JOURNAL OF MAGNETIC RESONANCE 76, 1 oo- 105 ( 1988) Electrochemical Generation of Free Radicals in an EPR Loop-Gap Resonator R. D. ALLENDOERFER,* W. FRONCISZ,~ G. C. FELIX, AND JAMES S. HYDE$ National Biomedical ESR Center, Department of Radiology, Medical College o.f Wisconsin, Milwaukee. Wisconsin 53226 Received May 8, 1987 An electrochemical cell for in situ generation of free radicals in a loop-gap resonator at X band is described. The cell is assembled from commercially available components and requires no glass blowing. The design is flexible and accommodates a variety of electrodes and electrode geometries. Remarkably small amounts of material are required. The filling factor is favorable for short-lived radicals. Performance of the cell is demonstrated by generation of the dianion radical of p-nitrobenzoic acid. 0 1988 Academic Press, Inc. Maki and Geske (I, 2) first demonstrated in situ electrochemical generation of free radicals for electron paramagnetic resonance studies. Shortly thereafter Piette, Ludwig, and Adams (3) developed an electrochemical cell for use with the Varian Multipurpose TELo2 microwave cavity. This cell remains in commercial production by Wilmad Glass Co. and by Bruker-Physik and is by far the most widely used geometry. The primary shortcomings of the Piette et al. cell are that the sensitivity is poor for short-lived radicals and simultaneous electrochemical EPR (SEEPR) experiments are not possible. Goldberg and Bard (4) introduced a modification of this cell wherein all the electrodes are contained within the EPR cavity, making SEEPR possible but the combination of fast solution flow and good potential control necessary for the obser- vation of short-lived radicals remained an unobtainable goal for this geometry. Al- lendoerfer et al. (5) developed an electrochemical cell for the TEol 1 mode cylindrical microwaye cavity that overcame these difficulties. Accurate SEEPR was possible at voltage sweep rates in excess of 150 mV/s (6) and EPR spectra for radicals with lifetimes of the order of 10 milliseconds were reported (7). The commercial IBM/Bruker en- hanced EPR electrochemistry accessory is based on this design. The loop-gap resonator was introduced into EPR spectroscopy by Froncisz and Hyde (8). It has subsequently been used by a number of groups. The design and applications of this resonator have been reviewed (9). Because this reduced size reso- nator fits in all EPR magnets and uses a dramatically smaller sample, which should lead to smaller electrolysis currents and concomitantly fewer potential control prob- * Permanent address: Department of Chemistry, SUNY University at Buffalo, Buffalo, New York 14214. 7 On leave from the Department of Biophysics, Institute of Molecular Biology, Jagiellonian University, Krakow 3 l-120, Poland. $ To whom correspondence should be addressed. 100 0022-2364188 $3.00 Copyright Q 1988 by Academic Press Inc. All rights of reproduction in any form reserved.