John W. Dolan
John Dolan has been writing "LC Troubleshooting" for LCGC for over 25 years. One of the industry's most respected professionals, John is currently a principal instructor for LC Resources, Walnut Creek, California He is also a member of LCGC's editorial advisory board.
Articles
Count the Cost, Part 2: Increasing Resolution by Increasing Retention
April 1st 2017We will discover how to find the “sweet spot” in terms of retention for a liquid chromatographic separation as well as how much retention change can be expected for a selected change in organic mobile-phase percentage or column temperature.
Count the Cost, Part I: Increasing Resolution by Increasing Column Efficiency
March 1st 2017When considering column efficiency, more is not always better. We look at some ways to quickly estimate the effects of changes in column length and particle diameter rather than trying the experiments in the laboratory.
Count the Cost, Part 1: Increasing Resolution by Increasing Column Efficiency
March 1st 2017When considering column efficiency, more is not always better. We look at some ways to quickly estimate the effects of changes in column length and particle diameter rather than trying the experiments in the laboratory.
Reader’s Question: Early Eluted Peak
November 1st 2016What could be causing a peak to be eluted before the column dead time? In last month’s “LC Troubleshooting” (1) we looked at problems two readers had with ghost peaks in gradient runs. This month, we’ll continue looking at submitted questions and examine one submitted by another reader of this column.
The Hydrophobic-Subtraction Model for Reversed-Phase Liquid Chromatography: A Reprise
September 1st 2016Reversed-phase liquid chromatographic columns can be compared quantitatively for differences in selectivity by means of the hydrophobic-subtraction model. This allows selection of columns that are either equivalent or different in selectivity. The present paper both presents a summary of this approach and shows in detail how to use it in practice.
The principal aim of this work was to provide a perspective with practical utility in streamlining the chromatographic method development in pharmaceutical industries based upon predicting the chromatographic retention times from molecular structures. Workflows were suggested with a focus on reversed-phase LC, IC, and HILIC as the three major techniques. Unlike HILIC, retention prediction in both reversed-phase LC and IC can benefit from the maturity of these techniques and the transparency of their retention mechanisms. In reversed-phase LC the solute coefficients in the hydrophobic subtraction model and in IC the a and b values in the linear solvent strength model can be the subject of modelling with their subsequent use in retention prediction. A workflow for HILIC can be based on the design of experiments approach, to account for all major contributors to the retention mechanism, and direct correlation of experimental retention times to the molecular descriptors.
