spectrometer, as reported in the literature ( 5 ) . The spectrometric resolution is about 1500 (10% valley), the total amount of the peak corresponding to a-ionone is about 0.7 pg (0.02% of the sample). In the scanning conditions described in the caption of Figure 5, this corresponds to about 10 nanograms of a-ionone to get the spectrum. Nevertheless, the spectrum is still very intense and the trace of the intermediate galvanometer is sufficient to unequivocally identify the compound. The spectrum of Figure 5 has been taken in routine work. Chromatographic efficiency and mass spectrometric sensitivity and resolution are both nearly as good as possible. The number of theoretical plates is about 6500. The possibility of using a wide range of flow rates is helpful for obtaining mass spectra of minor components in complex organic mixtures. In fact, columns of high loading capacity can be exploited. This allows the major peaks not to be overloaded and the minor ones to be present in sufficient amount to take interpretable mass spectra. The (5) K. Biernann, "Mass Spectrometry," Organic Chemical Applications,
McGraw-Hill, New York, N.Y., 1962.
main advantages of this system are that one can get mass spectra from a very complex chromatogram without the need of doing it at least twice, and that a very large range of linear gas velocities can be used without problems for the operating conditions of the mass spectrometer. With the apparatus here described, it is possible to eliminate the TIM recording of the chromatogram using for this scope the FID trace. By working with the repetitive scanning device operating continuously during the chromatogram and watching the output of the mass spectrometer on an oscilloscope, one has only to check the intensity of the spectrum, and when it reaches the right value, push the button to record the spectrum of a given chromatographic peak on the UV light oscillograph to get it displayed on paper. The results of the present work show also that many defects of coupling MS and GC, attributed to the separator, are actually dependent on the inefficiency of the gas lines of the interface. Received for review September 19, 1972. Accepted January 29, 1973.
Carbon Paste Electrode with a Wide Anodic Potential Range Jorgen Lindquist Department of Analytical Chemistry, University of Uppsala, S-75727 Uppsala 7 , Sweden
Increasing interest in electroanalysis and study of electrode processes at potentials more positive than can be reached by mercury has resulted in a great variety of electrode materials. The useful potential range and the magnitude and reproducibility of residual currents in different media are the most important parameters governing the selection of an electrode material. The need for a reproducible, easily renewable stationary electrode for the anodic region is great because the surface film formation and adsorption of reactants, intermediates, or products are all known to influence the reproducibility of peak current measurements. For this reason the carbon paste electrode ( I , 2 ) seems to be the most practical electrode for analytical work in the anodic region and it has also become widely used. The anodic limit for this electrode is about +1.3 volt (us. SCE) in acid aqueous media ( I ) , which is about the same as for glassy carbon, pyrolytic graphite, and wax impregnated electrodes (3-5). However, microcrystalline carbon products adsorb oxygen in different forms. This can be removed in different ways depending on which surface compound is of concern. One form can be removed as oxides a t very high tempera(1) C. Olson and R. N. Adarns. Anal. Chim. Acta, 22, 582 (1960). (2) R. N. Adarns, "Electrochemistry at Solid Electrodes," Marcel
Dekker, New York. N . Y . , 1969. (3) H . E. Zittei and F. J . Miller, Anal. Chem., 17, 200 (1965). (4) J. F. Aider, B. Fleet, and P. 0. Kane, J. Electroanal. Chem., 30, 427 (1971). (5) J. H . Morris and J . M. Schernpf, Anal. Chem., 31, 286 (1 959),
1006
ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, MAY 1973
tures; another can be removed as oxides by evacuation a t ordinary temperatures, and other forms can be pumped off as oxygen, etc. Such surface compounds might have an influence on the properties of carbon powder when used as a material for electrodes. Activated carbon and carbon black have a marked quinone-hydroquinone character ( 6 ) , which should affect the background current. Activated carbon is not suitable for preparing electrodes but graphite powder, which is mostly used, probably contains similar compounds but to a lesser extent. By removing the oxygen in a vacuum at high temperature and then blocking the surface of the carbon against further adsorption of oxygen, it was possible to obtain an improved carbon paste electrode.
EXPERIMENTAL Ten grams of graphite powder (Ringsdorff-Werke RW-A) was placed in a quartz tube 45 x 1.7 cm. The tube, closed a t one end, was bent to a right angle and connected to a 50-ml Erlenmeyer flask with a side arm for connection to a pump. In the Erlenmeyer flask were 5.6 grams of ceresin wax and 1.4 grams of paraffin oil (Merck's paraffin liquid for spectroscopy). The part of the quartz tube containing the graphite powder was placed in a tube furnace, the pressure reduced to less than lo-* mm Hg, and the temperature increased to about 1000 "C. At the same time, the Erlenmeyer flask was heated for a while to melt (6) V. A. Garten and D. E. Weiss, Aust. J. Chem., 8, 68 (1955)
Potential, volt v.SCE *1.0 95
* 1.5
+
0
+ Figure 1. Voltammogram of iodide in phosphate buffer at pH 7
*
+1J5 1
Potential,volt v.SCE t1.50 +&25
*LOO
*475
I
Figure 3. Voltammograms of 5, 10, 20, and 40 micromolar co2+ in pyrophosphate buffer at pH 6
Figure 2. Cyclic voltammogram of 6 X 10-3M Ce(lll) in 0.1M HzS04
the ceresin wax and drive off the dissolved oxygen. After 1 hour, the tube was removed from the furnace and the Erlenmeyer flask was placed on a steam bath. When the wax had melted, the graphite powder was poured into it and, after 30 minutes, the mixture was cooled and air admitted. The stiffened paste was pulverized and tamped into the electrode holder (7) which was then heated to 80 "C to produce a hon~ogenouspaste. The surface, with an area of 0.31 cm2, was prepared by cutting off the excess paste with a stretched length of a 0.2-mm piano wire, according to the method described earlier (7). The electrode was rotated during this procedure. Voltammograms were run by the conventional electroanalytical method with a stationary electrode configuration in an unstirred solution. The scan rate of 0.50 volt/min (if not otherwise stated) was produced by an operational amplifier circuit. The reference electrode was a saturated calomel electrode and the temperature was 23 f 1 "C.
RESULTS AND DISCUSSION The anodic potential range was improved. It increased by 300-500 mV as compared to other graphite electrodes. In 0.1M HzS04, the background is below 1 FA u p to a potential of about +1.7 volts us. SCE. The cathodic range did not change as compared to a wax impregnated graphite rod electrode, WIGE (5). Iodide. At smooth platinum electrodes and for pH values ranging from 1.8 to 7, voltammograms of I - (in the ( 7 ) J.
Lindquist,J. Electroanab Chern., 18, 204 (1968)
Conccnt rot ion [)J M )
Figure 4. Calibration graph for adenine in acetate buffer at pH 4.7
-
-
absence of Br- and C1-) show three steps, due to the 12, 12 IOH, and IOH IO3-, electrode processes Irespectively. The oxidation to IO3- is ill-defined because of the imminent discharge of the solvent (8). At this carbon paste electrode, the oxidation of '2 X 10-4M iodide in 0.1M phosphate buffer at p H 7 also seems to proceed in three steps of which the first two 12, and 12 IO3-. The secprobably are as follows: Iond peak is five times higher than the first one, which indicates the oxidation to iodate (Figure 1). This was also verified by coulometry a t constant potential and the product was analyzed. The first peak is better developed in acid media and, because of the extremely small residual current, it is possible to use the electrode for quantitative determinations down to lO-7M solutions. +
-
-
(8) G. Raspi, F. Pergola, and R. Guidelli.Anal. Chern.. 44, 472 (1972)
ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, MAY 1973
1007
Potential ,volt v SCE
6
I
Figure 5. Voltammograms of 1 ) lidocaine, 2) prilocaine, 3) caffeine, 4) cerium, 5) iodide, and 6) vitamin B2 in 0.1M H2S04
Cerium(II1). A cyclic voltammogram in 0.1M HzS04 shows that the system Ce(IV)/Ce(III) is not reversible (Figure 2). At a sweep rate of 0.3 volt/min, the half peak potentials were 1.30 and 1.27 volts, respectively. The formal potential with the carbon paste electrode was 1.183 V and with a platinum electrode 1.181 V (with C c e r r r r ,= Ccerrv, = 3 x 10-3M in 0.1M HzSO4). The time response for the platinum electrode was very slow a t this concentration because of the formation of platinum oxides (9). Cobalt(I1). The standard electrode potential of the couple Co3+/CoZ+ is as high as 1.60 V L'S. SCE ( I O ) . Co3+ (9) J. W. Ross and I . Shain, Anal. Chem.. 28,548 (1956). (10) W. M . Latirner. "Oxidation Potentials," Prentice-Hall, New York, N.Y.. 1953.
1008
ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, MAY 1973
therefore oxidizes water to oxygen. Figure 3 shows voltammograms of Co2t in pyrophosphate buffer a t pH 6 (0.1M Na4Pz07 + H3P04) with EPlz = 1.6 V. The high current indicates catalytic oxygen discharge peaks. In 0.1M H2S04,no peak was detected. Adenine. Usually the electrode surface was renewed before each recording of a voltammogram. However, many substances do not poison the surface and in such cases it is sufficient to rinse with distilled water. One such example is adenine. The peaks are very reproducible even a t low concentrations, and Figure 4 shows a calibration graph for adenine down to lO-7M solutions. Figure 5 shows some examples of well developed voltammograms (2 X 10-4M lidocaine gives a peak current of 10.2 PA; 2 x 10-4M prilocaine, 11.2 FA; 2 x 10-4 caffeine, 13.2 FA; 5 X 10-4M cerium, 10.9 FA; 5 X 10-4M iodide, 13.7 FA; and 2 X 10-4M vitamin Bz, 6.8 PA). At a pyrolytic graphite electrode, caffeine, for example, is observed only in 1M HOAc and in acetate buffers and occurs very close to the background discharge ( I 1 ) . Reproducibility. Repeated measurements of the peak current of 5 X 10-4M Ce(II1) in 0.1M HzS04, where a new surface was prepared in the usual way before every run, gave a relative standard deviation of 1.16%. This relatively high value probably is due to the difficulty of wetting the surface reproducibly. Pre-wetting it with Triton X-100 ( 1 % ) gave greater peak currents, but no improvements of the precision. Received for review August 3, 1972. Accepted January 29, 1973. (11) B. t i . Hansen and G . Dryhurst, J . Electroanai Chem.. 30, 407 (1971). (12) P.J . Elving and D. L. Smith. Anal. Chem., 32, 1849 (1960).
