Prediction of Analyte Retention for Ion Chromatography Separations Performed Using Elution Profiles Comprising Multiple Isocratic and Gradient Steps

Abstract

This study addresses the simulation of ion chromatographic (IC) separations performed under conditions where the elution profile consists of a sequence of isocratic and gradient elution steps (referred to as "complex elution profiles"). First, models for prediction of retention under gradient elution conditions in IC were evaluated using an extensive database of gradient elution retention data. It is shown that one such model is preferred on the basis that it can be used to predict gradient retention times on the basis of isocratic input data. A method is then proposed for using this model for complex elution profiles whereby each step of the elution profile is treated separately and analyte movement through the column is mapped. An empirically based algorithm for predicting peak width under complex elution conditions is also proposed. Evaluation of the suggested approaches was undertaken on a set of 24 analyte anions and 13 analyte cations on 5 different Dionex columns using a range of 5-step complex elution profiles that gave R2 values for correlations between predicted and observed retention times of 0.987 for anions and 0.997 for cations. The simulation of separations of anions and cations using a 3-step complex elution profile is demonstrated, with good correlation between observed and predicted chromatograms. The proposed approach is useful for the rapid development of separations when complex elution profiles are used in IC.

Column selection for comprehensive multidimensional ion chromatography

“…Fortunately, retention behaviour in IC is very well understood and the retention factor (k) of an analyte anion in IC under isocratic conditions is described by the linear solvent strength model [8]. We have recently shown that the linear solvent strength model can be used to predict analyte retention in ion chromatography separations performed using elution profiles comprising multiple isocratic and gradient steps [9].…”

“…Rather than program an individual gradient for each second dimension separation, they increased the second column eluent strength using a linear gradient over the duration of the first column separation time. Since the a and b values can be used to predict analyte retention in IC using isocratic or programmed gradient elution [9], a spreadsheet was developed in Microsoft Excel using the theory described in ref. [9] to give an estimate of the utilisation of the 2-D separation space when these two columns are employed.…”

“…Since the a and b values can be used to predict analyte retention in IC using isocratic or programmed gradient elution [9], a spreadsheet was developed in Microsoft Excel using the theory described in ref. [9] to give an estimate of the utilisation of the 2-D separation space when these two columns are employed. In this instance the first dimension retention times were manually entered into the spreadsheet to expedite method development of only the second dimension eluent profile.…”

“…In this instance the first dimension retention times were manually entered into the spreadsheet to expedite method development of only the second dimension eluent profile. However, it is not necessary to know the retention times of the first dimension separation in advance because these retention data can be predicted easily [9]. Figure 3 illustrates a typical output from the spreadsheet showing the predicted utilisation of the separation space from the coupling of an AS16 first dimension column to an AS20 second dimension column.…”

Methodology for porting retention prediction data from old to new columns and from conventional-scale to miniaturised ion chromatography systems

“…We have significant interests in ion chromatography (IC) separations as well as in the miniaturisation of IC systems and during the last decade we have developed strategies for retention prediction and the simulation and optimisation of IC that involve variations of the linear solvent strength retention equation [5][6][7][8][9][10]. These strategies are based on a database of carefully measured isocratic and linear gradient retention data obtained for a wide range of analytes and IC columns, with these data being "embedded" into the software used to simulate separations.…”