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Oscillating flow injection stripping potentiometry
ANALYTICA
CHIMICA ACTA ELSEVIER
Analytica
Chimica Acta 309 (1995) 293-299
Oscillating flow injection stripping potentiometry Spas D. Kolev ‘, Christopher W.K. Chow, David E. Davey School of Chemical Technology,
Unkersity
*,
Dennis E. Mulcahy
of South Australia, P.O. Box I, Ingle Farm, SA 5095, Australia
Received 17 October 1994; revised 31 January
1995; accepted 31 January
1995
Abstract A fully computerized flow injection (FI) manifold with a peristaltic pump allowing periodic alternation of the flow direction and incorporating a detector system capable of performing potentiometric stripping analysis (PSA) is outlined. The measuring technique has been named oscillating flow injection stripping potentiometry (OFISP). It combines the attractive features of both traditional flow injection analysis (FIA) and batch PSA and at the same time overcomes some of the most serious drawbacks of the latter resulting from the fact that potentiostatic deposition and chemical stripping occur in the same solution. An experimental study of the influence of the main parameters of the flow system on its behaviour was performed using Cu(I1) solutions in the pg I-’ concentration range as samples. The potentiostatic deposition was carried out on an Hg precoated glassy carbon electrode and Hg(I1) ions were utilized as oxidant in the chemical stripping step. Keywork
Flow injection;
Potentiometry
1. Introduction Potentiometric an
increasing
[l-7],
with
stripping
analysis
importance many
applications
in
trace
(PSA)
has gained
metal
analysis
of this technique
re-
metals are electroplated and concentrated onto a mercury film working electrode at an applied potential. The amalgamated metals are brought back from the working electrode into solution by an oxidant (e.g., Hg”+, 0,) present or additionally introduced into the sample solution. This results in a step-wise change in the potential of the working electrode, which now
ported
in the past
decade.
In such analysis,
* Corresponding author. ’ Permanent address: Faculty of Chemistry, University 1 James Bourchier Ave., BG-126 Sofia, Bulgaria.
of Sofia,
0003.2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0003-2670(95)00091-7
operates as an indicator electrode (Fig. 1). The resulting potential (E) versus time (t) curve can be considered as a normal redox titration curve containing both quantitative and qualitative information. The potential value at the plateau region is characteristic of the metal being stripped [ 1,2], while the elapsed time at the plateau potential is proportional to its initial concentration in the sample. Alternatively, the potential/time information can be collected and stored in the form of the time spent by the electrode at given potentials. This results in peak shaped pseudo-derivative curves, with each peak corresponding to a given analyte (Fig. 1) [8,9]. Mathematically this representation can be related to the E(t) curve mentioned above as the derivative dt/dE, with the area of a given peak equal to the corresponding stripping time. PSA data is relatively independent of the elec-
294
S.D. Koleu et al. /Analytica
Chimica Acta 309 (1995) 293-299
Fig. 1. Data presentation in PSA: (solid line) potential versus time, (dashed line) potential versus time spent by the electrode at a given potential (.Qp = electroplating potential).
trode surface area, so that reduction of the electrode size is a feasible and attractive option. This feature of the technique is an important prerequisite for miniaturization of the equipment without loss of sensitivity and simplifies the implementation of PSA as a detection technique in continuous flow [lo] and flow injection analysis (FIA) 1111. If PSA is performed in a FI manifold the processes of potentiostatic deposition and chemical stripping will take place in different sections of the
flowing stream. For this reason they will be virtually separated not only with respect to time as in the batch mode but also with respect to the medium. The sensitivity of analysis of different metal ions is thus improved and the resolution of overlapping stripping peaks increased compared to batch PSA [10,12]. Under FI conditions, the concentration of the oxidant (e.g., Hg2+) during stripping is not affected by the prior electroplating, and, thus improved reproducibility may be expected. Under FI conditions the background potential correction could easily be automated as well, The advantages offered by coupling PSA and FIA make the combined technique a powerful tool in the analysis of heavy metals [13,14]. Under normal FI conditions the contact time between the sample and the working electrode is relatively short, leading to lowered sensitivity in PSA, which is dependent on the plating time. However, if the flow rate is decreased to increase the contact time, a high sample throughput, one of the main advantages of FIA, will be lost. Furthermore, under the latter conditions the sensitivity will not improve as hoped, since at very low flow rates the convective component of radial mixing, which supplies the analyte to the electrode surface, will be reduced, and the generally slower process of diffusion will be domi-
I
I
5-d Potentiostat
Reference Electrode
Rotary Valve
Electrode
Electrode
I Carrier Fig. 2. Scheme of the experimental
setup.
SD. K&r
et al. /Analytica Chimica Acta 309 (1995) 293-299
nant. In the present paper an alternative approach which enhances the efficiency of the electroplating process for the analyte is outlined. It is similar to the technique applied by Wang et al. [15] for enhancement of the response of a stripping voltammetric flow injection system and is based on the periodic alternation of the flow direction during the potentiostatic step.
2. Experimental 2.1. Flow injection manifold The flow-injection manifold schematically represented in Fig. 2 includes a Minipuls 3 peristaltic pump (Gilson) fitted with Tygon” pump tubing. Polypropylene connectors and PTFE tubes (0.5 mm id.) were utilized throughout. A 4-way rotary valve with a sample loop size of 100 ~1 was employed as an injection device, with sample injection controlled by a pneumatic actuator, activated by a solenoid unit (Rheodyne 5041, 5701 and 7163, respectively). Radial mixing of the sample with the carrier solution was enhanced in a 1.5 m helically coiled PTFE tube of 11 mm coil diameter. The latter was jacketed, and acted as an in-stream deoxygenator with a steady nitrogen flow through the jacket. The flow cell employed in this manifold was a thin layer flow cell with 0.8 mm PTFE spacer with a BAS glassy carbon working electrode (3 mm diameter) (Bioanalytical Systems) at the base. A platinum auxiliary electrode (the exit tube) was built into the cell, and a Ag/AgCl (3 M KCl) electrode placed in the waste stream. 2.2. Hardware
and sojbvare
An IBM compatible 386-SX personal computer was used to control the experimental setup. Injection timing was controlled via the digital output of the computer. The potentiostat employed in this study was a BAS CV-1B cyclic voltammetry unit (Bioanalytical Systems). A 701A potentiometer (Orion) provided potential measurements, and a PCL-712 MultiLab Card (Advantech) gave data acquisition and control. An in-house designed, signal splitting unit, con-
295
sisting of three relay switches activated by software via the digital output port of the data acquisition card, controlled the application of the reduction potential across the electrodes, and separated the potentiostat signal from the potentiometer during the recording step. The potentiometer acted as a high input impedance amplifier for the electrode potential. The resolution in the potential measurements was 1 mV. The sampling rate of the system was set at uniform intervals of l/8000 s. Multichannel potentiometric monitoring was employed to give a differential data form, as previously mentioned [8,9]. The procedure employed was similar to that reported by Renman et. al. [12] for peak determination. All software was programmed in Microsoft Basic (Version 7.1). 2.3. Reagents All chemicals used were of analytical grade. Deionized water was used throughout the experiments. The carrier/stripping solution was 1 mg 1-l Hg(II1 nitrate solution in 0.1 M hydrochloric acid. Copper standards (Cu(I1) nitrate, Ajax Chemicals) in the concentration range O-1000 pg ll’ were prepared by serial dilution of a 1000 mg l- ’ Cu(I1) stock solution with 0.1 M HCl. 2.4. Mercury precoating The glassy carbon electrode was polished using the BAS PK-4 polishing kit (Bioanalytical Systems) and then rinsed with ethanol. A uniform mercury film was plated on the electrode by ten consecutive plating/stripping cycles, following normal practice [3,6,7], using a plating solution containing 100 mg 1-l Hg(I1) in 0.1 M HNO,. The plating solution was injected as a sample through the FI injection valve following the standard FI analysis procedure. 2.5. Procedure Initial deaeration of the carrier solution was carried out by bubbling high purity nitrogen gas into the mixture for one hour prior to analysis. Nitrogen purging is required to decrease the concentration of the dissolved oxygen which otherwise shortens the
S.D. Koleu et al. /Analytica
296
Chimica Acta 309 (1995) 293-299
stripping time prior to analysis. The oxygen concentration was further decreased in the in-stream deoxygenator (Fig. 2). 2.6. Manifold dispersion A series of experiments for selecting the proper timing parameters for the periodic alternation of the flow direction were performed using the amperometric option of the potentiostat. Samples containing only 0.1 M HCl solution were injected into the carrier stream under standard FI conditions, with the reduced Hg current reading providing the peak profile. The amperometric peaks were monitored at 3 different flow rates (1.0, 2.0 and 4.0 ml/min) with the potential of the working electrode poised at - 400 mV versus the reference. 2.7. PSA parameters The operational sequence of the analytical procedure controlled by software developed for this purpose, is shown in Fig. 3. Throughout the FI-PSA
I
Potentiostat on.
I
Tam Valve (ii Loading Position).
1 Sample.Pump on. (Sample Loading)
I
1
Turn Valve (Injecting).
I
J
1
Travelling Tie
I
I (T s) I
Alternating the Flow Direction.
Carrier Pump off.
60 Time (s) Fig. 4. Amperometric peak with five windows of different width monitored at 2.0 ml min-’ flow rate. (starting time of the 1 = 30.9 s, 2 = 32.7 s, 3 = 34.8 s, 4 = 36.8 s and oscillations: 5 = 44.0 s).
experiments, the potential during the plating step was set at -900 mV against the reference. A portion of the sample stream, termed a sample “window” in the subsequent discussion, was selected according to the amperometric results. The starting time and the length of each window, expressed in volume units (ml), were again determined according to the amperometric results (Fig. 4). During each experiment the flow direction was alternated by the pump allowing a preselected sample window (Fig. 4) to pass through the measuring cell an even number of times. One cycle incorporating a backward and forward reversal of the flow direction will be termed an oscillation in subsequent discussions. The effect of the window width, plating and stripping flow rate, and number of oscillations on the stripping time was studied by injecting 100 ~1 samples containing 500 ppb Cu(I1) in 0.1 M HCl. Calibration curves (stripping time vs. C&I) concentration) for four different numbers of oscillations (0, 4, 8, and 16) were obtained using 100-1000 ppb C&I) standards (Cu(I1) nitrate) at 267 ~1 window width and 2.0 ml/min plating flow rate.
+ (5 S) Potentiostat off. Record Potentiogram.
3. Results and discussion
1 Save Data. Plot Potentiogram.
Fig. 3. The operational sequence starting time of the oscillations).
of the FI-PSA
method
(T =
When defining the starting time of the oscillations and the duration of one oscillation for a given flow rate, information regarding the flow pattern in the
SD. Kolev et al. /Analytica
flow system is required. Such information can be provided by the residence time distribution function of the analyte upstream of the working electrode in the case of unidirectional flow (no oscillations). The amperometric peak heights obtained under such conditions are in fact proportional to this distribution function. The shape of the amperometric peaks (Fig. 4) indicates that convection plays an important role in the overall dispersion process for the manifold described, and the range of flow rates used in the experiments (1.0-4.0 ml/min). It was assumed that the residence time distribution information obtained for the Hg(II) ions could be used for other species frequently determined by potentiometric stripping analysis such as Cu(II). The factors affecting the magnitude of the stripping time were then examined. These included the number of oscillations, the window width, the flow rate during the oscillations, and the flow rate during stripping. The first three parameters were expected to affect the plating of the analyte, while the last parameter should influence only the stripping process. During stripping the flowing carrier solution supplies fresh solution with oxidant to the measuring cell. It is expected that the higher the flow rate during stripping the higher will be the oxidant (Hg(lI)) flux to the mercury film. As a result the stripping time will be reduced which has been experimentally observed by Schulze et al. [16]. The highest stripping time (maximum sensitivity) should be observed at stopped flow. The experimental results
01 0
1
2
3
4
Stripping Flow Rate (mUmin)
Fig. 5. Stripping time versus stripping flow rate for 64 s plating time (0 = experimental points, 0.267 ml window width, 2.00 ml/min transportation and oscillation flow rate, 4.00 ml/min flushing flow rate).
Chimica Acta 309 (1995) 293-299
297 Plating Time(s)
0
07 0
80
160
240
5
10
15
320
20
Number of Oscillations
Fig. 6. Influence of the number time (conditions as in Fig. 5).
of oscillations
on the stripping
presented in Fig. 5 confirm these conclusions. It was found that at very low flow rates the reproducibility of the measurements could be improved at the expense of slight decrease in sensitivity. The magnitude of the stripping time is also determined by the number of oscillations and the flow rate during stripping. The former parameter affects the plating of the analyte while the latter one influences only the stripping process. With the parameters affecting the plating process it could be expected that by increasing the number of oscillations starting from zero (i.e., conventional FI mode) up to a certain limit, the stripping time will also appreciably increase (Fig. 6). However the greater the number of oscillations, the greater will be the depletion of the analyte in the corresponding window (Fig. 4). As a result of this, the stripping time will asymptotically approach an upper limit corresponding to complete depletion of the analyte. Close to this region any increase in sensitivity can be achieved only by using a considerable number of oscillations, which will sharply decrease the sample throughput. It can be seen that by employing the oscillating flow injection mode the sensitivity can be increased several times (Fig. 6). The combined influence of the window width and the plating time (the time during which the oscillations take place) for different flow rates is illustrated in a three-dimensional form (Fig. 7a-cl. It can be
298
S.D. Kolev et al. /Analytica Chimica Acta 309 (1995) 293-299
Fig. 7. Dependence of the stripping time on the window width and the plating time for (A) 1.00 ml/min, (B) 2.00 ml/min, and (C) 4.00 ml/min oscillation flow rate.
seen that the dependence of the stripping time on the window width while the plating time is maintained constant leads to a maximum (Fig. 7). The position of this maximum shifts to lower values of the window width with increasing flow rate during the oscillation process. The plating time necessary for attaining the limiting sensitivity at a given oscillation flow rate was also decreased by increasing the flow rate. The three-dimensional graph allows the selection of the optimal sensitivity for a given flow rate region of window width and oscillation number. These regions provide relatively wide ranges of the
parameters involved, which leaves some flexibility in varying the parameters without deterioration in the sensitivity of the analysis. The sample throughput achieved with the experimental manifold varied in the range 11-25 h-‘. It depended on the flow rates during: (i) transportation of the sample plug to the measuring cell (transportation flow rate); (ii) the oscillations (oscillation flow rate); and (iii) flushing the manifold, prior to the next analysis (wash flow rate). All flow rates were considered as steady-state throughout the experiments assuming that the duration of the transitional periods occurring after injection and reversing the flow direction were negligible [17]. If the flow rates mentioned above are increased, the sample throughput will be increased as well. However, it should be realized that for each one of these flow rates there is an upper limit above which deterioration in the performance of the manifold will occur. At high transportation flow rates (e.g., 4.0 ml/min) narrow amperometric (current versus time) peaks were monitored. A slight error in determining the starting time for the oscillations (Fig. 4) would lead to a mismatch between the window and the actual residence time distribution of the analyte. Probably this was one of the main reasons for the decreased sensitivity observed during the experiments. When oscillation flow rate was increased the sensitivity tended to decrease (Fig. 7). Worsened reproducibility at high flow rates was attributed to deformations of the mercury film on the electrode surface, caused by the hydrodynamic effects accompanying the sudden change in the direction of the flow. Though the washing flow rate cannot affect the mercury film to the same extent as the oscillation flow rate, deformations at very high flow rates cannot be ruled out. To avoid sample carryover a wash period of 120 s at a 4.0 ml/min washing flow rate was used throughout the experiments. On the basis of the FI-PSA experimental results, 2.0 ml/min transportation and oscillation flow rates were found to give an acceptable compromise between the contrary requirements of high samplethroughput on one hand and satisfactory sensitivity and reproducibility, on the other. Calibration curves obtained for a varying number of oscillations with other parameters of the FI system constant, exhibit linearity (Fig. 8) except for the case
299
S.D. KoleL, et al. /Analytica Chimica Ada 309 (1995) 29.3-299
traces of metal ions in environmental, pharmaceutical samples.
food
and
Acknowledgements
0
500
loo0
Concentration (ppb) Fig. 8. Calibration curves for Cu(ll) for different oscillations (0, 4. 8, 16) (conditions as in Fig. 5).
numbers
of
with the highest number of oscillations studied (i.e., 16). This result demonstrates that oscillating FI-PSA can be used for straightforward determinations of metal ions.
The authors wish to acknowledge the contributions of T.C.W. Yeow and J.W.K. Wong in constructing the hardware, the financial support of the Department of Industry, Trade and Regional Development and the University of South Australia to S.D. Kolev, and that from the Department of Employment, Education and Training in the form of a scholarship to C.W.K. Chow.
References [II D. Jagner and A. Graneli, Anal. Chim. Acta, X3 (1976) 19. L21D. Jagner, Analyst, 107 (1982) 593. [31 D. Jagner, Anal. Chem., 50 (1978) 1924. [41 G.N. Chen, M. Mitri, G. Scollary and V. Vicentc-Beckett.
4. Conclusions
Chemistry
in Australia,
April (1994) 181.
[51 D. Jagner, E. Sahlin and L. Renman, Talanta, 41 (1994) 5 15.
On the basis of the results obtained it can be concluded that FIA and batch PSA may be successfully combined as a technique which is most appropriately named, oscillating flow injection stripping potentiometry (OFISP). This technique overcomes some of the main drawbacks of batch PSA caused by the depletion of the oxidant during the potentiostatic deposition process while introducing some attractive features of FIA, e.g., easy and relatively inexpensive automation of analysis, small sample volumes, high sample-throughput, and the possibility of on-line sample pretreatment. At the same time, due to the periodic alternation of the flow direction, the contact time between the working electrode and the analyte can be substantially increased compared to traditional FI applications, thus retaining the high sensitivity characteristic of batch PSA. All these features of the proposed technique suggest that it could be successfully used in the quantitative determination of
[61 C.W.K. Chow, D.E. Davey, M.R. Haskard, D.E. Mulcahy and T.C.W. Yeow, J. Chem. Educ., 71 (1994) 997.
[71 C.W.K. Chow, D.E. Davey and D.E. Mulcahy,
Chemom. Intell. Lab. Syst., in press. Bl .I. Mortensen, E. Ouziel, H.J. Skov and L. Kryger, Anal. Chim. Acta, 112 (1979) 297. Dl J.K. Christensen, L. Kryger, J. Mortensen and J. Rasmussen, Anal. Chim. Acta, 121 (1980) 71. 1101 L. Anderson, D. Jagner and M. Josefson, Anal. Chem., 54 (1982) 1371. 1111A. Hu, R.E. Dessy and A. Gram%, Anal. Chcm.. 55 (1983) 320. [I21 L. Renman, D. Jagner and R. Berglund, Anal. Chim. Acta, 188 (1986) 137. 1131 M.D. Luque de Castro and A. Izquicrdo. Electroanalysis, 3 (1991) 457. [141 W. Frenzel and P. Briitter, Anal. Chim. Acta. 179 (1986) 389. 2 [151 J. Wang, H. Huiliang and W. Kuhiak. Electroanalysis. (1990) 127. [I’51G. Schulze, E. Han, 0. Elsholz and W. Frenzel. Fresenius’ Z. Anal. Chem., 327 (1987) 15. [171 SD. Kolev and E. Pungor, Talanta, 34 (1987) 1OOY.
CHIMICA ACTA ELSEVIER
Analytica
Chimica Acta 309 (1995) 293-299
Oscillating flow injection stripping potentiometry Spas D. Kolev ‘, Christopher W.K. Chow, David E. Davey School of Chemical Technology,
Unkersity
*,
Dennis E. Mulcahy
of South Australia, P.O. Box I, Ingle Farm, SA 5095, Australia
Received 17 October 1994; revised 31 January
1995; accepted 31 January
1995
Abstract A fully computerized flow injection (FI) manifold with a peristaltic pump allowing periodic alternation of the flow direction and incorporating a detector system capable of performing potentiometric stripping analysis (PSA) is outlined. The measuring technique has been named oscillating flow injection stripping potentiometry (OFISP). It combines the attractive features of both traditional flow injection analysis (FIA) and batch PSA and at the same time overcomes some of the most serious drawbacks of the latter resulting from the fact that potentiostatic deposition and chemical stripping occur in the same solution. An experimental study of the influence of the main parameters of the flow system on its behaviour was performed using Cu(I1) solutions in the pg I-’ concentration range as samples. The potentiostatic deposition was carried out on an Hg precoated glassy carbon electrode and Hg(I1) ions were utilized as oxidant in the chemical stripping step. Keywork
Flow injection;
Potentiometry
1. Introduction Potentiometric an
increasing
[l-7],
with
stripping
analysis
importance many
applications
in
trace
(PSA)
has gained
metal
analysis
of this technique
re-
metals are electroplated and concentrated onto a mercury film working electrode at an applied potential. The amalgamated metals are brought back from the working electrode into solution by an oxidant (e.g., Hg”+, 0,) present or additionally introduced into the sample solution. This results in a step-wise change in the potential of the working electrode, which now
ported
in the past
decade.
In such analysis,
* Corresponding author. ’ Permanent address: Faculty of Chemistry, University 1 James Bourchier Ave., BG-126 Sofia, Bulgaria.
of Sofia,
0003.2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0003-2670(95)00091-7
operates as an indicator electrode (Fig. 1). The resulting potential (E) versus time (t) curve can be considered as a normal redox titration curve containing both quantitative and qualitative information. The potential value at the plateau region is characteristic of the metal being stripped [ 1,2], while the elapsed time at the plateau potential is proportional to its initial concentration in the sample. Alternatively, the potential/time information can be collected and stored in the form of the time spent by the electrode at given potentials. This results in peak shaped pseudo-derivative curves, with each peak corresponding to a given analyte (Fig. 1) [8,9]. Mathematically this representation can be related to the E(t) curve mentioned above as the derivative dt/dE, with the area of a given peak equal to the corresponding stripping time. PSA data is relatively independent of the elec-
294
S.D. Koleu et al. /Analytica
Chimica Acta 309 (1995) 293-299
Fig. 1. Data presentation in PSA: (solid line) potential versus time, (dashed line) potential versus time spent by the electrode at a given potential (.Qp = electroplating potential).
trode surface area, so that reduction of the electrode size is a feasible and attractive option. This feature of the technique is an important prerequisite for miniaturization of the equipment without loss of sensitivity and simplifies the implementation of PSA as a detection technique in continuous flow [lo] and flow injection analysis (FIA) 1111. If PSA is performed in a FI manifold the processes of potentiostatic deposition and chemical stripping will take place in different sections of the
flowing stream. For this reason they will be virtually separated not only with respect to time as in the batch mode but also with respect to the medium. The sensitivity of analysis of different metal ions is thus improved and the resolution of overlapping stripping peaks increased compared to batch PSA [10,12]. Under FI conditions, the concentration of the oxidant (e.g., Hg2+) during stripping is not affected by the prior electroplating, and, thus improved reproducibility may be expected. Under FI conditions the background potential correction could easily be automated as well, The advantages offered by coupling PSA and FIA make the combined technique a powerful tool in the analysis of heavy metals [13,14]. Under normal FI conditions the contact time between the sample and the working electrode is relatively short, leading to lowered sensitivity in PSA, which is dependent on the plating time. However, if the flow rate is decreased to increase the contact time, a high sample throughput, one of the main advantages of FIA, will be lost. Furthermore, under the latter conditions the sensitivity will not improve as hoped, since at very low flow rates the convective component of radial mixing, which supplies the analyte to the electrode surface, will be reduced, and the generally slower process of diffusion will be domi-
I
I
5-d Potentiostat
Reference Electrode
Rotary Valve
Electrode
Electrode
I Carrier Fig. 2. Scheme of the experimental
setup.
SD. K&r
et al. /Analytica Chimica Acta 309 (1995) 293-299
nant. In the present paper an alternative approach which enhances the efficiency of the electroplating process for the analyte is outlined. It is similar to the technique applied by Wang et al. [15] for enhancement of the response of a stripping voltammetric flow injection system and is based on the periodic alternation of the flow direction during the potentiostatic step.
2. Experimental 2.1. Flow injection manifold The flow-injection manifold schematically represented in Fig. 2 includes a Minipuls 3 peristaltic pump (Gilson) fitted with Tygon” pump tubing. Polypropylene connectors and PTFE tubes (0.5 mm id.) were utilized throughout. A 4-way rotary valve with a sample loop size of 100 ~1 was employed as an injection device, with sample injection controlled by a pneumatic actuator, activated by a solenoid unit (Rheodyne 5041, 5701 and 7163, respectively). Radial mixing of the sample with the carrier solution was enhanced in a 1.5 m helically coiled PTFE tube of 11 mm coil diameter. The latter was jacketed, and acted as an in-stream deoxygenator with a steady nitrogen flow through the jacket. The flow cell employed in this manifold was a thin layer flow cell with 0.8 mm PTFE spacer with a BAS glassy carbon working electrode (3 mm diameter) (Bioanalytical Systems) at the base. A platinum auxiliary electrode (the exit tube) was built into the cell, and a Ag/AgCl (3 M KCl) electrode placed in the waste stream. 2.2. Hardware
and sojbvare
An IBM compatible 386-SX personal computer was used to control the experimental setup. Injection timing was controlled via the digital output of the computer. The potentiostat employed in this study was a BAS CV-1B cyclic voltammetry unit (Bioanalytical Systems). A 701A potentiometer (Orion) provided potential measurements, and a PCL-712 MultiLab Card (Advantech) gave data acquisition and control. An in-house designed, signal splitting unit, con-
295
sisting of three relay switches activated by software via the digital output port of the data acquisition card, controlled the application of the reduction potential across the electrodes, and separated the potentiostat signal from the potentiometer during the recording step. The potentiometer acted as a high input impedance amplifier for the electrode potential. The resolution in the potential measurements was 1 mV. The sampling rate of the system was set at uniform intervals of l/8000 s. Multichannel potentiometric monitoring was employed to give a differential data form, as previously mentioned [8,9]. The procedure employed was similar to that reported by Renman et. al. [12] for peak determination. All software was programmed in Microsoft Basic (Version 7.1). 2.3. Reagents All chemicals used were of analytical grade. Deionized water was used throughout the experiments. The carrier/stripping solution was 1 mg 1-l Hg(II1 nitrate solution in 0.1 M hydrochloric acid. Copper standards (Cu(I1) nitrate, Ajax Chemicals) in the concentration range O-1000 pg ll’ were prepared by serial dilution of a 1000 mg l- ’ Cu(I1) stock solution with 0.1 M HCl. 2.4. Mercury precoating The glassy carbon electrode was polished using the BAS PK-4 polishing kit (Bioanalytical Systems) and then rinsed with ethanol. A uniform mercury film was plated on the electrode by ten consecutive plating/stripping cycles, following normal practice [3,6,7], using a plating solution containing 100 mg 1-l Hg(I1) in 0.1 M HNO,. The plating solution was injected as a sample through the FI injection valve following the standard FI analysis procedure. 2.5. Procedure Initial deaeration of the carrier solution was carried out by bubbling high purity nitrogen gas into the mixture for one hour prior to analysis. Nitrogen purging is required to decrease the concentration of the dissolved oxygen which otherwise shortens the
S.D. Koleu et al. /Analytica
296
Chimica Acta 309 (1995) 293-299
stripping time prior to analysis. The oxygen concentration was further decreased in the in-stream deoxygenator (Fig. 2). 2.6. Manifold dispersion A series of experiments for selecting the proper timing parameters for the periodic alternation of the flow direction were performed using the amperometric option of the potentiostat. Samples containing only 0.1 M HCl solution were injected into the carrier stream under standard FI conditions, with the reduced Hg current reading providing the peak profile. The amperometric peaks were monitored at 3 different flow rates (1.0, 2.0 and 4.0 ml/min) with the potential of the working electrode poised at - 400 mV versus the reference. 2.7. PSA parameters The operational sequence of the analytical procedure controlled by software developed for this purpose, is shown in Fig. 3. Throughout the FI-PSA
I
Potentiostat on.
I
Tam Valve (ii Loading Position).
1 Sample.Pump on. (Sample Loading)
I
1
Turn Valve (Injecting).
I
J
1
Travelling Tie
I
I (T s) I
Alternating the Flow Direction.
Carrier Pump off.
60 Time (s) Fig. 4. Amperometric peak with five windows of different width monitored at 2.0 ml min-’ flow rate. (starting time of the 1 = 30.9 s, 2 = 32.7 s, 3 = 34.8 s, 4 = 36.8 s and oscillations: 5 = 44.0 s).
experiments, the potential during the plating step was set at -900 mV against the reference. A portion of the sample stream, termed a sample “window” in the subsequent discussion, was selected according to the amperometric results. The starting time and the length of each window, expressed in volume units (ml), were again determined according to the amperometric results (Fig. 4). During each experiment the flow direction was alternated by the pump allowing a preselected sample window (Fig. 4) to pass through the measuring cell an even number of times. One cycle incorporating a backward and forward reversal of the flow direction will be termed an oscillation in subsequent discussions. The effect of the window width, plating and stripping flow rate, and number of oscillations on the stripping time was studied by injecting 100 ~1 samples containing 500 ppb Cu(I1) in 0.1 M HCl. Calibration curves (stripping time vs. C&I) concentration) for four different numbers of oscillations (0, 4, 8, and 16) were obtained using 100-1000 ppb C&I) standards (Cu(I1) nitrate) at 267 ~1 window width and 2.0 ml/min plating flow rate.
+ (5 S) Potentiostat off. Record Potentiogram.
3. Results and discussion
1 Save Data. Plot Potentiogram.
Fig. 3. The operational sequence starting time of the oscillations).
of the FI-PSA
method
(T =
When defining the starting time of the oscillations and the duration of one oscillation for a given flow rate, information regarding the flow pattern in the
SD. Kolev et al. /Analytica
flow system is required. Such information can be provided by the residence time distribution function of the analyte upstream of the working electrode in the case of unidirectional flow (no oscillations). The amperometric peak heights obtained under such conditions are in fact proportional to this distribution function. The shape of the amperometric peaks (Fig. 4) indicates that convection plays an important role in the overall dispersion process for the manifold described, and the range of flow rates used in the experiments (1.0-4.0 ml/min). It was assumed that the residence time distribution information obtained for the Hg(II) ions could be used for other species frequently determined by potentiometric stripping analysis such as Cu(II). The factors affecting the magnitude of the stripping time were then examined. These included the number of oscillations, the window width, the flow rate during the oscillations, and the flow rate during stripping. The first three parameters were expected to affect the plating of the analyte, while the last parameter should influence only the stripping process. During stripping the flowing carrier solution supplies fresh solution with oxidant to the measuring cell. It is expected that the higher the flow rate during stripping the higher will be the oxidant (Hg(lI)) flux to the mercury film. As a result the stripping time will be reduced which has been experimentally observed by Schulze et al. [16]. The highest stripping time (maximum sensitivity) should be observed at stopped flow. The experimental results
01 0
1
2
3
4
Stripping Flow Rate (mUmin)
Fig. 5. Stripping time versus stripping flow rate for 64 s plating time (0 = experimental points, 0.267 ml window width, 2.00 ml/min transportation and oscillation flow rate, 4.00 ml/min flushing flow rate).
Chimica Acta 309 (1995) 293-299
297 Plating Time(s)
0
07 0
80
160
240
5
10
15
320
20
Number of Oscillations
Fig. 6. Influence of the number time (conditions as in Fig. 5).
of oscillations
on the stripping
presented in Fig. 5 confirm these conclusions. It was found that at very low flow rates the reproducibility of the measurements could be improved at the expense of slight decrease in sensitivity. The magnitude of the stripping time is also determined by the number of oscillations and the flow rate during stripping. The former parameter affects the plating of the analyte while the latter one influences only the stripping process. With the parameters affecting the plating process it could be expected that by increasing the number of oscillations starting from zero (i.e., conventional FI mode) up to a certain limit, the stripping time will also appreciably increase (Fig. 6). However the greater the number of oscillations, the greater will be the depletion of the analyte in the corresponding window (Fig. 4). As a result of this, the stripping time will asymptotically approach an upper limit corresponding to complete depletion of the analyte. Close to this region any increase in sensitivity can be achieved only by using a considerable number of oscillations, which will sharply decrease the sample throughput. It can be seen that by employing the oscillating flow injection mode the sensitivity can be increased several times (Fig. 6). The combined influence of the window width and the plating time (the time during which the oscillations take place) for different flow rates is illustrated in a three-dimensional form (Fig. 7a-cl. It can be
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Fig. 7. Dependence of the stripping time on the window width and the plating time for (A) 1.00 ml/min, (B) 2.00 ml/min, and (C) 4.00 ml/min oscillation flow rate.
seen that the dependence of the stripping time on the window width while the plating time is maintained constant leads to a maximum (Fig. 7). The position of this maximum shifts to lower values of the window width with increasing flow rate during the oscillation process. The plating time necessary for attaining the limiting sensitivity at a given oscillation flow rate was also decreased by increasing the flow rate. The three-dimensional graph allows the selection of the optimal sensitivity for a given flow rate region of window width and oscillation number. These regions provide relatively wide ranges of the
parameters involved, which leaves some flexibility in varying the parameters without deterioration in the sensitivity of the analysis. The sample throughput achieved with the experimental manifold varied in the range 11-25 h-‘. It depended on the flow rates during: (i) transportation of the sample plug to the measuring cell (transportation flow rate); (ii) the oscillations (oscillation flow rate); and (iii) flushing the manifold, prior to the next analysis (wash flow rate). All flow rates were considered as steady-state throughout the experiments assuming that the duration of the transitional periods occurring after injection and reversing the flow direction were negligible [17]. If the flow rates mentioned above are increased, the sample throughput will be increased as well. However, it should be realized that for each one of these flow rates there is an upper limit above which deterioration in the performance of the manifold will occur. At high transportation flow rates (e.g., 4.0 ml/min) narrow amperometric (current versus time) peaks were monitored. A slight error in determining the starting time for the oscillations (Fig. 4) would lead to a mismatch between the window and the actual residence time distribution of the analyte. Probably this was one of the main reasons for the decreased sensitivity observed during the experiments. When oscillation flow rate was increased the sensitivity tended to decrease (Fig. 7). Worsened reproducibility at high flow rates was attributed to deformations of the mercury film on the electrode surface, caused by the hydrodynamic effects accompanying the sudden change in the direction of the flow. Though the washing flow rate cannot affect the mercury film to the same extent as the oscillation flow rate, deformations at very high flow rates cannot be ruled out. To avoid sample carryover a wash period of 120 s at a 4.0 ml/min washing flow rate was used throughout the experiments. On the basis of the FI-PSA experimental results, 2.0 ml/min transportation and oscillation flow rates were found to give an acceptable compromise between the contrary requirements of high samplethroughput on one hand and satisfactory sensitivity and reproducibility, on the other. Calibration curves obtained for a varying number of oscillations with other parameters of the FI system constant, exhibit linearity (Fig. 8) except for the case
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traces of metal ions in environmental, pharmaceutical samples.
food
and
Acknowledgements
0
500
loo0
Concentration (ppb) Fig. 8. Calibration curves for Cu(ll) for different oscillations (0, 4. 8, 16) (conditions as in Fig. 5).
numbers
of
with the highest number of oscillations studied (i.e., 16). This result demonstrates that oscillating FI-PSA can be used for straightforward determinations of metal ions.
The authors wish to acknowledge the contributions of T.C.W. Yeow and J.W.K. Wong in constructing the hardware, the financial support of the Department of Industry, Trade and Regional Development and the University of South Australia to S.D. Kolev, and that from the Department of Employment, Education and Training in the form of a scholarship to C.W.K. Chow.
References [II D. Jagner and A. Graneli, Anal. Chim. Acta, X3 (1976) 19. L21D. Jagner, Analyst, 107 (1982) 593. [31 D. Jagner, Anal. Chem., 50 (1978) 1924. [41 G.N. Chen, M. Mitri, G. Scollary and V. Vicentc-Beckett.
4. Conclusions
Chemistry
in Australia,
April (1994) 181.
[51 D. Jagner, E. Sahlin and L. Renman, Talanta, 41 (1994) 5 15.
On the basis of the results obtained it can be concluded that FIA and batch PSA may be successfully combined as a technique which is most appropriately named, oscillating flow injection stripping potentiometry (OFISP). This technique overcomes some of the main drawbacks of batch PSA caused by the depletion of the oxidant during the potentiostatic deposition process while introducing some attractive features of FIA, e.g., easy and relatively inexpensive automation of analysis, small sample volumes, high sample-throughput, and the possibility of on-line sample pretreatment. At the same time, due to the periodic alternation of the flow direction, the contact time between the working electrode and the analyte can be substantially increased compared to traditional FI applications, thus retaining the high sensitivity characteristic of batch PSA. All these features of the proposed technique suggest that it could be successfully used in the quantitative determination of
[61 C.W.K. Chow, D.E. Davey, M.R. Haskard, D.E. Mulcahy and T.C.W. Yeow, J. Chem. Educ., 71 (1994) 997.
[71 C.W.K. Chow, D.E. Davey and D.E. Mulcahy,
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