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Use the new Rheodyne Model 7120 Syringe Loading Sample Injector Valve in two ways. Use it to fill loops conventionally for maximum precision or in the partial loop variable volume filling mode with only 0.5 microliter sample loss. Here's the valve for maximum versatility in H PLC sample injection. Load the sample by syringe through built-in needle port. Works up to 7000 psi. The removable sample loop is available in sizes of 10 microliters up. This new Rheodyne Sample Injector Valve represents a significant improve ment over our own as well as other "universal injectors." From a price, performance and convenience aspect, there's no better injector available. Speaking of prices, the Model 7120 with a 20 microliter sample loop sells for $490. Sample loops from 10 microliters to 2 milliliters are offered. More information Ask us for complete data. Call or write Rheodyne Company, 2809 -10th St., Berkeley, CA 94710. Phone (415) 548-5374.
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underlies the popularity of the tech niques in question among research groups with widely diverse interests, ranging from trace analysis of environ mental pollutants (19) to kinetics of homogeneous chemical reactions and heterogeneous charge transfer (1417). On the other hand, this favorable aspect is coupled with the less appeal ing observation that the electrochemi cal cell represents a rather complex circuit element. This, together with the detailed physical chemical infor mation often sought, means that this conceptually simple measurement must be performed with greater than normal demands on precision and ac curacy, and repeated under a wide range of experimental conditions. All of this places major demands on mea surement technology. For example, in a quantitative kinetic-mechanistic study, a transient response to an im pulse or step function perturbation should be observed with good resolu tion over at least 2-3 time scale orders-of-magnitude, using 10-20 differ ent choices of dc potential characteriz ing the initial and/or final values of the input perturbation. If the frequen cy domain response is desired, the conventional approach is to scan the sinusoidal ac response as a function of dc potential (record an ac polarogram). This operation should be re peated using at least 10-15 different input frequencies to characterize the response spectrum over 2-3 decades. All of this implies a great deal of re dundancy and tedium when conven tional measurement approaches are employed, often tempting workers to acquire data with less detail than is really demanded by a given objective, forcing them to make certain a priori assumptions about system behavior. Fortunately, from relatively recent lit erature reports, one finds evidence that much or all of this tedium will be eliminated when on-line computerized FFT data processing is adopted as a conventional strategy. Fourier Transform Approach to Electrochemical Relaxation Spectral Measurements: Basic Principles The first step in a FFT approach to electroanalytical response spectral measurements involves computerized acquisition of time domain digital data arrays representing the applied potential, e(t), and current response, i(t). The Fourier spectra of these waveforms, Ε(ω) and Ι(ω), respective ly, then are computed by the FFT and correlated in a fashion appropriate to achieve the measurement objective. Normally, one is concerned with some aspect of the frequency domain cell admittance, Α (ω) (or impedance). / /
CIRCLE 1 8 4 O N READER SERVICE CARD
224 A · ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
the cell response is linear, Α(ω) is computed from the relationship (6, 7), Ι(ω)Ε*(ω) (1) Ε(ω)Ε*(ω) where Ε*(ω) is the complex conjugate of the applied potential Fourier spec trum, Ε (ω). Under conditions of most interest, an electrolysis step is contrib uting to interfacial charge transfer (a "faradic process"), giving rise to a "faradic admittance" component (14-17), which is manifested as a decidedly nonlinear circuit element, so that Equation 1 is not generally applicable. However, with sufficiently small am plitudes (a few millivolts), the nonlin ear properties are minimized to a suf ficient extent that, by conventional standards of accuracy and precision, the cell admittance may be approxi mated as linear, and Equation 1 be comes applicable. Knowledge of the small amplitude limit of the cell ad mittance suffices for most purposes to which the ERM is directed, and it is in this context where one finds the pre dominate FFT applications in electro chemistry (6-13). This is expected to be a continuing situation, since this particular application mode is the most obvious and powerful. Instead of spending many hours characterizing an admittance spectrum using pointby-point single-frequency sinusoidal measurements, as with the conven tional approach, one can employ an appropriate multiple-frequency ap plied potential waveform, FFT data processing, and Equation 1 to charac terize in a few seconds the admittance spectrum at all frequencies of interest simultaneously at a particular dc po tential. One can then "scan" the ad mittance spectrum as a function of dc potential to obtain the kind of data normally required for kinetic-mecha nistic studies, with the same ease (for the operator) as running a classical dc polarogram. Α(ω)
Within the restrictions of linear be havior, Equation 1 is independent of the type of waveform employed. Thus, in principle, the same computed Α(ω) will result from signals which are ape riodic transients, almost periodic, pe riodic or stochastic (7), provided that conditions are conducive to accurate measurement of the time domain waveforms. This expected indepen dence of waveform type has important implications for electroanalytical re laxation measurements. For example, previous workers normally have con sidered it necessary to operate within the frequency (or time) range where the analog potential perturbation con trol system (potentiostat) performed ideally within experimental uncertain ty (14, 15). This allows one to assume in rate law derivations that a specific, usually idealized (square impulse, step
