The Cogent UDC-Cholesterol™ HPLC Column is a unique TYPE-C™ silica hydride stationary phase that combines strong hydrophobic retention with shape-selective recognition. The cholesterol-based bonded phase provides selectivity that differs significantly from conventional C18 columns, making it particularly useful for separating structurally similar compounds, geometric isomers, steroids, lipids, fatty acids, and other hydrophobic analytes.
Unlike many traditional reversed-phase columns, UDC-Cholesterol can also be used in HILIC mode, giving chromatographers a versatile platform for both nonpolar and polar compound analysis.
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Choosing the Appropriate Separation Mode
Reversed-Phase (RP) - Recommended
Reversed-phase operation is the primary application for the UDC-Cholesterol stationary phase.
Typical Applications
- Steroids
- Lipids
- Fatty acids
- Hydrophobic pharmaceuticals
- Geometric isomers
- Structural isomers
- Natural products
- Highly hydrophobic compounds
Example Compounds (Structure shown below)
- Corticosterone
- Oleic Acid
- Elaidic Acid (trans isomer)
Unique Selectivity
The cholesterol ligand can provide shape recognition that differs from conventional C18 columns.
Selectivity may be influenced by:
- Mobile phase composition
- Temperature
- Molecular geometry
- Steric interactions
Increasing Retention in RP
To increase retention:
- Increase water content.
- Reduce organic solvent content.
HILIC - Mixed Mode Selectivity
The UDC-Cholesterol phase can also be used for highly polar compounds.
Typical Applications
- Amines
- Polar pharmaceuticals
- Highly water-soluble compounds
- Metabolites
- Compounds containing multiple hydroxyl groups
Example Compounds (Structures shown below)
- Tobramycin
- Metformin
Increasing Retention in HILIC
To increase retention:
- Increase organic solvent content.
- Reduce aqueous content.
Before Using the Column
Before installation:
- Purge all solvent lines of previous mobile phases.
- Purge the injector system.
- Verify all solvents are fresh and HPLC-grade.
- Degas solvents before use.
- Prepare fresh buffers daily.
Startup Instructions
Initial Conditioning
- Install the column using standard laboratory practices.
- Verify all fittings and tubing are properly connected.
- Condition the column with a 50:50 organic solvent/water mixture containing any intended additives.
- Equilibrate for approximately 30 minutes.
- Transition to your desired method conditions and allow stabilization.
Column Protection
For difficult sample matrices:
- Use inline filters whenever possible.
- Use guard columns whenever possible.
- Replace protection devices routinely.
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Reversed-Phase Method Development
Neutral Compounds
Recommended starting mobile phases:
- Acetonitrile / Water
- Methanol / Water
For most applications:
- Add 0.1% formic acid to both mobile-phase components.
This is especially important for LCMS methods.
Simple Samples
Begin with:
- 70-90% organic solvent
Increase water content until desired retention and separation are achieved.
Complex Samples
Suggested scouting gradient:
- 90% organic
- To 10% organic
- Over approximately 10 minutes
Adjust gradient composition and slope as needed.
Acidic Compounds
To improve retention:
- Use approximately 0.1% formic acid to suppress ionization.
Follow the same development strategy used for neutral compounds.
Basic Compounds
Many hydrophobic bases retain effectively in RP mode.
However:
- Small highly polar bases may show insufficient retention.
- These compounds often perform better in HILIC mode.
High-pH operation is generally not recommended.
HILIC Method Development
Polar Neutral Compounds
Highly polar neutral compounds may exhibit useful retention under HILIC conditions. The same general approach used for acidic compounds is often effective.
Acidic Compounds
To maximize retention:
- Ensure the analyte is appropriately ionized.
Recommended starting buffer:
- 10 mM ammonium formate
- or
- 10 mM ammonium acetate
Recommended starting pH:
- Approximately 6.5
Simple Samples
Begin with:
- 50% aqueous buffer
- 50% acetonitrile
Increase organic content as needed.
Complex Samples
Suggested scouting gradient:
- 90% acetonitrile
- To 20% acetonitrile
- Over approximately 10 minutes
Optimize according to sample complexity.
Basic Compounds
Recommended starting additives:
- 0.1% formic acid
- or
- 0.2% acetic acid
Both isocratic and gradient methods can be successful depending on sample complexity.
Using Temperature to Control Selectivity
One of the unique characteristics of the UDC-Cholesterol stationary phase is its ability to exhibit selectivity changes with temperature.
If desired selectivity is not achieved:
- Adjust column temperature.
- Evaluate retention and resolution changes.
- Re-optimize mobile-phase composition if needed.
This behavior can be particularly valuable for:
- Geometric isomers
- Structural isomers
- Steroid separations
- Lipid analysis
Troubleshooting Common Problems
Typical causes of chromatographic problems include:
- Inadequate equilibration
- Incorrect mobile-phase composition
- Inappropriate temperature selection
- Contaminated solvent lines
- Sample solvent mismatch
- Sample overload
- Failing fittings or tubing
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Storage Instructions
When analysis is complete:
- Flush the column with approximately 90:10 organic solvent/water.
- Ensure all system pressure has returned to zero.
- Disconnect and cap the column for storage.
Important
Never disconnect a column while pressure remains in the system. Pressure shock can damage both column hardware and stationary phase performance.
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Best Practices Summary
- Use UDC-Cholesterol primarily for hydrophobic compounds and shape-selective separations.
- Consider temperature as a method-development variable.
- Use HPLC-grade solvents only.
- Degas all solvents.
- Prepare buffers fresh daily.
- Increase water to increase RP retention.
- Increase organic solvent to increase HILIC retention.
- Use filters and guard columns whenever possible.
- Store in approximately 90:10 organic/water solvent.
- Remove all pressure before disconnecting the column.