W.E. van der Linden

MEMBRANE SEPARATION IN FLOW INJECTION ANALYSIS Gas Diffusion

A general expression is derived for the membrane transport process in a flow-through unit as commonly used in flow injection systems. The validity of the formulae was tested for gas-diffusion membranes by using compounds with different volatilities such as ammonia, carbon dioxide and acetic acid. Several microporous hydrophobic membranes were tested. A new module design is proposed.

pH Sensor Properties of Electrochemically Grown Iridium Oxide

The open-circuit potential of an electrochemically grown iridium oxide film is measured and shows a pH sensitivity between −60 and −80 mV/pH. This sensitivity is found to depend on the state of oxidation of the iridium oxide film; for a higher state of oxidation (or more of the oxide in the high valence state), the sensitivity is also higher. This high sensitivity can be explained on the basis of the extra proton release as a result of the acidic character of the porous hydrous oxyhydroxide, in combination with the redox behaviour. The response time to a pH step is measured and is found to depend mainly on the thickness of the oxide; it varies from 40 ms to 0.35 s due to the porous nature of the film. Drift measurements show that an iridium oxide film in reduced state is slowly oxidized by dissolved oxygen, whereas a pre-oxidized film in a pH = 4.01 buffer solution in contact with air shows a long-term drift of <0.3 mV/h.

Processing of signals from an ion-elective electrode array by a neural network

Neural network software is described for processing the signals of arrays of ion-selective electrodes. The performance of the software was tested in the simultaneous determination of calcium and copper(II) ions in binary mixtures of copper(II) nitrate and calcium chloride and the simultaneous determination of potassium, calcium, nitrate and chloride in mixtures of potassium and calcium chlorides and ammonium nitrate. The measurements for the Ca2+/Cu2+ determinations were done with a pH-glass electrode and calcium and copper ion-selective electrodes; results were accurate to ±8%. For the K+/Ca2+NO−3/Cl− determinations, the measurements were made with the relevant ion-selective electrodes and a glass electrode; the mean relative error was ±6%, and for the worst cases the error did not exceed 20%.

BOOTSTRAPPING PRINCIPAL COMPONENT REGRESSION MODELS

Bootstrap methods can be used as an alternative for cross‐validation in regression procedures such as principal component regression (PCR). Several bootstrap methods for the estimation of prediction errors and confidence intervals are presented. It is shown that bootstrap error estimates are consistent with cross‐validation estimates but exhibit less variability. This makes it easier to select the correct number of latent variables in the model. Using bootstrap confidence intervals for the regression vectors, it is possible to select a subset of the original variables to include in the regression, yielding a more parsimonious model with smaller prediction errors. The methods are illustrated using PCR, but can be applied to all regression models yielding a vector or matrix of regression coefficients.

Artificial neural networks as a tool for soft-modelling in quantitative analytical chemistry: the prediction of the water content of cheese

The application of artificial neural networks for the modelling of a complex process was examined. A real data set concerning the batch production of cheese from an actual plant was used to predict the resulting water content of the cheese from the milk composition and process parameters. Owing to the complex nature of the data and the limited number of available patterns, difficulties were encountered when the standard backward error propagation algorithm was applied and no solution was derived. Several adaptions to the algorithm as suggested in the literature were then examined, and several gave satisfactory solutions. The resulting mean of the absolute values of the absolute prediction errors was 0.25% and 0.29% for known and unknown patterns, respectively, with a worst case error of 0.8%.

Computerized conductometric determination of stability constants of complexes of crown ethers with alkali metal salts and with neutral molecules in polar solvents

A computerized conductometric procedure for the determination of stability constants of the complexes of crown ethers (15-crown-5, benzo-15-crown-5 and 12-crown-4) with alkali metal salts in polar solvents is described, based on a microcomputer-controlled titration system. For the control of the experiments from software, a modular computer program was written in FORTH computer language. The procedure is especially suitable for the study of 1:2 metal ion/ligand complexes, which occur frequently with the compounds used. For the study of the interaction between crown ethers and neutral molecules, an indirect procedure is outlined.

FLOW INJECTION ANALYSIS IN ON-LINE PROCESS CONTROL

Flow injection systems are serious candidates for a new generation of chemical on-line analyzers because there is a growing interest in instruments that combine versatility with the possibility of attaining high sampling frequencies. For real on-line applications the instrument and its component parts have to meet the highest standards with respect to reliability and maintenance. These aspects are considered in some detail, and some industrial applications are briefly discussed.

A possible approach to the optimization of flow injectin analysis

The theory of sample zone dispersion is well known for most cases of practical interest in flow injection analysis. This paper offers a theoretical analysis which allows for the optimal design of single-line flow systems. For various reactor types, a detailed analysis is provided in terms of physical constants, design parameters and constraints. It is shown that, within practical constraints and using a pressure drop of less than 1 bar, it is possible to operate flow systems at 100 samples per hour, with a residence time of 100 s and a reagent consumption of 8 μl for each determination. Further miniaturization of flow systems must rely on smaller detector volumes than those currently available, a situation not unlike that in liquid chromatography.

Electrochemistry of potentially bioreductive alkylating quinones Part 1. Electrochemical properties of relatively simple quinones, as model compounds of mitomycin- and aziridinylquinone-type antitumour agents

The influence of methyl-, hydroxy and amino substituents on the electrochemical behaviour of simple 1,4-naphtho-and 1,4-benzoquinones, model compounds of many quinoid antitumour agents, in aqueous media was studied. Significant changes in electrochemical behaviour were observed, potentially the result of a change in the electron density of the quinone moiety, pre- or post-protonation of substituents, hydrogen bond formation, tautomerization reactions and steric interactions between the quinone moiety and substituents. The information obtained was of benefit in the elucidation of the reduction mechanisms of quinoid antitumour agents such as aziridnylquinones and mitomycins.

Modelling of non-steady-state concentration profiles at ISFET-based coulometric sensor—actuator systems

Acid or base concentrations can be determined very rapidly by performing an acid—base titration with coulometrically generated OH− or H+ ions at a noble metal actuator electrode in close proximity to the pH-sensitive gate of an ion-sensitive field effect transistor (ISFET). The ISFET is used as the indicator electrode to detect the equivalence point in the titration curve. Typical values for the time needed to reach the equivalence point are 0.5–10 s for acid or base concentrations ranging from 0.5 × 10−3 to 20 × 10−3 mol l−1. A model is presented, giving an analytical description of the diffusion phenomena that occur with the sensor—actuator system. The results of this analytical model description, using linearized concentration gradients, are presented together with the results of numerical simulations. Both results are in good agreement with measurements.