Overview
Succinic acid and alpha-ketoglutarate are highly polar, acidic compounds that can present challenges during LC-MS analysis. When using a Cogent™ Diamond Hydride column with a HILIC method, analysts may occasionally observe little or no response in extracted ion chromatograms (EICs) despite proper chromatographic conditions.
In many cases, the issue is not related to the column but rather to the ionization conditions being used in the mass spectrometer.
Why Peaks May Not Be Detected
A common method configuration uses:
- Positive-ion electrospray ionization (ESI+)
- Formic acid as a mobile phase additive
- Acetonitrile-rich HILIC mobile phases
While these conditions work well for many analytes, they are often not ideal for strongly acidic compounds such as:
- Succinic acid
- Alpha-ketoglutarate
Because these molecules readily lose a proton, they generally ionize more efficiently in negative-ion mode rather than positive-ion mode.
As a result, little or no signal may be observed when monitoring these compounds under positive-ion conditions.
Recommended LC-MS Conditions
For improved performance, consider using:
Mobile Phase Additive
- 10 mM ammonium acetate
in both mobile phase A and mobile phase B.
Ammonium acetate provides a volatile, LC-MS-compatible buffering system that supports the ionized form of these acidic compounds.
Ionization Mode
Use:
- Negative-ion electrospray ionization (ESI−)
rather than positive-ion mode.
Under these conditions, succinic acid and alpha-ketoglutarate are typically observed as:
- [M-H]⁻ ions
which generally provides significantly improved detection sensitivity.
Benefits for HILIC Separations
Ammonium acetate can provide additional chromatographic benefits when analyzing acidic metabolites using Diamond Hydride columns.
Potential advantages include:
- Improved retention of polar acidic compounds
- Enhanced chromatographic reproducibility
- Better LC-MS compatibility
- Improved analyte ionization efficiency in negative-ion mode
These factors often contribute to greater sensitivity and more reliable quantitative performance.
Additional Troubleshooting Considerations
If analyte peaks remain absent after switching to negative-ion mode, verify:
- Correct mass transitions or extracted ion values
- Mobile phase preparation
- Instrument tuning parameters
- Sample concentration
- Injection volume
- Detector and source settings
Performing a direct infusion experiment can also help confirm that the analyte is ionizing efficiently under the selected MS conditions.
Key Takeaways
- Succinic acid and alpha-ketoglutarate are acidic compounds that may not respond well under positive-ion LC-MS conditions.
- Formic acid with positive-ion mode may produce little or no detectable signal.
- Negative-ion LC-MS is often the preferred ionization mode.
- Ammonium acetate is a useful LC-MS-compatible additive for these analytes.
- Improved retention and detection are often achieved using ammonium acetate and ESI− operation.
- These compounds are commonly detected as [M-H]⁻ ions in negative-ion mode.
