If the potential of the working electrode is monitored there will be a stepwise increase in the potential with time. The time duration of each potential step is proportional to the concentration of the relevant analyte in the sample. Both qualitative and also quantitative information can be obtained from the resulting potential (E) vs. time (t) curve. The shape of this curve is similar to that of curves obtained in chronopotentiometry. The time elapsed (stripping time) in each step is proportional to the amount of the given metal deposited at the electrode
[31. For OSP measurement, the achievement of reliable results is also highly dependent upon the transport of oxidant to the electrode surface, and, as a consequence, careful hydrodynamic control is crucial for reproducibility. Sensitivity is directly proportional to deposition time and indirectly proportional to oxidant concentration and oxidant transportation rate. It is a general practice to stir during both the deposition and stripping steps [3,8]. Therefore a well-defined hydrodynamic environment during the stripping step, such as that achieved by using a spinning electrode system [7] or by using a flow system [9,10], is essential for this technique. The authors believe that in the near future many laboratories will construct their own OSP equipment because of its sensitivity for metal ion analysis and the simplicity of the equipment required [4-61. This paper describes a practical comparison of the performance of three different detector systems. These are (a) a cell assembly containing a magnetic stirrer, (b) a stirrer cell with optical speed control adapted from a commercial ASV instrument (PDV 2000, Chemtronics), and (c) a flow system based upon a peristaltic pump and a thin layer flow cell.
2. Experimental In this study, the sensitivity and reproducibility of three different cell assemblies for OSP were compared. A diagram of the three cell assemblies is presented in Fig. 1. The sensitivity study was performed using a calibration in the range of O-1000 ppb and reproducibility of the stripping signal was examined using a 100 ppb copper solution by a repeat cycle of l-100 replicate analyses. In addition
33 (1998) 207-215
to the main comparison, various experimental parameters within the cell assembly were also studied. 2.1. Instrumentation The equipment employed in this study included a CV- 1B Cyclic voltammetry instrument (Bioanalytical Systems, IN, USA) as the potentiostat, a 701A Digital Ionalyzer (Orion Research, MA, USA) for potential measurement and a PCL-712 Multi-Lab Card (Advantech, Taiwan) for data acquisition and control. An IBM compatible 386-SX personal computer (Microbits, Adelaide, South Australia) was used to control the measurement system and store the recorded potentiogram. The potential was sampled at 8 kHz and the 1Zbit analogue-to-digital converter gives a 1 mV resolution. Details of this instrument have been reported elsewhere [6]. 2.2. Mercury precoating
and initial preparation
The glassy carbon working electrode was polished using a BAS PK-4 polishing kit (Bioanalytical Systems), then rinsed with ethanol and dried in air. It was then pre-plated with a mercury film from a 30 ppm Hg(I1) solution in 0.1 M HCl at - 900 mV for 5 min. 2.3. Electrode and cell assembly Three different electrode systems were studied in this experiment. (a> A cell assembly containing a magnetic stirrer with a 3 cm X 1 cm (length X height) flea (rotation rate approximately 200 rpm), a 3 mm diameter glassy carbon electrode (Chemtronics, Bentley, Western Australia), employed as the working electrode, a platinum wire, used as the auxiliary electrode, and a Ag/AgC1/3 M KC1 double junction electrode (HNU ISE 40-02-00, HNU Systems, USA) used as the reference. (b) A stirrer cell with optical speed control adapted from a commercial anodic stripping voltammetry instrument, the PDV 2000, (Chemtronics). This cell is designed as an integrated unit which consists of a glassy carbon (3 mm diameter) working electrode, reference electrode, auxiliary electrode, stirrer, sample inlet and drain [ll]. Four stirring rates, 500,
C. W.K. Chow et al. /Laboratory
1Ocm
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33 (1998) 207-215
209
Motor C
25 r
Stirrer
Auxiliary Electrode
Sa
Reference Electrode
Working Electrode Vent/
lcm
I
Pig. 1. The experimental arrangement of the three cell assemblies. (a) magnetic stirrer cell assembly, (b) cell with optically controlled adapted from the PDV2000, and (c) flow system based upon a peristaltic pump and a thin layer flow cell.
1000, 2000 and 4000 rpm, were selected in this comparison. (c) A flow system based upon a Gilson Minipuls 3 peristaltic pump (Gilson France, France) and a thin layer flow cell with glassy carbon (3 mm diameter) working electrode. In all cases, the working electrode was pre-plated with a mercury film from a 30 ppm Hg(I1) solution in 0.1 M HCI at - 900 mV for 5 min. Five different flow rates, 0.6, 1.2, 2.4, 3.6 and 4.8 ml/mm, were selected in this experiment.
2.4. Experimental
procedure
For the magnetic stirrer cell (Fig. la), a 50 ml aliquot of a 10 ppm Hg(I1) solution in 0.1 M hydrochloric acid (AR) was transferred to the analytical
stirrer
cell. Initial deaeration was carried out by bubbling high purity nitrogen gas into the mixture for 15 min so that the Hg*+ ion would be the sole oxidant. A sensitivity test in the range of O-1000 ppb was then performed with ten 50 ~1 additions of a 100 ppm copper nitrate standard solution. A 1-min plating time at - 900 mV was selected for the experiment. A further reproducibility study of the stripping signal of 100 ppb Cu was examined by a repeat cycle of l-100 replicate sets of analyses. For the optical speed controlled stirrer cell (Fig. lb), a 10 ml aliquot of a 10 ppm Hg(I1) solution was used and similar procedure was applied for both the sensitivity and reproducibility studies. For the flow cell (Fig. lc), the procedure was identical to the magnetic stirrer cell as it was used to hold the solution for the flow system.
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3. Results
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and discussion
3.1. Presentation
of the potentiogram
During the stripping step in OSP, the electrode potential is monitored at constant frequency; if a potential reading falls within a given potential range the count in the corresponding potential range is incremented by one. This method of recording the potential count versus potential is known as multichannel potentiometric monitoring [ 121. The recorded data can be presented either as an E vs. t curve [ 131 or in the form of peaks (which is equivalent to an inverse derivative of the conventional potentiogram) [14]. The use of multichannel potentiometric monitoring is extremely favourable for use in a computerised system. Once the raw data is obtained (Fig. 2a), then, as stated earlier, it can be processed into either an E vs. t curve [3] as illustrated in Fig. 2c or (a) 800 P S 0
600
-z .g 400 LI1 2 ‘& 200
-300
0
-100
-200
Potential (mV) “,, ‘, ‘X
r.---.--../. (cl
@)
-400
0.06
0.00
L-J
-zoo
-200
OI
-100
Potential(mV)
0.0
0.5
1.0
1.5
20
0 Stiippmg
Time(s)
Fig. 2. (a) The stripping signal for 100 ppb Cu in 0.1 M HCI obtained from the magnetic stirrer cell, using a I-min plating time as oxidant, recorded using the multichannel and 10 ppm Hg” potentiometric monitoring method, (b) the raw potentiogram of (a) after polynomial smoothing with 25.point window size and (c) the raw potentiogram of (a) after conversion to the conventional potential vs. time curve.
33 (1998) 207-215
as an inverse derivative potentiogram [14] as illustrated in Fig. 2b. In current literature terminology these are not distinguishable and both are called stripping potentiograms. However, the latter may be considered as an improvement of the former and is widely used in the more recent literature. Of course, presenting the potentiogram in the form of peaks is more favourable for the interpretation of results. In this paper, both forms are used. In order to minimise confusion, the E vs. t curve will be named the stripping curve while the inverse derivative potentiogram will be referred to stripping peaks. 3.2. Determination
of analytical parameters
The analytical parameter stripping time is defined by the time which elapses between two consecutive elemental ‘equivalence’ points in the conventional potentiogram (i.e., in the stripping curve>. The time is proportional to the concentration of the particular metal in the solution 131. Fig. 3a shows two methods for stripping time determination from a stripping curve. In Fig. 3a(i), the lines a and b are drawn parallel to the stripping curve just before and after the plateau. Line c is drawn parallel to the slope of the stripping curve on the plateau. The distance between the intersections of line c with lines a and b is then taken as stripping time. In Fig. 3a(ii), line c is replaced by line c’, which is drawn parallel to the time axis to intersect the stripping curve at the half potential point. In this case, the distance between the intersections of line c’ with lines a and b is taken as stripping time. Under normal conditions, the stripping times determined by these two methods do not differ [15]. However, when the stripping time is determined by computer, to develop an algorithm for the determination of stripping time from a conventional potentiogram is rather complicated. Cladera et al. [16] have reported a computerised method which made use of three regression equations to represent the three lines a, b and c on a stripping potentiogram and the stripping time was determined by solving the three equations simultaneously. The use of the stripping peak approach (Fig. 3b) to present the analytical result is more suited to a computerised system. Renman et al. [17] reported a method to determine the stripping time (peak area) and stripping potential which is particularly easy to
C.W.K.
Chow et al./L.aboratory a(ii)
4
Automation
(b)
-500 -400 -300
:
;” \
1s
0.02s
1
b’,,,
\ -b,
i
\
-200 -100
c’
i ‘,y_L ,‘a
0
T = stripping
-c\
~‘I-:-__;(. ‘,‘\ ‘,a
-
‘L L ,_>
.
i/ P&Area=‘5 \
time
Fig. 3. Potentiograms for 100 ppb Cu in 0.1 M HCI, using a l-mm plating time, and 10 ppm Hg2+ as oxidant: means of presenting PSA data: showing (a) the stripping curves (i) and (ii) are two difference methods to determine stripping time, and (b) the stripping peak.
implement. A simplified method which involves a preliminary manual examination of the stripping curve or peak to determine potential ranges for the elements involved can also be used. The number of data points within the predetermined potential range for an element is then counted, and divided by the sampling rate. This method gives essentially the same stripping time as found by the literature methods mentioned and is less demanding on programming [4]. 3.3. Effect of mass transfer rate For stripping analysis, such as ASV with stirred deposition and static stripping, the sensitivity of the measurement is dependent upon the duration (deposition time) and the mass transfer rate in the deposition step. In general a higher stirring or flow rate, which improves the mass transfer rate, improves the sensitivity, as more analyte can be transported to the working electrode. In OSP, the sensitivity also depends on the concentration of the oxidising agent and on the transportation rate of the oxidising species to the mercury-coated glassy carbon electrode/solution interface during the stripping step [ 15,181. Therefore, the effect of mass transfer rate during the analysis is an overall process on the sensitivity. In general, slowing down the mass transfer rate, or use of static conditions during the stripping process, can enhance the sensitivity of the measurement [ 181. However, in this section, the same mass transfer rate was used for both plating and stripping steps. In
and Information Management
33 (1998) 207-215
211
addition, as forced convention stripping scheme was adopted and thus the reproducibility of the signal was dependent upon the hydrodynamics during the stripping step. When constant hydrodynamic conditions can be maintained by the detector system during the stripping step, a better reproducibility of the system can be obtained. The voltammetric cell assembly (Fig. lb) contains an optically controlled stirrer which provides an accurate stirring rate control in the range of 50-4000 ‘pm. This apparatus should be an improved cell assembly compared to the magnetic stirrer cell (Fig. la). In OSP measurement, if the deposition and stripping stirring rates are equal, the stripping time should be independent of stirring rate provided that constant hydrodynamic conditions are maintained [8]. However, examination of the stripping peaks in Fig. 4a reveals that they were affected by the stirring rates, even when equal stirring rates for deposition and stripping were employed. In Fig. 4a, the stripping peaks for 1000 and 2000 rpm are very similar and they are also similar to the stripping peak obtained using the magnetic stirrer cell (Fig. 2a). The peak areas of the stripping peaks for 500 and 4000 rpm are obviously larger (longer stripping time) and distorted compared with the peaks obtained using 1000 and 2000 rpm stirring rates. The relationship of stripping time (peak area) with stirring rate is shown in Fig. 4b and can be interpreted by separating the graph into three regions. Region b, between lOOO-
\ &d!L (b)
(a) 0.04
4000 rpm
z
0.03
a :
2000 rpm
: F ~ 0.02 ._ 0. -9 67 001
1000 ‘pm
500 rpm
D
)
b
0
-300
:
-200
-100
Potential (mV)
0
0
2000
4000
stirring Rate (r.p.m.)
Fig. 4. (a) The stripping potentiograms for 100 ppb Cu in 0.1 M HCl, using a I-min plating time, and 10 ppm Hg’+ as oxidant with various stirring rates. (b) The effect of stirring rate on stripping potentiometric signals for 100 ppb of Cu in 0.1 M HCl, using a I-min deposition time and 10 ppm Hg*+ as oxidant. Note: the stripping peaks were processed using polynomial smoothing with a 25.point window size.
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steady state value at a flow rate approaching 3.6 ml/min. In addition, there is an obvious difference in the shape of the stripping peak obtained between the batch (Fig. 2b and Fig. 4a) and flow (Fig. 5a) systems. This deviation may be due to the fact that the stripping step was carried out in a different hydrodynamic condition between the two systems.
/ 3.6 mLhi”\l\
0.0
I, 0
-300
-200
-100
0
33 (1998) 207-215
2
4
6
Flow Rate (mL/min)
Potential (mV)
Fig. 5. (a) The stripping potentiograms for 100 ppb Cu in 0.1 M HCl, using a I-min plating time and 10 ppm Hg** as oxidant with various flow rates. (b) The effect of flow rate on stripping potentiometric signals for 100 ppb of Cu in 0.1 M HCl, using a 1-min deposition time and 10 ppm Hg*+ as oxidant. Note: the stripping peaks were processed using polynomial smoothing with a 25-point window size.
2000 rpm, is a region with stripping time less influenced by the stirring rate. However, when stirring rate was set either below, this (region a) or above it (region c), the stripping time was found to be dependent upon stirring rate. This indicated a relatively constant hydrodynamic condition was obtained in the mid stirring range of the operation range of the stirrer. This is interpreted as resulting from the onset of turbulent flow which decreases the mass transfer rates during both the deposition and stripping steps as regions a and c [ 15,191. Both the movement of elements toward the electrode surface during deposition and the transportation rate of the oxidant toward the electrode surface, during stripping are affected. Thus the results shown in Fig. 4 relate to the overall influence of stirring rate on the stripping time and the influence of stirring rate on the individual steps cannot be analysed separately. When the experiment was carried out using the flow cell with a peristaltic pump, a similar dependence of stripping time upon flow rate was observed. An initial study indicated that this flow cell could only allow a 5 ml/min flow rate. When flow rates higher than this were selected, leakage resulted. In Fig. 5a, the shape of the stripping peak improved as flow rate increased. In the study carried out at the same deposition and stripping flow rates using 100 ppb copper (Fig. 5b) the magnitude of the copper stripping signal (peak area) was dependent upon the flow rate. The stripping time tends to come to a
3.4. The relationship tration
of stripping
time and concen-
The results of the sensitivity study of the three different cell assemblies in the range of O-1000 ppb in 0.1 M HCl using a 1-min deposition time and 10 ppm Hg2+ as oxidant are presented in Table 1. The magnetic stirrer cell has no control over the stirring rate, therefore, a single setting was used. Two different modes of stripping, stirred and static, were studied. For stirred stripping, the stirrer was operated during both the deposition and stripping steps. For static stripping, the stirrer was turned off after the deposition step and a settling period of 30 s was set before the potentiostatic circuit was disconnected to allow stripping to proceed. The correlation coefficient, R2, of the calibration plots for copper is 0.996 in stirred stripping mode and 0.998 in static stripping mode. Marginally better linearity was obtained when stripping was carried out in static solution.
Table 1 A comparison of the sensitivity and correlation coefficient, R’, for the calibration plots of copper for the three cell assemblies (Fig. 1) with various experimental parameters Sensitivity
R2
hs/ppb) Magnetic stirrer cell Stirred stripping Static stripping Cell with optically controlled stirrer (stirred deposition and stripping) Rotation rate 500 rpm 1000 rpm 2000 rpm 4000 rpm Flow cell with peristaltic pump (all deposition flow rates at 4.8 ml/min) 4.8 ml/min Stripping flow rate 0.6 ml/min Static
12.7 167.0
0.996 0.998
14.4 12.4 12.0 16.9
0.997 0.998 0.998 0.996
7.5 13.4 80.3
0.998 0.986 0.985
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For the optical speed control cell, four stirring rates, 500, 1000, 2000 and 4000 rpm were selected. Due to the limitation of this instrument, only stirred stripping mode was investigated. All show good linearity, with sensitivities in the order 4000 > 500 > 2000 = 1000 rpm (Table 1). Table 1 shows the correlation coefficient, R2, of the calibration plots [20,21]. Marginally better R* values were obtained when 1000 and 2000 rpm were used (middle of the operating range) compared to 500 and 4000 rpm (outer limits of the operating range). This indicates that by selection of rotation rates in the range of 1000-2000 rpm, the electroactive species are transported to and from the electrode surface in a more reproducible manner, yielding better precision compared to the cases of rates of 500 and 4000 rpm. The three calibration plots for copper in the range of O-1000 ppb with a fixed deposition flow rate, 4.8 ml/mm, and three different stripping flow rates, 4.8, 0.6 and 0 (stopped flow) ml/min show good linearity, with sensitivities in the order 0 (stopped flow) > 0.6 > 4.8 ml/min stripping flow rates (Table 1). This indicates that for a fixed deposition flow rate, the lower the stripping flow rate, the higher the sensitivity. When stripping is carried in a flowing solution, the mass transfer of the oxidant takes place mostly by forced convection rather than by diffusion (stopped flow). A well-defined hydrodynamic pattern in the vicinity of the electrode surface is created by the flow, which is stable towards mechanical disturbances in the cell. Therefore, electroactive species are transported to and from the working electrode surface in a highly reproducible manner, which is reflected in the R* value of the calibration plots for stripping carried out in flowing solution and static solution, respectively. Table 1 gives a summary of sensitivity obtained with the three electrode systems for copper under various experimental conditions. There is great improvement in sensitivity for stripping carried out in the static solution compared to the stirred stripping or flow situations. This applies to both the magnetic stirrer cell and flow cell. Considering R* of the calibration plots, better values were obtained for stripping carried out in flowing solution (Table 1). This indicates that the flow cell provides a highly reproducible and stable means of solute transport. However, when compar-
33 (1998) 207-215
213
ing the R* of the calibration plots obtained for static and stirred stripping (magnetic stirrer cell), better values were obtained for static compared to stirred stripping. This shows that this cell cannot provide a stable means of solute transport when stirring is continuous during the stripping step. Static stripping, with a diffusion controlled transport mechanism, provides better performance. In addition, when comparing the sensitivities achievable using the two cell assemblies, magnetic stirrer cell and flow cell, in the static stripping mode, it is clear that the use of a magnetic stirrer gives a larger sensitivity compared to the flow cell. This indicates that more analyte was plated onto the electrode during the deposition step (as the stripping step for both cell assemblies was carried out under static conditions). Thus, the stirred solution has higher mass transfer rate than that in the flowing solutions under our selected experimental conditions. 3.5. Reproducibility
study
As mentioned earlier, the reproducibility of the signal was dependent upon the hydrodynamics during the stripping step. In this section, the same mass transfer rate (stirring or flow) was used for both plating and stripping steps. A comparison of relative standard deviations (r.s.d.1 of the stripping signal obtained using the three cell assemblies was made.
Table 2 A comparison of the reproducibility of the stripping signals for the three cell assemblies with various experimental parameters using the average relative standard deviation of seven groups of 10 replicate measurements
Magnetic
Stripping mode
Average r.s.d. (%)
Stirred Stirred
Stirred Static
5.59 2.56
500 ‘pm 1000 ‘pm 2000 rpm 4000 rpm
500 rpm 1000 rpm 2000 ‘pm 4000 rpm
3.73 2.06 1.80 4.04
4.8 ml/min 4.8 ml/min
4.8 ml/min Static
0.56 0.76
stirrer cell
Cell with optically controlled stirrer Rotation rate
Flow cell Flow rate
Deposition mode
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2 ._ + ._2 8 ‘2 WI
2-w
0,
1 0 Number
50
100
of Cycles
0 Number
50
100
of Cycles
0 Number
50
100
of Cycles
Fig. 6. A comparison of different cell configurations: (a) magnetic stirrer cell, (b) optically controlled stirrer cell with two selected stirring rates, 1000 and 2000 rpm, and (c) flow system using a flow cell and peristaltic pump with a 4.8 ml/mm deposition and stripping flow rates. All data for 100 ppb Cu in 0.1 M HCl using a l-mm plating time and 10 ppm Hg*+ as oxidant,
The data were obtained through 100 replicate cycles of analysis using each cell assembly and seven groups of ten consecutive measurements starting from the 31st runs were used to compute the average r.s.d. figures. The first thirty runs were treated as a conditioning process for the mercury film electrode. This selection was based on examination of the graphical presentations for copper through 100 replicate cycles of analysis. This procedure eliminates the problem of signal drift over the period of the experiment. The results for this comparison are presented in Table 2. A graphical comparison is also presented in Fig. 6. Stirring is a major cause of poor reproducibility for stripping techniques with conventional cell assemblies [22]. In Fig. 6a, a graphical comparison of copper stripping signals is made. It shows that when stripping is carried out using the magnetic stirrer cell, a very large scattering of results is observed. This comparison confirms strongly the need for hydrodynamic control during the stripping step. The comparison using the r.s.d. of the signals in Table 2 reveals that stirred stripping yields a larger r.s.d. compared to the static stripping mode. The precision of stirred stripping by the optically controlled stirrer shows less scatter (Table 21, but signal drift is observed during the replicate runs (Fig. 6b). By way of explanation, it should be noted that although the speed of the stirrer is optically monitored and adjusted by a feed-back control loop, the DC motor may not be designed for work of this long duration (100 cycles took approximately 100 min) in the continuous stirring mode. However, if the drift can be eliminated, the signals are less scattered compared to those obtained with the magnetic stirrer. The r.s.d. figures from Table 2 are excellent for the flow cell in this comparison. Poorly controlled
hydrodynamic conditions, such as irregular stirring, have a large influence on the consistency of the measurement, especially during the stripping step. In the flowing solution, the transport of oxidant towards the electrode is by convection rather than diffusion. Under controlled hydrodynamic conditions, a convection-controlled transport mechanism will provide highly reproducible transport compared to the diffusion-controlled mechanism [ 181. This is reflected by the r.s.d. values for stripping carried out in flowing solution compared to those obtained in static solution. This also correlates with the R* of the calibration plots under the same condition reported earlier (Table 1). In Fig. 6c, the measurements obtained using the flow cell show both consistency and excellent stability over the period of the experiment. 4. Conclusion Though these experiments, it has been confirmed that each of the three cell assemblies studied can be used for OSP measurements, all yielding excellent linear calibrations. The crucial role of hydrodynamics has been highlighted. When the magnetic stirrer cell was used, the static stripping procedure enhanced the sensitivity of the measurement compared to stirred stripping. The repeat cycle experiment was a preliminary investigation of the suitability of OSP for application to on-line heavy metal monitoring. The use of a magnetic stirrer cell in performing stirred stripping gave larger scatter than the other approaches, ostensibly due to the poor reproducibility of the stirring rate. The use of the cell with an optically controlled stirrer was regarded as a very good way to improve the sample handling aspect of this technique as the cell is integrated in one unit
C. W.K. Chow et al./Laboratory
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with sample inlet and drain. The glassy carbon electrode is located at the bottom of the cell and above the drain. With this arrangement, the working surface of the electrode is protected from exposure to the atmosphere even during a change of sample [ 111. However, it may be vulnerable to precipitate formation. The module’s limited flexibility regarding stirring rate selection restricted the adaptation of this stirrer into our OSP system. The use of the optically controlled stirrer resulted in less scatter than the magnetic stirrer but gave some disappointing results (signal drift) during replicate runs. The use of a flow cell and peristaltic pump provided all the flexibility needed for OSP measurement. The sensitivity can be improved using the combination of fast deposition flow rate and slow stripping flow rate. In addition, the technique of matrix exchange can be easily implemented in a flow system [23]. From the excellent results obtained in the replicate runs, we can conclude that the use of a flow system is highly favourable in OSP measurement.
Acknowledgements The authors wish to thank Mr. J.W.K. Wong and Dr. T.C.W. Yeow for the design and fabrication of the signal-splitting unit and the associated electronics, and for writing software used in the measurement setup.
References [l] A.J. Bard, L.R. Faulkner, Electrochemical methods: mentals and applications, Wiley, New York, 1980.
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[21 AI. Vogel, Revised By J. Bassett, R.C. Denney, G.H. Jeffery, J. Mendhan, Textbook of Quantitative Inorganic Analysis, 4th edn., Longman, New York, 1986. [31 D. Jagner, A. Graneli, Anal. Chim. Acta 83 (1976) 19-26. [41 C.W.K. Chow, D.E. Davey, M.R. Haskard, D.E. Mulcahy, T.C.W. Yeow, J. Chem. Educ. 71 (1994) 997-1000. 151 C.W.K. Chow, D.E. Davey, D.E. Mulcahy, Laboratory Automation and Information Management 31 (1995) 77-88. [61 C.W.K. Chow, D.E. Davey, D.E. Mulcahy, T.C.W. Yeow, Anal. Chim. Acta 307 (1995) 15-26. 171 D. Jagner, E. Sahlin, L. Renman, Talanta 41 (1994) 512-522. [81 D. Jagner, Analyst 107 (1982) 593-599. 191 L. Anderson, D. Jagner, M. Josefson, Anal. Chem. 54 (1982) 1371-1376. 1101 W. Frenzel, G. Schulze, Analyst 112 (1987) 134-137. [ill A.W. Mann, Anodic stripping voltammetry for rapid, routine trace metal analysis, Proceedings of the Ninth Australian Symposium on Analytical Chemistry, Royal Australian Chemical Institute, Analytical Chemistry Division, 2, 1987, 604-607. [la J. Mortensen, E. Ouziel, H.J. Skov, L. Kryger, Anal. Chim. Acta 112 (1979) 297-312. 1131 A. Graneli, D. Jagner, M. Josefson, Anal. Chem. 52 (1980) 2220-2223. [141 J.K. Christensen, L. Kryger, J. Mortensen, J. Rasmussen, Anal. Chim. Acta 121 (1980) 71-83. 1151 D. Jagner, K. Aren, Anal. Chim. Acta 100 (1978) 375-388. I161A. Cladera, J.M. Estela, V. Cerda, Talanta 37 (1990) 689693. [I71 L. Renman, D. Jagner, R. Berglund, Anal. Chim. Acta 188 (1986) 137-150. [181 C. Labar, L. Lamberts, Anal. Chim. Acta 132 (1981) 23-33. [I91 A.W. Bott, Current Separations 12 (1993) 21-25. DO1 J.C. Miller, J.N. Miller, Statistics For Analytical Chemistry, Ellis Horwood, Chichester, 1984. 1211 R.C. Graham, Data analysis for the chemical sciences: a guide to statistical techniques, VCH Publishers, New York, 1993. t221 P.M. Bersier, J. Howell, C. Bruntlett, Analyst 119 (1994) 219-232. 1231 A. Hu, R.E. Dessy, A. Granell, Anal. Chem. 55 (1983) 320-328.
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33 (1998) 207-215
Comparison of detector systems in oxidative stripping potentiometry Christopher W.K. Chow
*,
David E. Davey, Dennis E. Mulcahy
Analysis and Sensors Group, School of Chemical Technology, Received
University of South Australia, The Levels, South Australia 5095, Australia
15 January
1998; accepted
13
June 1998
Abstract
For oxidative stripping potentiometry (OSP) measurement, the achievement of reliable results is highly dependent upon the transport of oxidant to the electrode surface and, as a consequence, careful hydrodynamic control is crucial for reproducibility. Two types of detector system, batch and flow, with three different electrode systems, a standard voltammetric magnetic stirrer cell assembly, an voltammetric cell with optically controlled stirrer and a thin layer flow cell with peristaltic pump were compared. In the sensitivity studies, the static stripping procedure enhanced the sensitivity of the measurement compared to forced convection stripping. The use of a flow cell and peristaltic pump provided all the flexibility needed for OSP measurement. The sensitivity can be improved using the combination of fast deposition and slow stripping flow rates. From the viewpoint of reproducibility of the signals, excellent results were obtained for the use of the flow cell with peristaltic pump. 0 1998 Elsevier Science B.V. All rights reserved. Keywo&c
Oxidative
stripping
potentiometry;
Different detector systems: Sensitivity;
1. Introduction
Oxidative stripping potentiometry (OSP) is a relatively new technique for trace metal analysis particularly when seen against the long history of other electrochemical methods [ 1,2]. Since the first report of this technique in 1976 [3], numerous applications to a variety of analyses have appeared in the literature. The technique is based on aspects of the already well established anodic stripping voltammetry (ASV). OSP is, however, rapidly coming to be a preferred technique for trace metal analysis and it requires
* Corresponding author. Australian Water Quality Centre, Private Mail Bag, Salisbury, South Australia, 5108, Australia. Fax: + 61-8-825-90228; E-mail: [email protected]
Reproducibility;
Copper analysis
simpler equipment [4-61. Apart from the excellent detection limits and sensitivities, stripping potentiometry also has the advantages of multi-element capability, allowing the determination of several elements in the same sample and the option of speciation by proper selection of the experimental conditions [7]. OSP, makes use of the same preconcentration step as ASV but follows it with a different stripping step. The amalgamated metals are brought back from the surface of the working electrode into solution by a chemical oxidation. Alternative oxidants include dissolved oxygen, Hg*+, MnOCr O*-, Fe3’ and Ce4+. All metals amalgamatld’ at tie ‘working electrode will be re-oxidised at a rate dependent upon the rate of transport of the oxidant to the working elec-
0925.5281/98/$ - see front matter 0 1998 Elsevier Science B.V. All rights reserved. PII: Sl381-141X(98)00006-9
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Automation and Information Management
trode. If the potential of the working electrode is monitored there will be a stepwise increase in the potential with time. The time duration of each potential step is proportional to the concentration of the relevant analyte in the sample. Both qualitative and also quantitative information can be obtained from the resulting potential (E) vs. time (t) curve. The shape of this curve is similar to that of curves obtained in chronopotentiometry. The time elapsed (stripping time) in each step is proportional to the amount of the given metal deposited at the electrode
[31. For OSP measurement, the achievement of reliable results is also highly dependent upon the transport of oxidant to the electrode surface, and, as a consequence, careful hydrodynamic control is crucial for reproducibility. Sensitivity is directly proportional to deposition time and indirectly proportional to oxidant concentration and oxidant transportation rate. It is a general practice to stir during both the deposition and stripping steps [3,8]. Therefore a well-defined hydrodynamic environment during the stripping step, such as that achieved by using a spinning electrode system [7] or by using a flow system [9,10], is essential for this technique. The authors believe that in the near future many laboratories will construct their own OSP equipment because of its sensitivity for metal ion analysis and the simplicity of the equipment required [4-61. This paper describes a practical comparison of the performance of three different detector systems. These are (a) a cell assembly containing a magnetic stirrer, (b) a stirrer cell with optical speed control adapted from a commercial ASV instrument (PDV 2000, Chemtronics), and (c) a flow system based upon a peristaltic pump and a thin layer flow cell.
2. Experimental In this study, the sensitivity and reproducibility of three different cell assemblies for OSP were compared. A diagram of the three cell assemblies is presented in Fig. 1. The sensitivity study was performed using a calibration in the range of O-1000 ppb and reproducibility of the stripping signal was examined using a 100 ppb copper solution by a repeat cycle of l-100 replicate analyses. In addition
33 (1998) 207-215
to the main comparison, various experimental parameters within the cell assembly were also studied. 2.1. Instrumentation The equipment employed in this study included a CV- 1B Cyclic voltammetry instrument (Bioanalytical Systems, IN, USA) as the potentiostat, a 701A Digital Ionalyzer (Orion Research, MA, USA) for potential measurement and a PCL-712 Multi-Lab Card (Advantech, Taiwan) for data acquisition and control. An IBM compatible 386-SX personal computer (Microbits, Adelaide, South Australia) was used to control the measurement system and store the recorded potentiogram. The potential was sampled at 8 kHz and the 1Zbit analogue-to-digital converter gives a 1 mV resolution. Details of this instrument have been reported elsewhere [6]. 2.2. Mercury precoating
and initial preparation
The glassy carbon working electrode was polished using a BAS PK-4 polishing kit (Bioanalytical Systems), then rinsed with ethanol and dried in air. It was then pre-plated with a mercury film from a 30 ppm Hg(I1) solution in 0.1 M HCl at - 900 mV for 5 min. 2.3. Electrode and cell assembly Three different electrode systems were studied in this experiment. (a> A cell assembly containing a magnetic stirrer with a 3 cm X 1 cm (length X height) flea (rotation rate approximately 200 rpm), a 3 mm diameter glassy carbon electrode (Chemtronics, Bentley, Western Australia), employed as the working electrode, a platinum wire, used as the auxiliary electrode, and a Ag/AgC1/3 M KC1 double junction electrode (HNU ISE 40-02-00, HNU Systems, USA) used as the reference. (b) A stirrer cell with optical speed control adapted from a commercial anodic stripping voltammetry instrument, the PDV 2000, (Chemtronics). This cell is designed as an integrated unit which consists of a glassy carbon (3 mm diameter) working electrode, reference electrode, auxiliary electrode, stirrer, sample inlet and drain [ll]. Four stirring rates, 500,
C. W.K. Chow et al. /Laboratory
1Ocm
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33 (1998) 207-215
209
Motor C
25 r
Stirrer
Auxiliary Electrode
Sa
Reference Electrode
Working Electrode Vent/
lcm
I
Pig. 1. The experimental arrangement of the three cell assemblies. (a) magnetic stirrer cell assembly, (b) cell with optically controlled adapted from the PDV2000, and (c) flow system based upon a peristaltic pump and a thin layer flow cell.
1000, 2000 and 4000 rpm, were selected in this comparison. (c) A flow system based upon a Gilson Minipuls 3 peristaltic pump (Gilson France, France) and a thin layer flow cell with glassy carbon (3 mm diameter) working electrode. In all cases, the working electrode was pre-plated with a mercury film from a 30 ppm Hg(I1) solution in 0.1 M HCI at - 900 mV for 5 min. Five different flow rates, 0.6, 1.2, 2.4, 3.6 and 4.8 ml/mm, were selected in this experiment.
2.4. Experimental
procedure
For the magnetic stirrer cell (Fig. la), a 50 ml aliquot of a 10 ppm Hg(I1) solution in 0.1 M hydrochloric acid (AR) was transferred to the analytical
stirrer
cell. Initial deaeration was carried out by bubbling high purity nitrogen gas into the mixture for 15 min so that the Hg*+ ion would be the sole oxidant. A sensitivity test in the range of O-1000 ppb was then performed with ten 50 ~1 additions of a 100 ppm copper nitrate standard solution. A 1-min plating time at - 900 mV was selected for the experiment. A further reproducibility study of the stripping signal of 100 ppb Cu was examined by a repeat cycle of l-100 replicate sets of analyses. For the optical speed controlled stirrer cell (Fig. lb), a 10 ml aliquot of a 10 ppm Hg(I1) solution was used and similar procedure was applied for both the sensitivity and reproducibility studies. For the flow cell (Fig. lc), the procedure was identical to the magnetic stirrer cell as it was used to hold the solution for the flow system.
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3. Results
Automarion and Information Management
and discussion
3.1. Presentation
of the potentiogram
During the stripping step in OSP, the electrode potential is monitored at constant frequency; if a potential reading falls within a given potential range the count in the corresponding potential range is incremented by one. This method of recording the potential count versus potential is known as multichannel potentiometric monitoring [ 121. The recorded data can be presented either as an E vs. t curve [ 131 or in the form of peaks (which is equivalent to an inverse derivative of the conventional potentiogram) [14]. The use of multichannel potentiometric monitoring is extremely favourable for use in a computerised system. Once the raw data is obtained (Fig. 2a), then, as stated earlier, it can be processed into either an E vs. t curve [3] as illustrated in Fig. 2c or (a) 800 P S 0
600
-z .g 400 LI1 2 ‘& 200
-300
0
-100
-200
Potential (mV) “,, ‘, ‘X
r.---.--../. (cl
@)
-400
0.06
0.00
L-J
-zoo
-200
OI
-100
Potential(mV)
0.0
0.5
1.0
1.5
20
0 Stiippmg
Time(s)
Fig. 2. (a) The stripping signal for 100 ppb Cu in 0.1 M HCI obtained from the magnetic stirrer cell, using a I-min plating time as oxidant, recorded using the multichannel and 10 ppm Hg” potentiometric monitoring method, (b) the raw potentiogram of (a) after polynomial smoothing with 25.point window size and (c) the raw potentiogram of (a) after conversion to the conventional potential vs. time curve.
33 (1998) 207-215
as an inverse derivative potentiogram [14] as illustrated in Fig. 2b. In current literature terminology these are not distinguishable and both are called stripping potentiograms. However, the latter may be considered as an improvement of the former and is widely used in the more recent literature. Of course, presenting the potentiogram in the form of peaks is more favourable for the interpretation of results. In this paper, both forms are used. In order to minimise confusion, the E vs. t curve will be named the stripping curve while the inverse derivative potentiogram will be referred to stripping peaks. 3.2. Determination
of analytical parameters
The analytical parameter stripping time is defined by the time which elapses between two consecutive elemental ‘equivalence’ points in the conventional potentiogram (i.e., in the stripping curve>. The time is proportional to the concentration of the particular metal in the solution 131. Fig. 3a shows two methods for stripping time determination from a stripping curve. In Fig. 3a(i), the lines a and b are drawn parallel to the stripping curve just before and after the plateau. Line c is drawn parallel to the slope of the stripping curve on the plateau. The distance between the intersections of line c with lines a and b is then taken as stripping time. In Fig. 3a(ii), line c is replaced by line c’, which is drawn parallel to the time axis to intersect the stripping curve at the half potential point. In this case, the distance between the intersections of line c’ with lines a and b is taken as stripping time. Under normal conditions, the stripping times determined by these two methods do not differ [15]. However, when the stripping time is determined by computer, to develop an algorithm for the determination of stripping time from a conventional potentiogram is rather complicated. Cladera et al. [16] have reported a computerised method which made use of three regression equations to represent the three lines a, b and c on a stripping potentiogram and the stripping time was determined by solving the three equations simultaneously. The use of the stripping peak approach (Fig. 3b) to present the analytical result is more suited to a computerised system. Renman et al. [17] reported a method to determine the stripping time (peak area) and stripping potential which is particularly easy to
C.W.K.
Chow et al./L.aboratory a(ii)
4
Automation
(b)
-500 -400 -300
:
;” \
1s
0.02s
1
b’,,,
\ -b,
i
\
-200 -100
c’
i ‘,y_L ,‘a
0
T = stripping
-c\
~‘I-:-__;(. ‘,‘\ ‘,a
-
‘L L ,_>
.
i/ P&Area=‘5 \
time
Fig. 3. Potentiograms for 100 ppb Cu in 0.1 M HCI, using a l-mm plating time, and 10 ppm Hg2+ as oxidant: means of presenting PSA data: showing (a) the stripping curves (i) and (ii) are two difference methods to determine stripping time, and (b) the stripping peak.
implement. A simplified method which involves a preliminary manual examination of the stripping curve or peak to determine potential ranges for the elements involved can also be used. The number of data points within the predetermined potential range for an element is then counted, and divided by the sampling rate. This method gives essentially the same stripping time as found by the literature methods mentioned and is less demanding on programming [4]. 3.3. Effect of mass transfer rate For stripping analysis, such as ASV with stirred deposition and static stripping, the sensitivity of the measurement is dependent upon the duration (deposition time) and the mass transfer rate in the deposition step. In general a higher stirring or flow rate, which improves the mass transfer rate, improves the sensitivity, as more analyte can be transported to the working electrode. In OSP, the sensitivity also depends on the concentration of the oxidising agent and on the transportation rate of the oxidising species to the mercury-coated glassy carbon electrode/solution interface during the stripping step [ 15,181. Therefore, the effect of mass transfer rate during the analysis is an overall process on the sensitivity. In general, slowing down the mass transfer rate, or use of static conditions during the stripping process, can enhance the sensitivity of the measurement [ 181. However, in this section, the same mass transfer rate was used for both plating and stripping steps. In
and Information Management
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211
addition, as forced convention stripping scheme was adopted and thus the reproducibility of the signal was dependent upon the hydrodynamics during the stripping step. When constant hydrodynamic conditions can be maintained by the detector system during the stripping step, a better reproducibility of the system can be obtained. The voltammetric cell assembly (Fig. lb) contains an optically controlled stirrer which provides an accurate stirring rate control in the range of 50-4000 ‘pm. This apparatus should be an improved cell assembly compared to the magnetic stirrer cell (Fig. la). In OSP measurement, if the deposition and stripping stirring rates are equal, the stripping time should be independent of stirring rate provided that constant hydrodynamic conditions are maintained [8]. However, examination of the stripping peaks in Fig. 4a reveals that they were affected by the stirring rates, even when equal stirring rates for deposition and stripping were employed. In Fig. 4a, the stripping peaks for 1000 and 2000 rpm are very similar and they are also similar to the stripping peak obtained using the magnetic stirrer cell (Fig. 2a). The peak areas of the stripping peaks for 500 and 4000 rpm are obviously larger (longer stripping time) and distorted compared with the peaks obtained using 1000 and 2000 rpm stirring rates. The relationship of stripping time (peak area) with stirring rate is shown in Fig. 4b and can be interpreted by separating the graph into three regions. Region b, between lOOO-
\ &d!L (b)
(a) 0.04
4000 rpm
z
0.03
a :
2000 rpm
: F ~ 0.02 ._ 0. -9 67 001
1000 ‘pm
500 rpm
D
)
b
0
-300
:
-200
-100
Potential (mV)
0
0
2000
4000
stirring Rate (r.p.m.)
Fig. 4. (a) The stripping potentiograms for 100 ppb Cu in 0.1 M HCl, using a I-min plating time, and 10 ppm Hg’+ as oxidant with various stirring rates. (b) The effect of stirring rate on stripping potentiometric signals for 100 ppb of Cu in 0.1 M HCl, using a I-min deposition time and 10 ppm Hg*+ as oxidant. Note: the stripping peaks were processed using polynomial smoothing with a 25.point window size.
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steady state value at a flow rate approaching 3.6 ml/min. In addition, there is an obvious difference in the shape of the stripping peak obtained between the batch (Fig. 2b and Fig. 4a) and flow (Fig. 5a) systems. This deviation may be due to the fact that the stripping step was carried out in a different hydrodynamic condition between the two systems.
/ 3.6 mLhi”\l\
0.0
I, 0
-300
-200
-100
0
33 (1998) 207-215
2
4
6
Flow Rate (mL/min)
Potential (mV)
Fig. 5. (a) The stripping potentiograms for 100 ppb Cu in 0.1 M HCl, using a I-min plating time and 10 ppm Hg** as oxidant with various flow rates. (b) The effect of flow rate on stripping potentiometric signals for 100 ppb of Cu in 0.1 M HCl, using a 1-min deposition time and 10 ppm Hg*+ as oxidant. Note: the stripping peaks were processed using polynomial smoothing with a 25-point window size.
2000 rpm, is a region with stripping time less influenced by the stirring rate. However, when stirring rate was set either below, this (region a) or above it (region c), the stripping time was found to be dependent upon stirring rate. This indicated a relatively constant hydrodynamic condition was obtained in the mid stirring range of the operation range of the stirrer. This is interpreted as resulting from the onset of turbulent flow which decreases the mass transfer rates during both the deposition and stripping steps as regions a and c [ 15,191. Both the movement of elements toward the electrode surface during deposition and the transportation rate of the oxidant toward the electrode surface, during stripping are affected. Thus the results shown in Fig. 4 relate to the overall influence of stirring rate on the stripping time and the influence of stirring rate on the individual steps cannot be analysed separately. When the experiment was carried out using the flow cell with a peristaltic pump, a similar dependence of stripping time upon flow rate was observed. An initial study indicated that this flow cell could only allow a 5 ml/min flow rate. When flow rates higher than this were selected, leakage resulted. In Fig. 5a, the shape of the stripping peak improved as flow rate increased. In the study carried out at the same deposition and stripping flow rates using 100 ppb copper (Fig. 5b) the magnitude of the copper stripping signal (peak area) was dependent upon the flow rate. The stripping time tends to come to a
3.4. The relationship tration
of stripping
time and concen-
The results of the sensitivity study of the three different cell assemblies in the range of O-1000 ppb in 0.1 M HCl using a 1-min deposition time and 10 ppm Hg2+ as oxidant are presented in Table 1. The magnetic stirrer cell has no control over the stirring rate, therefore, a single setting was used. Two different modes of stripping, stirred and static, were studied. For stirred stripping, the stirrer was operated during both the deposition and stripping steps. For static stripping, the stirrer was turned off after the deposition step and a settling period of 30 s was set before the potentiostatic circuit was disconnected to allow stripping to proceed. The correlation coefficient, R2, of the calibration plots for copper is 0.996 in stirred stripping mode and 0.998 in static stripping mode. Marginally better linearity was obtained when stripping was carried out in static solution.
Table 1 A comparison of the sensitivity and correlation coefficient, R’, for the calibration plots of copper for the three cell assemblies (Fig. 1) with various experimental parameters Sensitivity
R2
hs/ppb) Magnetic stirrer cell Stirred stripping Static stripping Cell with optically controlled stirrer (stirred deposition and stripping) Rotation rate 500 rpm 1000 rpm 2000 rpm 4000 rpm Flow cell with peristaltic pump (all deposition flow rates at 4.8 ml/min) 4.8 ml/min Stripping flow rate 0.6 ml/min Static
12.7 167.0
0.996 0.998
14.4 12.4 12.0 16.9
0.997 0.998 0.998 0.996
7.5 13.4 80.3
0.998 0.986 0.985
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For the optical speed control cell, four stirring rates, 500, 1000, 2000 and 4000 rpm were selected. Due to the limitation of this instrument, only stirred stripping mode was investigated. All show good linearity, with sensitivities in the order 4000 > 500 > 2000 = 1000 rpm (Table 1). Table 1 shows the correlation coefficient, R2, of the calibration plots [20,21]. Marginally better R* values were obtained when 1000 and 2000 rpm were used (middle of the operating range) compared to 500 and 4000 rpm (outer limits of the operating range). This indicates that by selection of rotation rates in the range of 1000-2000 rpm, the electroactive species are transported to and from the electrode surface in a more reproducible manner, yielding better precision compared to the cases of rates of 500 and 4000 rpm. The three calibration plots for copper in the range of O-1000 ppb with a fixed deposition flow rate, 4.8 ml/mm, and three different stripping flow rates, 4.8, 0.6 and 0 (stopped flow) ml/min show good linearity, with sensitivities in the order 0 (stopped flow) > 0.6 > 4.8 ml/min stripping flow rates (Table 1). This indicates that for a fixed deposition flow rate, the lower the stripping flow rate, the higher the sensitivity. When stripping is carried in a flowing solution, the mass transfer of the oxidant takes place mostly by forced convection rather than by diffusion (stopped flow). A well-defined hydrodynamic pattern in the vicinity of the electrode surface is created by the flow, which is stable towards mechanical disturbances in the cell. Therefore, electroactive species are transported to and from the working electrode surface in a highly reproducible manner, which is reflected in the R* value of the calibration plots for stripping carried out in flowing solution and static solution, respectively. Table 1 gives a summary of sensitivity obtained with the three electrode systems for copper under various experimental conditions. There is great improvement in sensitivity for stripping carried out in the static solution compared to the stirred stripping or flow situations. This applies to both the magnetic stirrer cell and flow cell. Considering R* of the calibration plots, better values were obtained for stripping carried out in flowing solution (Table 1). This indicates that the flow cell provides a highly reproducible and stable means of solute transport. However, when compar-
33 (1998) 207-215
213
ing the R* of the calibration plots obtained for static and stirred stripping (magnetic stirrer cell), better values were obtained for static compared to stirred stripping. This shows that this cell cannot provide a stable means of solute transport when stirring is continuous during the stripping step. Static stripping, with a diffusion controlled transport mechanism, provides better performance. In addition, when comparing the sensitivities achievable using the two cell assemblies, magnetic stirrer cell and flow cell, in the static stripping mode, it is clear that the use of a magnetic stirrer gives a larger sensitivity compared to the flow cell. This indicates that more analyte was plated onto the electrode during the deposition step (as the stripping step for both cell assemblies was carried out under static conditions). Thus, the stirred solution has higher mass transfer rate than that in the flowing solutions under our selected experimental conditions. 3.5. Reproducibility
study
As mentioned earlier, the reproducibility of the signal was dependent upon the hydrodynamics during the stripping step. In this section, the same mass transfer rate (stirring or flow) was used for both plating and stripping steps. A comparison of relative standard deviations (r.s.d.1 of the stripping signal obtained using the three cell assemblies was made.
Table 2 A comparison of the reproducibility of the stripping signals for the three cell assemblies with various experimental parameters using the average relative standard deviation of seven groups of 10 replicate measurements
Magnetic
Stripping mode
Average r.s.d. (%)
Stirred Stirred
Stirred Static
5.59 2.56
500 ‘pm 1000 ‘pm 2000 rpm 4000 rpm
500 rpm 1000 rpm 2000 ‘pm 4000 rpm
3.73 2.06 1.80 4.04
4.8 ml/min 4.8 ml/min
4.8 ml/min Static
0.56 0.76
stirrer cell
Cell with optically controlled stirrer Rotation rate
Flow cell Flow rate
Deposition mode
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2 ._ + ._2 8 ‘2 WI
2-w
0,
1 0 Number
50
100
of Cycles
0 Number
50
100
of Cycles
0 Number
50
100
of Cycles
Fig. 6. A comparison of different cell configurations: (a) magnetic stirrer cell, (b) optically controlled stirrer cell with two selected stirring rates, 1000 and 2000 rpm, and (c) flow system using a flow cell and peristaltic pump with a 4.8 ml/mm deposition and stripping flow rates. All data for 100 ppb Cu in 0.1 M HCl using a l-mm plating time and 10 ppm Hg*+ as oxidant,
The data were obtained through 100 replicate cycles of analysis using each cell assembly and seven groups of ten consecutive measurements starting from the 31st runs were used to compute the average r.s.d. figures. The first thirty runs were treated as a conditioning process for the mercury film electrode. This selection was based on examination of the graphical presentations for copper through 100 replicate cycles of analysis. This procedure eliminates the problem of signal drift over the period of the experiment. The results for this comparison are presented in Table 2. A graphical comparison is also presented in Fig. 6. Stirring is a major cause of poor reproducibility for stripping techniques with conventional cell assemblies [22]. In Fig. 6a, a graphical comparison of copper stripping signals is made. It shows that when stripping is carried out using the magnetic stirrer cell, a very large scattering of results is observed. This comparison confirms strongly the need for hydrodynamic control during the stripping step. The comparison using the r.s.d. of the signals in Table 2 reveals that stirred stripping yields a larger r.s.d. compared to the static stripping mode. The precision of stirred stripping by the optically controlled stirrer shows less scatter (Table 21, but signal drift is observed during the replicate runs (Fig. 6b). By way of explanation, it should be noted that although the speed of the stirrer is optically monitored and adjusted by a feed-back control loop, the DC motor may not be designed for work of this long duration (100 cycles took approximately 100 min) in the continuous stirring mode. However, if the drift can be eliminated, the signals are less scattered compared to those obtained with the magnetic stirrer. The r.s.d. figures from Table 2 are excellent for the flow cell in this comparison. Poorly controlled
hydrodynamic conditions, such as irregular stirring, have a large influence on the consistency of the measurement, especially during the stripping step. In the flowing solution, the transport of oxidant towards the electrode is by convection rather than diffusion. Under controlled hydrodynamic conditions, a convection-controlled transport mechanism will provide highly reproducible transport compared to the diffusion-controlled mechanism [ 181. This is reflected by the r.s.d. values for stripping carried out in flowing solution compared to those obtained in static solution. This also correlates with the R* of the calibration plots under the same condition reported earlier (Table 1). In Fig. 6c, the measurements obtained using the flow cell show both consistency and excellent stability over the period of the experiment. 4. Conclusion Though these experiments, it has been confirmed that each of the three cell assemblies studied can be used for OSP measurements, all yielding excellent linear calibrations. The crucial role of hydrodynamics has been highlighted. When the magnetic stirrer cell was used, the static stripping procedure enhanced the sensitivity of the measurement compared to stirred stripping. The repeat cycle experiment was a preliminary investigation of the suitability of OSP for application to on-line heavy metal monitoring. The use of a magnetic stirrer cell in performing stirred stripping gave larger scatter than the other approaches, ostensibly due to the poor reproducibility of the stirring rate. The use of the cell with an optically controlled stirrer was regarded as a very good way to improve the sample handling aspect of this technique as the cell is integrated in one unit
C. W.K. Chow et al./Laboratory
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with sample inlet and drain. The glassy carbon electrode is located at the bottom of the cell and above the drain. With this arrangement, the working surface of the electrode is protected from exposure to the atmosphere even during a change of sample [ 111. However, it may be vulnerable to precipitate formation. The module’s limited flexibility regarding stirring rate selection restricted the adaptation of this stirrer into our OSP system. The use of the optically controlled stirrer resulted in less scatter than the magnetic stirrer but gave some disappointing results (signal drift) during replicate runs. The use of a flow cell and peristaltic pump provided all the flexibility needed for OSP measurement. The sensitivity can be improved using the combination of fast deposition flow rate and slow stripping flow rate. In addition, the technique of matrix exchange can be easily implemented in a flow system [23]. From the excellent results obtained in the replicate runs, we can conclude that the use of a flow system is highly favourable in OSP measurement.
Acknowledgements The authors wish to thank Mr. J.W.K. Wong and Dr. T.C.W. Yeow for the design and fabrication of the signal-splitting unit and the associated electronics, and for writing software used in the measurement setup.
References [l] A.J. Bard, L.R. Faulkner, Electrochemical methods: mentals and applications, Wiley, New York, 1980.
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[21 AI. Vogel, Revised By J. Bassett, R.C. Denney, G.H. Jeffery, J. Mendhan, Textbook of Quantitative Inorganic Analysis, 4th edn., Longman, New York, 1986. [31 D. Jagner, A. Graneli, Anal. Chim. Acta 83 (1976) 19-26. [41 C.W.K. Chow, D.E. Davey, M.R. Haskard, D.E. Mulcahy, T.C.W. Yeow, J. Chem. Educ. 71 (1994) 997-1000. 151 C.W.K. Chow, D.E. Davey, D.E. Mulcahy, Laboratory Automation and Information Management 31 (1995) 77-88. [61 C.W.K. Chow, D.E. Davey, D.E. Mulcahy, T.C.W. Yeow, Anal. Chim. Acta 307 (1995) 15-26. 171 D. Jagner, E. Sahlin, L. Renman, Talanta 41 (1994) 512-522. [81 D. Jagner, Analyst 107 (1982) 593-599. 191 L. Anderson, D. Jagner, M. Josefson, Anal. Chem. 54 (1982) 1371-1376. 1101 W. Frenzel, G. Schulze, Analyst 112 (1987) 134-137. [ill A.W. Mann, Anodic stripping voltammetry for rapid, routine trace metal analysis, Proceedings of the Ninth Australian Symposium on Analytical Chemistry, Royal Australian Chemical Institute, Analytical Chemistry Division, 2, 1987, 604-607. [la J. Mortensen, E. Ouziel, H.J. Skov, L. Kryger, Anal. Chim. Acta 112 (1979) 297-312. 1131 A. Graneli, D. Jagner, M. Josefson, Anal. Chem. 52 (1980) 2220-2223. [141 J.K. Christensen, L. Kryger, J. Mortensen, J. Rasmussen, Anal. Chim. Acta 121 (1980) 71-83. 1151 D. Jagner, K. Aren, Anal. Chim. Acta 100 (1978) 375-388. I161A. Cladera, J.M. Estela, V. Cerda, Talanta 37 (1990) 689693. [I71 L. Renman, D. Jagner, R. Berglund, Anal. Chim. Acta 188 (1986) 137-150. [181 C. Labar, L. Lamberts, Anal. Chim. Acta 132 (1981) 23-33. [I91 A.W. Bott, Current Separations 12 (1993) 21-25. DO1 J.C. Miller, J.N. Miller, Statistics For Analytical Chemistry, Ellis Horwood, Chichester, 1984. 1211 R.C. Graham, Data analysis for the chemical sciences: a guide to statistical techniques, VCH Publishers, New York, 1993. t221 P.M. Bersier, J. Howell, C. Bruntlett, Analyst 119 (1994) 219-232. 1231 A. Hu, R.E. Dessy, A. Granell, Anal. Chem. 55 (1983) 320-328.