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Carr P.W.,University of Minnesota | Dolan J.W.,LC Resources | Neue U.D.,Waters Corporation | Snyder L.R.,LC Resources
Journal of Chromatography A | Year: 2011

In reversed-phase chromatography (RPC), the restricted retention of " bulky" solutes can occur in one of two ways, giving rise to either " shape selectivity" or " steric interaction." Starting with data for 150 solutes and 167 monomeric type-B alkylsilica columns, the present study examines the steric interaction process further and compares it with shape selectivity. The dependence of column hydrophobicity and steric interaction on column properties (ligand length and concentration, pore diameter, end-capping) was determined and compared. The role of the solute in steric interaction was found to be primarily a function of solute molecular length, with longer solutes giving increased steric interaction. We find that there are several distinct differences in the way shape selectivity and steric interaction are affected by separation conditions and the nature of the sample. Of particular interest, steric interaction exhibits a maximum effect for monomeric C18 columns, and becomes less important for either a C1 or C30 column; shape selectivity appears unimportant for monomeric C1-C18 columns at ambient and higher temperatures, but becomes pronounced for C30 - as well as polymeric columns with ligands ≥C8. One hypothesis is that shape selectivity involves the presence or creation of cavities within the stationary phase that can accommodate a retained solute (a primarily enthalpic process), while steric interaction mainly makes greater use of spaces that pre-exist the retention of the solute (a primarily entropic process). The related dependence of hydrophobic interaction on column properties was also examined. © 2011 Elsevier B.V.


Dolan J.W.,LC Resources
LC-GC Europe | Year: 2016

A reliable autosampler is one key requirement for unattended operation of a liquid chromatograph. © 2016, Advanstar Communications Inc. All rights reserved.


Dolan J.W.,LC Resources
LC-GC Europe | Year: 2015

How to distinguish between liquid chromatography (LC) column overload and detector overload. For many applications of liquid chromatography (LC), the goal is to measure very small quantities of the analytes of interest. In such cases, the problems associated with too much sample are rarely encountered. However, problems at the other end of the scale exist when more of an analyte is present than can be accommodated by the LC system. This situation is usually referred to as overload. Overload most commonly occurs as column overload or detector overload. This month’s “LC Troubleshooting” article looks at these two overload types, their causes, and possible solutions to correct the problem. © 2015 Advanstar Communications Inc. All rights reserved.


Dolan J.W.,LC Resources
LC-GC Europe | Year: 2015

Unexpected results from calibration standards create confusion in a clinical liquid chromatography (LC) method. © 2015, Advanstar Communications Inc. All rights reserved.


Dolan J.W.,LC Resources
LC-GC Europe | Year: 2014

How large an injection can you make if the injection solvent is not matched to the mobile phase?. © 2014, Advanstar Communications Inc. All Rights reserved.


Dolan J.W.,LC Resources
LC-GC Europe | Year: 2014

If scaling isocratic separations is so simple, why is gradient scaling so confusing?. © 2014, Advanstar Communications Inc. All rights reserved.


Snyder L.,LC Resources
Journal of Planar Chromatography - Modern TLC | Year: 2012

"Localization" refers to the non-covalent attachment of molecules of solute or solvent to strong sites that form part of the adsorbent surface. This phenomenon and its significance in adsorption chromatography are reviewed, including its various effects on sample retention and resolution. © Akadémiai Kiadó.


Snyder L.R.,LC Resources | Dolan J.W.,LC Resources
Journal of Chromatography A | Year: 2013

Several different conditions can be varied to improve selectivity for reversed-phase chromatography (RPC). A reexamination of literature data suggests that changes in selectivity due to a change of column or mobile phase pH are largely replicated by changes in temperature or mobile phase composition (concentrations of acetonitrile and/or methanol). This suggests a reconsideration of the role of mobile phase pH and the column during method development. © 2013 Elsevier B.V.


Dolan J.W.,LC Resources
LC-GC Europe | Year: 2016

Different techniques of liquid chromatography (LC) mobile-phase mixing can give different results and have different problems. © 2016 Advanstar Communications Inc. All rights reserved.


Dolan J.W.,LC Resources
LCGC North America | Year: 2013

Gradient methods can offer unique solutions to separation problems, but transferring a gradient method from the literature, between laboratories, or even within the same laboratory can be a challenging process.

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