Instrumental configurations for the determination of sub-micromolar concentrations of electroactive species with carbon,gold and platinum microdisk electrodes in static and flow-through cells

Microelectrodes should provide a greater analytical sensitivity than electrodes of conventional size. However, the detection of micromolar or lower concentrations with microdisk electrodes requires measurement of femtoamp currents, which is outside the range of most commercially available instrumentation. The novel use of a picoammeter or femto-ammeter as a current amplifier permits commercial instrumentation to be used with microdisk electrodes. Such instrumentation incorporating a picoammeter or femtoammeter is limited by the relatively slow rise time; this places restrictions on scan rates in all voltammetric techniques and on pulse widths in transient techniques such as differential pulse and square wave voltammetry. Because of the small currents, the ohmic (iR) drop is very small and polarization of the reference electrode is unimportant; thus a two-electrode format without a potentiostat can be used. Consequently, a microprocessor-based function generator and data storage system, in conjunction with a pico- or femto-ammeter, is satisfactory in providing inexpensive, versatile and very sensitive instrumentation for voltammetric detection with microdisk electrodes. Convenient methods for fabricating platinum, gold and carbon microdisk electrodes for use in stationary and flowing solution configurations are also presented.     

Tests for specific nonparametric relations between two distribution functions with applications

Let (X,Y) be a random vector and let G and H be the marginal distributions of X and Y, respectively. In this paper, we propose two tests, one of Kolmogorov‐Smirnov type and the other of Wilcoxon type, for the null hypothesis Ψ(G) = H against the alternative Ψ(G) < H, where Ψ() is a function such that Ψ(G) is a distribution function. The tests are based on the empirical distribution functions of the observations on X and Y, which are dependent. We obtain their asymptotic null distributions. A suspected relationship between the distribution functions of two dependent outcomes can be specified as a hypothesis to be tested in examples like the load sharing models, record values, and auction bidding models. As an application, we consider in detail the problem of testing the effect of load sharing in two component parallel systems.