Overview
Cogent™ Diamond Hydride columns are frequently used in HILIC and HILIC-like separations of polar compounds. Many methods employ mobile phase additives such as formic acid, acetic acid, ammonium formate, or ammonium acetate to optimize retention, peak shape, selectivity, and LC-MS performance.
However, analysts occasionally wish to simplify mobile phase preparation by eliminating additives altogether. This may be done to study retention mechanisms, evaluate intrinsic column selectivity, reduce method complexity, or support specialized detection requirements.
Diamond Hydride columns are fully compatible with additive-free mobile phases consisting only of acetonitrile and water.
Are Mobile Phase Additives Required?
No.
Mobile phase additives are not required for safe use of the Cogent™ Diamond Hydride column.
The stationary phase can be operated using:
- Acetonitrile and water only
- Acetonitrile-rich HILIC mobile phases
- Gradient methods
- Isocratic methods
Running additive-free mobile phases will not damage the column or shorten its service life.
When Additive-Free Mobile Phases May Be Useful
Methods without additives can be useful when:
- Evaluating fundamental HILIC retention behavior
- Studying intrinsic stationary phase selectivity
- Minimizing mobile phase complexity
- Avoiding additives that may interfere with certain detection techniques
- Developing highly simplified chromatography methods
These approaches can provide valuable information during method development and optimization.
Impact on Peak Shape
Although additives are not required, they often improve chromatographic performance.
Many analytes exhibit:
- Improved peak symmetry
- Reduced tailing
- More consistent retention
- Better reproducibility
when small amounts of acidic or buffered modifiers are present.
Without additives, some compounds may display:
- Broader peaks
- Increased tailing
- Reduced efficiency
- Changes in selectivity
The impact is highly dependent on analyte chemistry.
Why Additives Often Improve Performance
Mobile phase modifiers can influence chromatography by:
- Controlling analyte ionization
- Reducing secondary interactions
- Improving peak symmetry
- Stabilizing retention behavior
- Enhancing LC-MS response
Common additives include:
- Formic acid
- Acetic acid
- Ammonium acetate
- Ammonium formate
For many applications, only low concentrations are required.
Effects Vary by Analyte
The benefit of additives depends on the properties of the compound being analyzed.
Factors include:
- Functional groups
- Ionization state
- Molecular polarity
- Hydrogen-bonding characteristics
- Hydrophobicity
Some compounds show little change when additives are removed, while others exhibit substantial differences in peak shape, retention, or selectivity.
Because of this variability, method screening remains an important part of development.
Optimizing Additive-Free Methods
If additive-free operation is desired, several strategies can help improve performance.
Adjust Gradient Slope
A steeper gradient may:
- Sharpen peaks
- Reduce peak broadening
- Improve symmetry
Evaluate Multiple Operating Regions
Diamond Hydride columns can provide both:
- HILIC retention behavior
- Reversed-phase-like retention behavior
Testing both retention regions may identify better selectivity and peak shape for a particular analyte.
Optimize Injection Conditions
Evaluate:
- Injection solvent composition
- Injection volume
- Sample concentration
These factors can significantly affect peak quality.
Compare With Additive-Based Methods
Running side-by-side comparisons can help determine whether removing additives affects:
- Precision
- Retention stability
- Peak shape
- Method robustness
Method Development Considerations
For many HILIC applications, formic acid or other volatile additives remain a useful starting point because they frequently improve method robustness and detector response.
However, additive-free methods can perform very well for selected compounds and may provide a simpler workflow when properly optimized. The best approach is often determined experimentally during method development.
Key Takeaways
- Cogent™ Diamond Hydride columns do not require mobile phase additives for safe operation.
- Acetonitrile/water mobile phases can be used without damaging the column.
- Additives often improve peak shape, reproducibility, and retention consistency.
- The effect of additives is highly analyte-dependent.
- Additive-free methods are useful for studying HILIC retention mechanisms and simplifying mobile phase preparation.
- Gradient optimization and injection-condition adjustments can help improve additive-free methods.
