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Retention Mechanisms
in Chromatography
| The
main goals of our research on retention mechanisms in chromatography
are to provide microscopic-level insight into chromatographic separation
processes and to predict retention times using novel simulation
techniques and transferable force fields. |
Simulating the Chromatographic
Column
This figure shows a snapshot of one of our recent reversed-phase
liquid chromatography simulations with a dimethyl octadecylsilane (C18)
stationary phase and aqueous mobile phase. This chromatographic system
was studied to determine the microscopic-level structure of the bonded
phase in contact with the aqueous mobile phase with different type amd
amount of organic modifier (methanol or acetonitrile). This study will
provide insight into the experimentally observed phenomenon of retention
loss when using mobile phases with high water content.
Gas-liquid and reversed-phase liquid chromatography are
the principal methods for the analysis and separation of organic and biological
molecules, as evidenced by their extensive use throughout the scientific
community. In industry alone, more than half the cost of manufacturing
arises from separation processes, with chromatography playing a central
role, in particular, for high-value-added products like pharmaceuticals.
However, despite such wide-spread use, many fundamental questions on the
retention process remain unanswered, largely due to the complexity of
chromatographic systems and the lack of microscopic-level information.
Unanswered questions regarding the theory of retention include:
- What is the relative importance of partition and adsorption equilibria?
- To what extent does the mobile phase alter the structure and properties
of the stationary-phase?
- Do dangling hydroxyl groups at the surface of the silica substrate
influence retention?
Without these answers, there is great difficulty, first, in predicting
the retention properties of any solute and, then, in designing novel stationary
phases with improved retention characteristics. In a recent article in
the journal, Analytical Chemistry, trying to do so was compared
to climbing Mt. Everest:
Retention by theory
"It is considered the
'Mt. Everest' of separation science: Given just a structure and
experimental conditions, accurately predict the retention characteristics
of a solute molecule. J. Ilja Siepmann and colleagues at
the University of Minnesota and Rohm and Haas take on the challenge
and, using powerful molecular simulations and transferable force
fields, obtain microscopic pictures of the partitioning of 10
alkanes between a helium vapor phase and a squalene liquid phase
in gas-liquid chromatography. The analysis is made even more difficult
because the alkanes include two sets of topological isomers, such
as 2,5-dimethylhexane and 3,4-dimethylhexane, which are constructed
from the same set of methyl, methylene, and methine 'building
blocks'."
Anal. Chem. A-pages 185A
(2000)
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Chemistry Department
Research News:
- September
8, 1999: Simulating Retention in Chromatography
- November
14, 2001: Influence of Analyte Overloading on Retention in Gas-Liquid
Chromatography
- July
9, 2003: Temperature Dependence of Transfer Properties: Importance
of Heat Capacity Effects
- January
12, 2008: Understanding Retention in Reversed-Phase Liquid Chromatography
Recent Chromatography Publications:
-
J.L. Rafferty, J.I. Siepmann, and M.R. Schure,
- 'Size and shape effects on the retention of polycyclic aromatic
hydrocarbons in reversed-phase liquid chromatography,'
- Anal. Chem., submitted for publication.
-
J.L. Rafferty, J.I. Siepmann, and M.R. Schure,
- 'Investigation of the driving forces for retention in
reversed-phase liquid chromatography: Monte Carlo simulations of
solute partitioning between n-hexadecane and various
aqueous-organic mixtures,'
- Fluid Phase Equil., 300, ASAP article (2009).
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J.L. Rafferty, J.I. Siepmann, and M.R. Schure,
- 'The effects of chain length, embedded polar groups, pressure, and
pore shape on structure and retention in reversed-phase liquid
chromatography: Molecular-level insights from Monte Carlo simulations,'
- J. Chromatogr. A, 1216, 2320-2331 (2009).
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J.L. Rafferty, J.I. Siepmann, and M.R. Schure,
- 'Influence of bonded-phase coverage in reversed-phase liquid
chromatography via molecular simulation. II. Effects on solute
retention,'
- J. Chromatogr. A, 1204, 20-27 (2008).
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J.L. Rafferty, J.I. Siepmann, and M.R. Schure,
- 'Influence of bonded-phase coverage in reversed-phase liquid
chromatography via molecular simulation. I. Effects on chain
conformation and interfacial properties,'
- J. Chromatogr. A, 1204, 11-19 (2008).
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