Computer-assisted simulation and optimisation of retention in ion chromatography
Section snippets
Ion chromatography
Ion chromatography (IC) is a powerful analytical technique for the separation and determination of inorganic and low molecular weight organic ions. IC falls under the general category of liquid–solid chromatographic methods, in which a liquid mobile phase (or eluent) is passed over a solid stationary phase (SP) and usually then through a suppression device before entering a flow-through detector (typically a conductivity type). The sample to be separated is introduced into the flowing eluent
Isocratic elution
The most fundamental elution regime in IC is isocratic elution, whereby the eluent composition remains constant throughout the entire separation. The constant eluent strength typically leads to several general elution problems in separating mixtures containing analytes with widely differing distribution coefficients. Co-elution, insufficient peak capacity and excessive separation time of the later eluting peaks are typical problems observed in isocratic separations (Fig. 1) [2].
Gradient elution
Gradient elution
Retention time modelling in IC
To perform optimisation in silico, the main factors that determine the resolution in a chromatographic separation must be modelled mathematically. The resolution is given by [21]:where tR1 and tR2 are the retention times of the adjacent peaks, and w1 and w2 are the base widths of both peaks. It is important to note that the peak widths at half height can also be used to calculate the resolution of a peak pair.
From Equation (1), it is clear that both retention times and peak
Peak width modelling
In the isocratic elution mode, peak width is affected by peak broadening processes [53], [54], and it can be predicted using the rearranged theoretical plate count expression:where N is the theoretical plate number of the analyte.
Peak width in gradient separations is controlled by two major factors: peak broadening and band compression. Increasing solvent strength in gradient elution tends to increase the trailing edge of the analyte band relative to the leading edge, which results in
Optimisation of chromatographic conditions
Retention time and peak width modelling allows in silico optimisation of IC separations with a two-step procedure. First, a search area (minimum and maximum boundaries) for each parameter influencing the separation (such as initial eluent concentration and gradient slope) needs to be defined. Next, a set of experimental conditions within the defined search area is then selected, followed by assessing the quality of the potential separation. This process is repeated until the potential
Simulation and optimisation software
‘Virtual Column Separation Simulator’, marketed by ThermoFisher Scientific, is currently the only commercial simulation and optimisation tool available for IC method development. It was originally developed at the Australian Centre of Research on Separation Science (ACROSS) in collaboration with the Dionex Corporation [46].
Virtual Column Separation Simulator provides rapid optimisation as well as a simulation tool for IC separations on two different column diameters (4 and 2 mm). It can predict
Conclusions, challenges and perspectives
Time-consuming trial-and-error approaches are no longer used to develop methods of IC separations with the emergence of robust and reliable computer-assisted optimisation procedures. Computer-assisted optimisation allows users to achieve a desired separation in a short time and to select the requirements of the target separation. The predictive accuracy of the chosen retention model is key to the success of the optimisation, which is somewhat less dependent on the accuracy peak width model used.
Acknowledgements
This research was supported by the Australian Research Council's Discovery funding scheme (project DP0663781) and a Federation Fellowship (FF0668673) to PRH, Singapore GSK-EDB Trust Fund Project ‘Large-scale Chromatography with Green Solvents: Fundamentals and Novel Processes’.
References (64)
- et al.
Gradient elution in high-performance liquid chromatography : II. Practical application to reversed-phase systems
J. Chromatogr. A
(1979) - et al.
Multilinear gradient elution optimisation in reversed-phase liquid chromatography using genetic algorithms
J. Chromatogr. A
(2006) - et al.
Effect of column temperature on high-performance liquid chromatographic behaviour of inorganic polyphosphates : II. Gradient ion-exchange chromatography
J. Chromatogr. A
(1985) Computer-assisted retention prediction for high-performance liquid chromatography in the ion-exchange mode
J. Chromatogr. A
(1989)- et al.
Gradient elution in liquid chromatography : II. Retention characteristics (retention volume, band width, resolution, plate number) in solvent-programmed chromatography – theoretical considerations
J. Chromatogr. A
(1974) - et al.
Ion-exchange chromatography of nucleotides on polyethyleneimine cellulose columns: analysis of maize grain extracts
Anal. Biochem
(1968) Prediction of the behaviour of oligonucleotides in high-performance liquid chromatography and capillary electrophoresis
J. Chromatogr. B. Biomed Sci. Appl
(1993)- et al.
Computer-assisted retention prediction system for oligonucleotides in gradient anion-exchange chromatography
J. Chromatogr. A
(1988) - et al.
Simultaneous determination of creatinine, creatine, and UV-absorbing amino acids using dual-mode gradient low-capacity cation-exchange chromatography
J. Chromatogr. A
(2005) - et al.
Development of an ion chromatographic gradient retention model from isocratic elution experiments
J. Chromatogr. A
(2006)