John Madden

Critical comparison of retention models for optimisation of the separation of anions in ion chromatography. I. Non-Suppressed anion chromatography using phthalate eluents and three different stationary phases

A series of mathematical models describing analyte retention behaviour in non-suppressed ion chromatography of anions has been compared in order to assess their suitability for inclusion in computer optimisation software for determining the optimal eluent composition for a desired separation. The series of models comprised the linear solvent strength model (using both the dominant equilibrium approach and the competing ion effective charge approach), the dual eluent species model, the Kuwamoto model, the extended dual eluent species model and the multiple species eluent/analyte model, together with a new empirical model, the end points model. An extensive set of experimental retention data obtained for 15 anions (acetate, fluoride, iodate, bromate, chloride, nitrite, bromide, chlorate, nitrate, iodide, oxalate, sulfate, sulfite, thiosulfate and phosphate) on three columns (Waters IC Pak A, Hamilton PRP-X100 and Vydac 302 IC) using phthalate eluents of varying concentration and pH was used to evaluate the ability of each model to predict retention factors. Statistical comparison of the predicted retention factors with those obtained experimentally showed that the performance of the theoretical models improved with the complexity of the model, but none of the theoretical models could give sufficiently reliable prediction of retention factors (especially for divalent analyte ions) for the model to be used in optimisation software. However, the empirical end points model (in which a linear relationship is assumed between log k′ and log [eluent], but the slope of the relationship is determined empirically) gave satisfactory performance, with correlation coefficients for all analytes of 0.9953, 0.9840 and 0.9919 for the Hamilton PRP-X100, Vydac 302 IC and Waters IC Pak A columns, respectively.

Prediction of retention times for anions in ion chromatography using Artificial Neural Networks

SummaryAn Artificial Neural Network (ANN) was investigated as a method to model retention times of anions in nonsuppressed and suppressed ion chromatography (IC) using a range of eluents and stationary phases, with the results being compared to those obtained using mathematical retention models. The optimal ANN architecture was determined for six specific IC cases of increasing complexity. Analysis of the retention times predicted using the ANN and those predicted by the mathematical models showed that the ANN approach yielded superior performance in all of the above cases. The use of a limited training data set configured in a central composite experimental design was suitable for application of the ANN to non-suppressed IC but was not applicable to suppressed IC, for which a more extensive training data set was necessary.

Prediction of retention times for anions in linear gradient elution ion chromatography with hydroxide eluents using artificial neural networks.

The feasibility of using an artificial neural network (ANN) to predict the retention times of anions when eluted from a Dionex AS11 column with linear hydroxide gradients of varying slope was investigated. The purpose of this study was to determine whether an ANN could be used as the basis of a computer-assisted optimisation method for the selection of optimal gradient conditions for anion separations. Using an ANN with a (1, 10, 19) architecture and a training set comprising retention data obtained with three gradient slopes (1.67, 2.50 and 4.00 mM/min) between starting and finishing conditions of 0.5 and 40.0 mM hydroxide, respectively, retention times for 19 analyte anions were predicted for four different gradient slopes. Predicted and experimental retention times for 133 data points agreed to within 0.08 min and percentage normalised differences between the predicted and experimental data averaged 0.29% with a standard deviation of 0.29%. ANNs appear to be a rapid and accurate method for predicting retention times in ion chromatography using linear hydroxide gradients.

Critical comparison of retention models for the optimisation of the separation of anions in ion chromatography: II. Suppressed anion chromatography using carbonate eluents

Seven theoretical retention models, namely the linear solvent strength model (using the dominant equilibrium approach and competing ion effective charge approach), the dual eluent species model, the Kuwamoto model, the extended dual eluent species model, the multiple species eluent/analyte model and the empirical end-points model, were used to describe the retention behaviour of anions in suppressed ion chromatography (IC). An extensive set of experimental retention data was gathered for 24 anions (fluoride, formate, bromate, chloride, hexanesulfonate, bromide, chlorate, nitrate, iodide, thiocyanate, perchlorate, sulfite, succinate, sulfate, tartrate, selenate, oxalate, tungstate, phthalate, molybdate, chromate, thiosulfate and phosphate) on a Dionex AS4A-SC column using carbonate eluents of varying concentration and HCO3-:CO3(2-) ratios. Statistical comparison of the predicted and experimentally obtained retention factors showed that the performance of the theoretical models improved with the complexity of the model. However the empirical model (in which a linear relationship is assumed between the logarithm of retention factor and the logarithm of eluent strength, but the slope is determined empirically) gave the most consistent performance across the widest range of anions. The empirical end-points model was also shown to be the most satisfactory model due to its low knowledge requirements and easy solution. Compared with non-suppressed IC (see Part I), the retention behaviour in suppressed IC was found to be easier to model by all retention models.

Critical comparison of retention models for optimisation of the separation of anions in ion chromatography III. Anion chromatography using hydroxide eluents on a Dionex AS11 stationary phase

Three ion chromatography (IC) retention models, namely the linear solvent strength model (LSSM), empirical end points model (EEPM) and three-point curve fitting using DryLab from LC Resources were evaluated in terms of their ability to predict retention factors for inorganic anions separated on a Dionex AS11 column using electrolytically generated hydroxide eluents. Extensive experimental retention data were gathered for 21 anions (fluoride, acetate, formate, bromate, chloride, nitrite, methanesulfonate, bromide, chlorate, nitrate, iodide, thiocyanate, succinate, sulfate, tartrate, oxalate, tungstate, phthalate, chromate, thiosulfate and phosphate) using hydroxide eluents of varying concentration. Although the purely theoretical LSSM was found to give adequate performance, the EEPM (in which a linear relationship is assumed between the logarithm of retention factor and the logarithm of eluent strength, but the slope is determined empirically) and DryLab performed better, with DryLab giving the best accuracy and precision of the three models. The EEPM and DryLab were also shown to have advantages in terms of their low knowledge requirements and ease of solution. Compared with IC using dual eluent species, the retention behaviour in IC using single eluent species was found to be easier to model by both theoretical and empirical approaches.

Measuring industry importance: an Australian application

The focus of this paper is an empirical examination of the importance of an industry in terms of its contribution to regional employment. It uses a closed input-output model. Four alternative measures of importance are presented and explored in the framework of a 58-industry input-output model of the Australian State of Tasmania. The four measures are compared to each other, to direct employment and to a multiplier-based rule-of-thumb. Our preferred measure is one which takes into account both direct effects and the strength of backward linkages. The rule-of-thumb is found to be highly correlated to this measure.

Inundative release of coccinellid beetles into eucalypt plantations for biological control of chrysomelid leaf beetles

Abstract 1 Inundative augmentative releases of adult coccinellid beetles were assessed for their potential to effectively supplement biological control of outbreak populations of the Eucalyptus leaf beetle Chrysophtharta bimaculata in Eucalyptus nitens plantations.