A Laboratory Experiment in Peak Resolution Using Operational Amplifiers Gregory Gilmartin, Shaw Kong Chang, and G e q e W. Harrlngton Temple University, Philadelphia, PA 19122 The purpose of this experiment is to offer the student an experience in a simple application of operational amplifiers that will improve the accuracy of an analytical experiment. A similar result could, of course, be obtained using a microprocessor-based instrument or an instrument interfaced to a computer. This experiment, however, allows the student the opportunity to build a simple circuit that is easily understood and see a n immediate application. When analyzing a multicomponent system using an instrumental technique that yields peak shaped responses, such as differential pulse polarography, the peak for one analyte may appear as a shoulder on the peak of a second analyte. If there is a large concentration difference between the two components the height of the peakdue to the analyte at the lower concentration may be difficult to measure. An example of this is shown in Figure 1,which is a differential pulse polarogram of a solution of Cd(I1) and Cu(I1) in 2.0 M KSCN. The larger peak is due to the Cd(I1). The shoulder peak is due to the reduction of Cu(1) to Cu(0); the Cu(1) peak is the second stage of the two-step reduction of Cu(I1) in this supporting electrolyte. If the leading edge of the Cd(I1) peak is recognized as essentiallv a ramo. . . then subtraction of that ramo from the potential;egionencompassing thecopper peakshould result ina re.solvcd Cu(1) oeak.The height of this resolved peakcan then he more easiG measured fr%mthe flat baseline than it could be from the sloping baseline. The circuitry and procedure given below permits this type of resolution.
0 - ,300
-.5W - ,700 E vs. Ag/AgCI (volts) Figure 1.Oiflerentiaipulse polarogram of a mixture of copper and cadmium in 2.0 MKSCN. 276
Journal of Chemical Education
Experimental The differential pulse polarograms were obtained using an IBM model EC225 voltammetric analyzer. The operating conditions were: working electrode,dropping mercury; counter electrode, platinum; reference electrode,AgIAgC1, sat. KCI; drop time, one second; pulse height, 60 mV; scan rate, 2 mV/s. Any comparable potentiostat having differential pulse capability could be used.
VRS
-
V.. V,,-Recorder
U~=Lln=545KH
Figure 2. Operational amplifier circuit diagrams for ramp generator (U,) and subtractor circuit (LIZ).
E vs. AglAgCl (volts) Figure 3. Differential pulse polamgram of 1.93 X 10-'MCd(l1) X IO-'MCU(II) in 2.0 M KSCN. Only the copper peak and the leading edge of me cadmium peak is shown.
The operational amplifiers were Analog Devices No. 545KH. All resistors were *1% and the capacitor was a high-quality, low-leakage polystyrene capacitor. The circuit was breadhoarded on an E&L Instruments model CDP-01 IC breadboard and power supply. The constant voltage source was a Heath Model No. EU 80-A Voltage Reference Source (VRS). All reagents were analytical grade. Solutions were deaerated with HP nitrogen that was passed through a vanadous scrubber and saturated at the vapor pressure of the supporting electrolyte. For the experiment as described here the initial concentrations of Cd(I1)andC~(I1) were 1.93 X lO-'Mand 1.45 X 10-W, respectively. The metal ion solutions were prepared in 2.0 MKSCN. Procedure The circuit should be assembled as shown in the schematic in Figure 2. The current output from the palarographic analyzer should he connected to point Vi,. The output of UI should be connected to the Y input of the recorder. The initial voltage should be set at -0.432 V versus AgIAgCI. With the switch "s" closed the scan is started and the polarogram shown in Figure 3A should be obtained. The voltage of the VRS(Vi,) is now calculated from eq 1 (below)by determining the desired slope from the polarogram (VW in eq 1) and solving for Vj. The scan should be repeated by opening switch "S" at the same time the "start" switch on the EC225 is depressed. Apesk suchas that shown in Figure 3Bshould be recorded. The scan is stopped at the point at which the baseline is fully established. When the scan is stopped the switch "a" is closed. If the baseline is not flat the voltage of the VRS should he adjusted and scans repeated until a flat haseline is achieved. Variations in circuit components may cause the actual VRS setting to differ slightly from the calculatedvalue. The scan may now be repeated several times to establish the reproducibility of the resolved peak. The four peaks shown in Figure 3 were produced in this fashion. Incremental amounts of Cu(I1)solution should he added and the resolved polaragram recorded after each addition using the procedure given above. To obtain the results shown here 20 pL aliquots of the copper solution were added. In each case the polarogram should be recordedwith and withoutthe integratingcircuit (U,). The peakheightsof each Cu peak should he measured and plotted versus concentration. A typical plot is shown in Figure 4. The corrected points refer to the oeak heiehts of the eaooer .. .oeak after ramo subtraction.The uncorrertcdpoints were read t'rtrm theuriginnl polnmyr.m. It i~rlcnrfrom t l w figure that the wattcr is reduced ronsiderahlyntter whtmrtim. Discussion The electronic circuit consists of two parts. The first is a n integrator shown by U1in the top of Figure 2. T h e output of this circuit is given by eq 1.
where R = innut resistance. V;. = innut voltage. and C = feedback capacitance. ~ h e the d input to an integrator is a constant voltage, the output is s i m ~ l va ramp. The slope of the ramp is varied by changing the-input ioltage. ~ e u c e ,
Figure 4.
Plots of peak height versus copper concentration for corrected and
uncorrected peak heights.
varying the setting of the VRS allows one to select a ramp of any desired slope. T h e integrator is connected, to one input of a n adder circuit (U2).The input and feedback resistors are equal in magnitude hence the circuit has unity gain. The second input of the adder circuit (Vi,J is the output of the i-e converter of the EC225. Thus, the ramp and the signal from the EC225 are combined in the adder circuit yielding the result shown in the figures. It should be apparent from Figure 3 that the height of the Cu peak would be very difficult to measure since the peak maximum cannot be located. Suhtracting out the ramp portion of the larger Cd peak results in a Cu peak in which the maximum can easily be found and thus, the height measured. I n differential pulse polarography a plot of peak heieht versus concentration should be linear. T h e two plots shown in Figure 4 show the improvement in linearity ;sing the subtractor circuit. The plots, incidentally, do not pass through the origin since zero response for this system would be that copper concentration a t which the copper peak disappears in-the Cd peak, not zero copper concentration.
Volume 63
Number 3
March 1986
277
