Focus
Square Wave Voltammetry A Challenger to Differential Pulse Polarography Emerges The introduction to a recent technical paper by P. A. Boudreau and S. P. Péroné indicated that an electroanalytical technique known as square wave voltammetry (SWV) "may replace conventional differential pulse polarography [DPP] for quantitative determinations of electroactive species in solution." [Anal. Chem., 1979,57, 811] To the chemists who use differential pulse polarography in their work, such a "replacement" would represent a significant development. Not even the name of the technique is definite at this point. Donald E. Smith of Northwestern University reportedly recommended at the ACS/ CSJ Chemical Congress in Honolulu earlier this year that it be called "rectangular wave voltammetry." But whether it is called "square" or "rectangular," it is sure to gain in importance in the coming years, though its complete replacement of DPP is apparently far from assured, and even some of the purported advantages of SWV are very much in dispute. The idea for what is now known as SWV originated with G. C. Barker, who reported on the application of a square wave to a single drop of mercury from a dropping mercury electrode (DME) at the Congress on Analytical Chemistry in Industry at St. Andrews. The year was 1957. The theory developed by Louis Ramaley and Matthew S. Krause, Jr. in 1969 [Anal. Chem., 1969, 41, 1362 and 1365] was basically a modification of Barker's original treatment of the technique. Ramaley and Krause presented analytical results for SWV at a hanging Hg drop electrode (HMDE), but did not apply the technique to a DME at that time. In 1977 J. H. Christie, John A.
Turner, M. Vukovic, and R. A. Osteryoung ressurected SWV. Dr. Christie tells the story: "We were aware of the work of Ramaley and Krause when it was originally done, and I had some reservations about its applicability. But Osteryoung had always had the idea that he would like to be able to do a complete experiment on a single drop." The fruit of this idea was a pair of original papers that appeared in November of that year [Anal. Chem., 1977, 49, 1899 and 1904]. Christie et al. optimized the electroanalytical response by altering the waveform Ramaley and Krause had used, and changed some of the nomenclature in developing the theoretical framework for SWV at the DME. Proponents of SWV emphasize its advantages over DPP, which is currently unrivaled as the electroanalytical workhorse technique. J. H. Christie of the U.S. Geological Survey in Denver, Colo., explains: "The major advantage of square wave is it's much faster, because you effectively perform a complete potential scan on each drop of Hg. You get the advantage of being able to do a more rapid experiment, and you can lower the signalto-noise ratio of the current-potential curve by repetitive scanning and signal averaging." Perhaps analytical speed is SWV's greatest claim to fame. In comparative analyses of Pb, Cd, and EDTA, Janet Osteryoung of the State University of New York at Buffalo found that "with DPP a scan of 500 mV required 250 s (over 4 min), while with square wave only 10 s was required." Robert Osteryoung, also of SUNY Buffalo, emphasizes the versatility of square wave voltammetry, and its
ability to generate information on electrode kinetics as well as analytical information. According to him, "One may difference the current in SWV just as in DPP, giving rise to the familiar peak-shaped curves of both techniques, or one may look at the "forward" and "reverse" currents in the square wave procedure to give rise to kinetic information much like that one obtains in a complete cyclic voltammogram." In fact, one of Osteryoung's students recently extended the theory of square wave voltammetry to include electron transfer kinetics and first-order preceding, following, and catalytic reactions [O'Dea, John Joseph, Ph.D. Dissertation, Colorado State University, Fort Collins, Colo., 1979.] "Analytically the techniques are almost identical in terms of sensitivity," Robert Osteryoung concludes. "But square wave is much faster than differential pulse. And it will be very useful at solid electrodes. Differential pulse has problems at solid electrodes because it's slow. Square wave is also useful for studies of kinetic processes, whereas differential pulse is almost exclusively an analytical technique." On the other hand, some strong arguments have been raised against most of these advantages. It seems there are several factors which may work against wholesale replacement of DPP by SWV. For instance, the question has been raised: "Why replace one technique with another of equivalent sensitivity?" In addition, the sensitivity of the square wave technique is reduced for irreversibly reduced substances as compared to reversibly reduced materials. Howard Siegerman of EG&G
ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980 · 229 A
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Princeton Applied Research Corp. (EG&G PARC) wonders "if SWV doesn't resemble ac polarography in its inferiority relative to D P P for the analysis of irreversibly reduced substances. In both SWV and ac polarography a rapidly changing modulation signal is applied to t h e working electrode, and the slow electron transfer kinetics associated with irreversibly reduced substances prevents one from determining these materials with t h e same sensitivity as D P P . " In addition, while t h e ability to extract kinetic data from the SWV experiment may be of interest to theoreticians, by far the greatest use of polarography (to use t h e general term) is for analysis. T h u s t h e applicability of SWV to the study of electrode kinetics may provide minimal driving force for the abandonment of D P P in favor of SWV. Even the time advantage in being able to scan rapidly with SWV may be misleading. T o fully benefit from this advantage, one must perform a single scan on a single mercury drop. But if one must use multiple scans to accomplish signal averaging and noise reduction, t h e time advantage of SWV may be lost. And if t h e analyst has to do any sample workup prior to t h e actual measurement—such as digestion or extraction—the additional few minutes required for t h e D P P scan may be considered negligible relative to t h e time required for the analytical scheme as a whole. Finally, Dr. Siegerman of EG&G PARC points out t h a t "to implement SWV with high speed scans and signal averaging capability, along with producing a hard copy record of t h e analytical results, more expensive instrumentation would be required than is currently available for D P P . Would analysts be willing to pay the price?" In any case, the rate-determining step in the further development of SWV is t h e availability of commercial instrumentation. Robert Osteryoung and J a n e t Osteryoung of SUNY Buffalo and Chaim Yarnitzky of the Israel Institute of Technology in Haifa constructed an SWV instrument a few years ago (Anal. Chem., submitted), and there are reports t h a t Bascom Instruments Division of EIC, Inc. is interested in manufacturing a similar instrument. B u t neither Bascom nor EG&G PARC is willing to discuss its plans for development of SWV instrumentation. Some of the advantages of SWV are generally acknowledged, but D P P will probably remain t h e method of choice for t h e near future. Even so, we will no doubt be hearing more about SWV in t h e coming years. Perhaps only time will tell how much potential it really has.
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Cannabinoid Analysis in Physiological Fluids A C S Symposium Series No. 98 Joe A. Vinson, Editor University of Scranton Based on a symposium sponsored by the Division of Analytical Chemistry of the American Chemical Society. An indispensable reference for law enforcement officials, forensic scientists, and analytical chemists, this practical new volume thoroughly evaluates recent advances in physiological fluid analysis that will aid in the detection of marijuana use in humans. Written by experts in the field, this symposium discusses new analytical procedures using gas chromatography, mass spectroscopy, radioimmunoassay, high-pressure liquid chromatography, and thin-layer chromatography. CONTENTS Metabolic Transformations · GLC and HPLC Analyses • Detection and Quantitation · GC CI-MS Analyses · Mass Fragmentographic Technique · General Approach to the Analysis of Cannabmoids from Physiological Sources · Quantitation by Probability Based Matching G C M S · HPLC Analysis · Radioimmunoassay of Cannabinoid Compounds · Antisera Raised Against Tetrahydrocannabinol · HPLC Analyses in Human Plasma · Detection and Quantitation by Dansylation and Double Labeling
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