M through 1 X lo4 M. The slope of the calibration curve was 0.506 f 0.035 pA/pM. The minimum detectable difference between the background and sample solution estimated by paired measurements (5) was 0.025 pM, which is over an order of magnitude lower than in the dc polarographic method (1 7).A voltammogram of a 5.0 X lo4 M Ti(IV) solution is presented in Figure 10. ACKNOWLEDGMENTThe authors thank Janet Osteryoung for helpful discussion.(1) Brdicka, R.; Wlesner, K.Concentration vs. distance profiles are fundamental to a variety of electrochemical processes but have not been observed previously at resolutions better than 25 pm. I n the present work, a collimated He-Ne laser beam was passed parallel to a planar electrode surface and then magntfied and imaged onto an array detector. Each pixel of the detector

12) Schmldt, D. E.; Giese, R. W.; Conron, D.; Karger, 8. L. Anal. Chem. lQB0. 52. 177-182. ----, --. (13) Pinkerton, T. C.; Hagestam, I. H. US. Patent, 6646 153, 1984. (14) Bethell, G. S.; Ayerer, J. S.; Hancock, W. S.; Hearn, M. T. J . 8/01. Chem. 1979, 254, 2572-2574. (15) Lasson, P. 0.; Glad, M.; Hasson, L.; Mansson, M. 0.; Ohlson, S.; Mosbach, U. (21) Haroon, V.; Keith, D. A. J . Chromatogr. 1983, 276, 445-450. (22) Lunde, P. K. M.; Rane, A.; Yaffe, S. J.; et al. C/h. Pharm. Ther. 1970, (23) Miller, T. D.; Plnkerton, T. C. Anal. Chim. Acta, In press. Sir:We report here a spectrophotometric technique for determining concentration vs. distance profiles of absorbing solution species generated at an electrode surface. The technique can accurately describe diffusion layers as thin as 8 pm, after only 50 ms of electrolysis. In this paper, we describe the apparatus and its performance and discuss its potential for providing new information about mass transport and reaction mechanisms. The fundamental importance of mass transfer to electrochemistry has spawned a variety of theoretical and experimental examinations into diffusion (1, 2), convection (3-51, and migration (6) as mechanisms for the transport of redox species to an electrode. For several well-defined mass transport situations, the Faradaic current may be predicted from theories of diffusion and hydrodynamics, but in many cases solutions are not readily available. Diffusion to microelectrode arrays (7,8), mass transport in flowing streams (9, IO), and mixed convection/migration conditions are examples of cases where concentration vs. distance profiles are not available, and the Faradaic current is not accurately predictable from theory. Several approaches have been pursued to experimentally observe concentration-vs. distance profiles near an electrode, including interferometric methods (10-15) based on refractive index gradients near an electrode and on UV-vis absorption by electrogenerated species (16-18). The present approach is a spatidy resolved UV-vis absorption measurement which permits concentration vs. distance profiles to be obtained with better resolution and shorter time scales than those from previous methods. The cross section of a beam passing parallel to a planar electrode surface is magnified and imaged onto a photodiode array detector. Each diode samples a discrete distance from the electrode, and Beer's law may be used to directly determine a spatially resolved concentration profile.The cell, electrodes, and chemical systems were identical with those described previously for diffractive spectroelectrochemistry (19). Trianisylamine (TAA) in acetonitrile was oxidized at a platinum electrode to TAA+. at +0.8 V vs. aqueous SCE. The diffusion coefficient of TAA (1.25 X 10" cm2/s) and molar absorptivity for TAA+. (11OOO & 200 M-' cm-' , at 633 nm) have been reported previously (19). The optical apparatus shown in Figure 1 consists of a 2Ox magnifier producing an image of the beam cross section on the face of a 1024 element photodiode array. L1 ...

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