Overview
When developing HPLC methods using Cogent™ Diamond Hydride columns in HILIC mode, chromatographers often predict retention based on the ionization state of functional groups within a molecule.
In many cases, ionized compounds become more polar and exhibit stronger retention in HILIC separations. However, some molecules contain multiple charged functional groups that can influence overall polarity in unexpected ways.
As a result, retention behavior may differ from what would be predicted by examining a single functional group in isolation.
Example: S-Adenosyl Methionine (SAMe)
A commonly observed example involves S-adenosyl methionine (SAMe) analyzed using a Cogent™ Diamond Hydride column.
Under otherwise identical chromatographic conditions:
- A mobile phase containing formic acid may produce stronger retention.
- A mobile phase containing ammonium acetate may produce lower retention.
At first glance, this may appear counterintuitive.
Why the Expected Retention Increase May Not Occur
One possible assumption is that a carboxyl group becomes ionized when using ammonium acetate, making the molecule more polar and therefore more strongly retained in HILIC mode. However, compounds containing multiple charged functional groups can behave differently.
In the case of SAMe:
- The sulfur-containing functional group carries a permanent positive charge.
- The carboxyl group may become negatively charged under certain mobile phase conditions.
- These opposite charges may partially offset one another.
This charge balancing effect can reduce the molecule's effective polarity and alter its interaction with the stationary phase. As a result, the compound may exhibit lower retention even though one functional group has become ionized.
Understanding Overall Molecular Polarity
Retention in HILIC methods is influenced by the overall behavior of the molecule rather than the charge state of a single functional group.
Important factors include:
- Total molecular charge
- Charge distribution
- Zwitterionic character
- Hydrogen bonding capability
- Solvation effects
- Mobile phase composition
Because these variables interact simultaneously, retention predictions based solely on pKa values or ionization state can sometimes be misleading.
Troubleshooting Strategies
When unexpected retention changes occur:
- Evaluate the complete molecular structure rather than a single functional group.
- Compare different acidic mobile phase additives.
- Investigate the influence of mobile phase pH and buffer composition.
- Monitor changes in selectivity as well as retention.
Alternative additives that may be useful during method development include:
- Formic acid
- Acetic acid
- Trifluoroacetic acid (TFA)
- Other volatile acidic modifiers compatible with the analytical objective
Small changes in additive chemistry can significantly alter retention and selectivity.
Key Takeaways
- Ionization does not always increase retention in HILIC chromatography.
- Molecules containing both positive and negative charges may exhibit unexpected retention behavior.
- Effective molecular polarity depends on the entire molecular structure.
- Ammonium acetate and acidic modifiers can influence retention differently.
- Evaluating multiple mobile phase additives is often helpful during method development.
- Retention behavior should be interpreted using overall charge distribution rather than individual functional groups alone.
