Probing the kinetic performance limits for ion chromatography. I. Isocratic conditions for small ions

doi:10.1016/j.chroma.2010.05.066

Journal of Chromatography A

Volume 1217, Issue 31, 30 July 2010, Pages 5057-5062

https://doi.org/10.1016/j.chroma.2010.05.066 Get rights and content

Abstract

The first use of the kinetic plot method to characterise the performance of ion-exchange columns for separations of small inorganic anions is reported. The influence of analyte type (mono- and divalent), particle size (5 and 9 μm), temperature (30 and 60 °C) and maximum pressure drop upon theoretical extrapolations was investigated using data collected from anion-exchange polymeric particulate columns. The quality of extrapolations was found to depend upon the choice of analyte, but could be verified by coupling a series of columns to demonstrate some practical solutions for ion chromatography separations requiring relatively high efficiency. Separations of small anions yielding 25–40,000 theoretical plates using five serially connected columns (9 μm particles) were obtained and yielded deviations of <15% from the kinetic plot predictions. While this approach for achieving high efficiencies results in a very long analysis time (t₀ = 21 min), separations yielding approximately 10,000 theoretical plates using two serially connected columns (t₀ < 5 min) were shown to be more practically useful for isocratic separations when compared to use of a single column operated at optimum linear velocity (t₀ > 10 min).

Introduction

In the realm of liquid chromatographic techniques, ion chromatography (IC) is generally considered to be a separation technique yielding relatively low separation efficiencies when compared to reversed-phase liquid chromatographic methodologies. This is due partly to limitations in the mechanical strength of small polymeric particles and non-metallic instrumental components such as the column housing and extra-column tubing [1], [2]. Despite these limitations, developments over the last 30 years in stationary phase design, including a reduction in particle size from 25 to <10 μm and concomitant increase in ion-exchange capacity, have vastly improved IC performance to the point where typical separations of small anions can yield efficiencies of 5–10,000 theoretical plates in <30 min [1]. While this is adequate performance for most applications requiring separation of <20 components, it does not necessarily reflect the current performance possibilities of the technique.

Approaches to improving chromatographic performance to satisfy analytical goals most commonly revolve around resolution optimisation. In the case of IC, existing approaches to separation prediction using in silico methods with commercially available software (Virtual Column™, Dionex) already serve to minimise time spent on method development by use of solvent strength modelling for selectivity optimisation [3]. However, such an approach cannot be employed for predicting separation performance outside the range of standard column lengths, flow-rates and pressure drop conditions. Such an optimisation requires use of kinetic performance methods first introduced by Giddings [4], developed extensively by Poppe [5] and more recently advanced by Desmet et al. [6].

While the range of IC separation media and instrumentation places some physical limitations on the obtainable efficiency values for this technique, the kinetic performance limits are certainly under-explored. Fortunately for this purpose, an accessible and diverse range of kinetic plots has been introduced recently by Desmet et al. [6] for the unbiased optimisation and comparison of liquid chromatography systems with different support formats under kinetically optimised conditions. Although application of this approach has been limited thus far to a small range of reversed-phase [7], [8], [9], [10] and hydrophilic interaction (HILIC) [11] separations of small organic molecules and some pharmaceuticals, there is no theoretical impediment to employing this method for other modes of liquid chromatography, including ion-exchange chromatography.

Predicting any improvements in the kinetic performance for liquid chromatography is best achieved by considering an appropriate number of experimental variables including flow-rate, temperature, column length and pressure drop limitations. Such an approach should allow a reliable appraisal of the current performance limitations of anion-exchange chromatography with particular attention paid to high efficiency separations and methods for rapid analysis requiring comparatively low efficiency.

Using experimental H, u₀ data and measurements of column permeability, kinetic plots were produced to characterise the current performance limits of IC with reference to particle size, available pressure drop, temperature and a number of other practical considerations. Furthermore, the practical application of long columns and increased pressure with currently available instrumentation was also broadly assessed both theoretically and experimentally.

Section snippets

Reagents

All chemicals used were of analytical reagent grade and were used as supplied by Sigma–Aldrich (Sydney, Australia) unless stated otherwise. The eluent was prepared using deionised 18.2 MΩ water from a Millipore Milli-Q water purification system (Bedford, MA, USA). Working standards were prepared in potassium hydroxide mobile phase or deionised water from 1000 mg/L stock solutions from their respective sodium salts.

Chromatography

Dionex ICS-3000 and ICS-2000 ion chromatographic instruments with conductivity

Selection of column and analytes

The wide range of commercially available columns for IC separations of small anions has been driven by requirements for different separation selectivity depending upon the application. Having chosen to undertake this study with entirely standard equipment, a range of polymeric particulate anion-exchange columns was considered. A relatively high capacity stationary phase was chosen as it was available in a commonly available particle size for IC (9 μm) and as a shorter column packed with 5 μm

Conclusions

The use of the KP method for characterising the current performance limitations of IC separations of small inorganic anions has been demonstrated for some commercially available instrumentation and columns. Some modifications to the standard instrumental setup were used to develop a robust approach to predict performance across a wide range of conditions. Practically constrained plots could then be used to present a range of solutions for IC separations requiring very high efficiency or speed.

Acknowledgements

This work was supported by the Australian Research Council's Discovery funding scheme (project numbers DP0663781 and DP0987318). The support of Dionex Corporation is gratefully acknowledged, in particular the loan of columns used in this work. The authors wish to acknowledge the valuable discussions with Prof. Gert Desmet.

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