Predictive Elution Window Stretching and Shifting as a Generic Search Strategy for Automated Method Development for Liquid Chromatography

Abstract

ABSTRACT:We report on the possibilities of a new method development (MD) algorithm that searches the chromatographic parameter space by systematically shifting and stretching the elution window over different parts of the time-axis. In this way, the search automatically focuses on the most promising areas of the solution space. Since only the retention properties of the first and last eluting compounds of the sample need to be (approximately) known, the algorithm can be directly applied to samples with unknown composition, and the proposed solutions are not sensitive to any modeling errors. The search efficiency of the algorithm has been evaluated on an extensive set of random-generated in silico samples covering a broad range of different retention properties. Compared to a pure grid-based search, the algorithm could reduce the number of missed components by 50% and more. The algorithm has also been applied to solve three different real-world separation problems from the pharmaceutical industry. All problems could be successfully solved in a very short time (order of 12 h of instrument time).O ne of the most time-consuming tasks in analytical liquid chromatography is method development (MD). This is the search for the chromatographic operating conditions (type of organic modifier and stationary phase, temperature, gradient profile, pH, ionic strength, etc.) leading to the complete resolution of the sample in all its individual constituents. 1−9 Because of the high probability for peak overlap 10 and the sensitive dependency of the retention time of the individual analytes on, for example, the employed gradient slope, the MD process still involves a lot of trial and error and can take up to several weeks of work.To speed up this work, fully or semiautomated MD software has been developed over the years. 10−16 Roughly spoken, the automated MD strategies described in literature are either search-based (e.g., using the Simplex method) 17 or modelbased (e.g., Drylab, 18 Chromsword). 19 In the present study, the properties of a hybrid method, further referred to as the predictive elution window stretching and shifting method (PEWS 2 ), were investigated. This method explores the chromatographic parameter space in a pure search-mode but also uses the information on the retention properties of some of the peaks (e.g., the first and the last peak of the chromatogram and/or the first and last peaks of its most problematic zone). This information is used to make a model-based prediction of the gradient slope (expressed here as βt 0 = (ϕ e − ϕ 0 )·t 0 /t G ) and initial gradient composition ϕ 0 values that should be imposed to shift and stretch the elution window of the sample in a controlled manner over different parts of the time-axis. Hence, instead of optimizing the gradient by searching in the (ϕ 0 ,βt 0 )-space, the PEWS 2 -method searches directly in the (k first ,k last )-space. Since the values of k first and k last directly determine how wide the elution window is and whether the components elute early or late, t...

Use of individual retention modeling for gradient optimization in hydrophilic interaction chromatography: Separation of nucleobases and nucleosides

“…As such, this study provides an example of a case where individual retention modeling and computer-optimization can be used as a way to optimize the gradient conditions in the HILIC mode provided a non-linear model such as the Neue and Kuss model can be used. For compounds whose retention mechanism is more complex than that of the presently studied set, search approaches depending less strongly on an accurate retention modeling, such as the predictive elution window shifting and stretching (PEWS 2 ) method, should be preferred to optimize the gradient conditions in the HILIC separation mode [11,12].…”

“…using the Simplex method) [7], model-based (e.g. Drylab [8], Chromsword) [9] or based on a combination of both (design of experiments (DoE), multiple linear regression (MLR) [10], predictive elution window stretching and shifting method (PEWS 2 ) [11]). Recently, the PEWS 2 method was successfully applied for the gradient optimization in hydrophilic interaction chromatography (HILIC) [12].…”

“…In a previous study, involving samples with pteridines, sugars and a drug mixture of venlafaxine, tramadol and their metabolites in HILIC we observed strongly non-linear retention relationships, that were so complex that they could not be modeled using any of the existing non-linear retention models. As a consequence, the traditional retention modeling approach [22] that is so successful and widespread used in RPLC, could not be applied and we had to switch to a model-guided search approach (so-called PEWS 2 -technique [11,12]). For a sample containing compounds behaving more "nicely", individual retention modeling using a non-linear 3-parameter model (mixed model and the empirical model from Neue and Kuss) could be used to optimize the gradient conditions.…”

Separation Optimization of a Mixture of Ionized and Non-Ionized Solutes under Isocratic and Gradient Conditions in Reversed-Phase HPLC by Means of Microsoft Excel Spreadsheets

“…The most popular trial-and-error approach has several disadvantages since it is often time-consuming, usually requires a large number of preliminary experiments, and might not be fully efficient. Model-based techniques (fully or semi-automated software programs) can be used in the process of searching for desired RPLC separations [1][2][3][4][5]. These methods usually provide very successful separations based on a series of preliminary experiments.…”

Possibilities of retention modeling and computer assisted method development in supercritical fluid chromatography

“…In this case, normal phase retention behavior takes place using a polar stationary phase (such as silica, diol, amino, cyano, amide). On the other hand, using an apolar stationary phase (such as C 8 , C 18 , phenylhexyl), reversed-phase retention behavior is expected, as in the absence of H 2 O, the interactions between the compounds and the stationary phase are favored, limiting the contribution of the apolar CO 2 in the mobile phase. When other stationary phases (such as alkyl bonded phases with hydrophilic end-capping or polar embedded group) are used, intermediate behavior can be expected [7].…”