(zl - zZ)/(zz - zs), regardless of whether Reaction 2 takes place or not. This conclusion contrasts with the interpretation of the experimental value for the ratio of the height of the oxidation wave from I- to Iz to the height of the successive oxidation wave from I2 t o 1 0 3 - in 1M HCIOa, as given by Beran and Bruckenstein (5). We have seen that, if the standard rate constant for the electrode process Equation 1b is sufficiently high, then the boundary value problem relative to the partial cathodic wave A P +. A S involves exclusively the linear combinations cp and I) and therefore is formally identical both when the solution contains either A1 or A z . Hence, under the present conditions, the second cathodic wave Az +. As obtained from a solution of A 1 of concentration C1* is identical both in shape and in height to the unique cathodic wave given by a solution of Az of concentration C:* = I1C1*/h. If the substance AP is not stable in solution or if its preparation is difficult, we can therefore advantageously study the properties of the A z + A l step starting from a solution of A1 without worrying about kinetic complications due to Reaction 2. The preceding considerations hold even if Equilibrium 2 is
perfectly mobile and may be easily extended to the case in which A1 is electroreduced in more than two steps. An eventual deviation from the preceding theoretical expectations may be attributed to the existence of a notable difference among the diffusion coefficients of the various species. Thus, for example, the two successive reversible steps I- + Iz and IZ--c IC1 of the overall oxidation wave of I- to IC1 in hydrochloric acid medium on smooth platinum, exhibit different heights ( 9 , 10) on account of the noticeable difference among the diffusion coefficients of I-, Iz, IzCl-, ICl, and ICIz-. Institute of Analytical Chemistry University of Florence Florence, Italy
ROLANDOGUIDELLI
RECEIVED for review October 19, 1970. Accepted June 8, 1971. (9) A. L. Beilby and A. L. Crittenden, J . Phys. Chem., 64, 177 (1960). (10) G. Piccardi and R. Guidelli, ibid.,72, 2782 (1968).
A Digital Potentiostat SIR: Reported, for the first time, is an instrument for potentiostatic control of an electrochemical cell employing a new technique-digital current feedback. Digital current pulses, which can be scaled to read-out directly on a scaler the weight of sample oxidized or reduced, pass through the auxiliary electrode of the cell and are used to maintain a controlpotential. The potential-control rise-time of this device is limited primarily by the R C characteristics of the cell and, to a minor extent, by the size of the digital current driver and the dead time between pulses. This development entirely obviates the need for elaborate phase-correcting networks presently required to prevent potentiostatic overshoot or oscillation. In addition, current-time information appears at the output of the device in the form of a pulse train and is thus directly computer compatible. The use of a current-to-voltage converter and a voltage-to-frequency converter required for computer compatibility with conventional analog systems is eliminated. Since the instrument operates simultaneously as a potentiostat, a current-to-frequency converter, and subsequently as a digital integrator, a new term-digipotentiogrator, abbreviated DPI-has been coined t o identify it. The title of this article may, however, find more general acceptance. The further advantages of this new design are improved sensitivity and stability, design simplicity, and a large reduction in costs for components and assembly. Extensive use of integrated circuits is made. Also the instrument size and weight are small and the power requirements are quite low. The arrangement shown in Figure 1 is used to digitally maintain a control-potential and t o digitally measure the charge transfer which occurs in an electrochemical reaction. Two charge sources are connected to the auxiliary electrode. One source injects charge digitally, while the other source, which is a constant current driver, extracts charge on a continuous basis. Both drivers are designed to deliver up to 1 m A of average current and to perform independently of the potential of the auxiliary electrode. 1718
The cell reference electrode is connected to the positive terminal of a differential comparitor through a unity gain impedance coupling amplifier and the desired control-potential is applied to the negative terminal. Any positive output from the differential comparitor-balance detector initiates operation of the digital charge pump for fixed time periods and at rates determined by the charge demands of the cell. A local 500-kHz crystal clock furnishes digital injector drive intervals up to the clock frequency and therefore provides very good digital resolution. Since the charge demand sampling is made only during the times between possible injector drive intervals, the occurrence of a partial current drive interval is prevented. The small value capacitor C1is attached from the reference electrode to the grounded working electrode to maintain potential-control when the cell is driven through its electrocapillary maximum potential. Figure 2 illustrates the current and voltage response in time to a -500 mV to ground square-wave applied t o the control input of the DPI when employing a capacitive-resistive simulated cell load. The digital current feedback pulses, which are initially applied at the clock frequency and thus appear as a blur on the time base selected, drive the control potential from -500 mV to ground. Once ground is reached, only occasional pulses are required to maintain the control-potential. These pulses have a n average frequency of about 4 kHz and a displacement in the control-potential of approximately 10 mV. Note that, in this system, the descent time is limited only by the current capability of the digital charge source and by the dead time between pulses. The use of a IO-mA charge source would, for example, reduce the descent time by a factor of 10. Figure 3 shows a repeat of the above experiment but now using a modified Matson et ai. cell (1) and employing 0.01M (1) W. R. Matson, D. K. Roe, and D. E. Carritt, ANAL.CHEM., 37, 1594 (1965).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
d Reference electrode control potenliol
I
i, =Constant current flow
1
,, :g!i+ A+ pump
Figure 1. Digipotentiogmtor @PI) block diagram
Figure 2. DPI transient response for a dummy cell Upper trace: Current pulses Lower trace: Control-potential response Time base: 250 Mecldivisian
Figure 4.
Anodic-stripping voltammogram
-
0.01M KCI, 61 ppb. Pb2+ Conditions: -1.000 V os. SCE starting potential. Ineremental ramp step 5 mV, ungated analyzer input employed. DwelII time = SoluUon:
1sec/channel
Figure 3. DPI transient response for an electrochemical cell (1). Parameters the same as for Figure 2
KCI as the supporting electrolyte. The potential errcursion is less than that shown in Figure 2 because the squa re-wave is now applied against a SCE. Although the descent!time is somewhat longer owing to the solution resistance ,and the dynamically changing cell capacitance, it is adequat ely fast for our present purposes. A pulsed current extractioiI rate of roughly 8 kHz is required to satisfy the background current h i s fredemand at a control-potential of 0.0 V vs. SCE. ‘I quency is greater than the electron-transfer rate of electroactive substances and is therefore of no consequence. The current drift of this device amounts to only 0.01I% over a period of 1.4 hours. The study was made by implosing a control-potential of - 1.000 V against the simulated cel1. The average count rate was 24,000 for a IO-sec current Gimpling interval. The internal potential control of the DPI is better than 1 mV.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
4b
1719
Figure 4 is an anodic-stripping voltammogram taken with the DPI employing the required peripherial gear described in a previous paper ( 2 ) and using the cell mentioned earlier (1). The presented instrument is a unipolar device because it was developed solely for anodic-stripping analysis in conjunction with an ongoing environmental study. A more sophisticated bipolar instrument is presently under development and will be reported in the near future. One should note that direct grounding of the working electrode of the cell eliminates bothersome induced currents present in the floating working electrode designs. Location of the drop separation time when applying the future instrument t o polarography will be simplified. A drop separation signal can easily be derived by sensing the abrupt current-pulse spacing changes which occur at the end of the life of a drop. In conclusion, this approach to direct conversion of charge to a digital number appears to be quite generally applicable. The same principles which resulted in the development of the digital integrator (2) and the digital nuclear spectrometer (3) have now been applied to electrochemistry. If the pulsed (2) R. G. Clem and W. W. Goldsworthy, ANAL.CHEM.,43, 918 (1971). (3) W. W. Goldsworthy, Nucl. Instrum. Methods, 94, 221 (1971).
current source in the present application were replaced with a broad spectrum pulsed light source and the electrochemical cell was replaced with a phototube, direct digital conversion of spectrometric data for an absorber in the pulsed light path would result. ACKNOWLEDGMENT
The authors thank Dr. E. H. Huffman and Dr. E. K. Hyde for their support in this undertaking. WILLIAMW. GOLDSWORTHY RAYG. CLEM‘ Nuclear Chemistry Division and Lawrence Radiation Laboratory University of California Berkeley, Calif. 94720 RECEIVED for review June 1, 1971. Accepted July 13, 1971. Work performed under the auspices of the U. S. Atomic Energy Commission. 1
To whom correspondence should be sent.
Alternate or Simultaneous Electron Impact-Chemical Ionization Mass Spectrometry of Gas Chromatographic Effluent SIR: G a s chromatography-electron impact ionization mass spectrometry (GC-EIMS) is by now a well established analytical technique ( I ) . Gas chromatography-chemical ionization mass spectrometry (GC-CIMS) is, o n the other hand, a recent development (2, 3). In GC-CIMS the carrier gas for gas chromatography on entering the ion source becomes the reactant gas for chemical ionization and the carrier-reactant gas may be introduced directly (2) into the ion source without an interfacing splitter or separator. Methane has thus far been used mostly as the carrier-reactant gas although GCCIMS is far from being limited to this gas. In general, chemical ionization (CI) ( 4 ) mass spectra have fewer ions and more abundant high mass ions than electron impact ionization (EI) mass spectra although this is not always the case. However, C I mass spectra alone are of limited value in such application as structure elucidation of organic compounds if they show too few characteristic fragment ions. E1 and CI are in fact complementary and should be used as intimately as possible; furthermore these two modes of ionization are highly compatible (5) and can be used intimately. A mass spectrometer with two ion sources, one E1 and one CI, is described herein. The two-source instrument allows for the Watson, in “Ancillary Techniques of Gas Chrornatography,” L. S . Ettre and W. H. McFadden, Ed., WileyInterscience, New York, N. Y., 1969, pp 145-225. (2) G. P. Arsenault, J. J. Dolhun, and K . Biernann, Chem. Commun., 1970, 1542. ( 3 ) D. M. Schoengold and B. Munson, ANAL. CHEM.,42, 1811
(1) J. T.
(1970).
first time alternate or simultaneous EI-CI mass Spectrometry of G C effluent. Figure 1 shows a line drawing of the modified QUAD 300 quadrupole mass spectrometer. The CI and E1 sources are in series and have a ground lens (G) in common. The GC effluent enters the CI chamber (I) which is gas tight and is a t a pressure of ca. 0.5 torr. The G C effluent then exits through the electron entrance and ion exit apertures of the CI chamber (I) into the vacuum envelope surrounding it which is at ca. Torr and in which the E1 source is located. The 4 X pressure in the quadrupole mass filter is maintained at 3 X Torr under these conditions. Alternate E1 and CI mass spectra of G C effluent are obtained by turning the power off
CI
E1
Source Source
Quadrupole -Mass Filter
-*r
\
;C Pumping I System
#I
I
Pumping System
I
#2
(4) M. S. B. Munson and F. H. Field, J . Amer. Chem. Soc., 88, 2621 (1966). (5) G . P. Arsenault, in “Biochemical Applications of Mass Spec-
Figure 1. Line drawing of m a s spectrometer fitted with two ion sources-not drawn to scale
trometry,’’ G. R. Waller, Ed., Wiley-Interscience, New York, N. Y., in press, 1971.
F: filament; R : repeller; D: drawout lens; I: ionization chamber; G: ground lens as well as differentially pumped :awture
1720
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
