I recently coupled an old MSD to an SFC system and after tuning the MSD, I tried a sample run in positive mode. This is when I observed my MS peaks were going down. These negative peaks would usually indicate that the signal from the background or solvent is greater than the signal from the analyte at that mass-to-charge ratio (m/z). This can be due to various factors, including background ions, solvent suppression, or incorrect detector polarity.
What Causes Negative Peaks in ESI-MS?
1. Background Ion interference and analyte Suppression
A background ion, often from the solvent or other contaminants, might have a higher signal than the analyte at certain mass-to-charge ratios (m/z). If the analyte elutes and effectively “turns off” the background ion, a negative peak can appear. The analyte itself might be interfering with the ionization of the background ions, leading to a decrease in their signal. This can be particularly noticeable in ESI-MS (electrospray ionization mass spectrometry).
2. Solvent and pH Effects
The composition of the solvent and its pH play a significant role in ionization efficiency. Acidic solvents favor positive ion formation, while basic solvents enhance negative ionization. If the solvent conditions are not optimized, negative peaks may appear due to incomplete ionization.
3. Adduct Formation and Fragmentation
In negative ion mode, molecules can form adducts with counterions such as chloride (Cl⁻) or acetate (CH₃COO⁻), leading to unexpected mass shifts. Additionally, fragmentation patterns in negative ion mode can differ from those in positive mode, sometimes resulting in lower signal intensity or unexpected peak behavior.
4. Instrumental and Tuning Parameters
The electrospray voltage, capillary temperature, and gas flow rates influence ionization efficiency. Improper tuning of these parameters can lead to reduced ion transmission, causing negative peaks or signal suppression. Also, If the recorder connections are reversed, all peaks might appear negative due to incorrect polarity.
How to Minimize Negative Peaks?
To reduce the occurrence of negative peaks in ESI-MS, consider the following strategies:
- Clean the system and ion source by flushing with water and IPA to remove residual solvent. Clean the ESI source to eliminate potential background ion interference.
- Optimize solvent composition and pH to favor the desired ionization mode and enhance ion formation.
- Optimize instrument parameters (e.g., cone voltage, collision energy) to maximize the signal from the analyte and minimize suppression. Verify that the recorder connections are correctly polarized.
- Check background ions to account for ion suppression effects and ensure that background ions are not interfering with analyte ions.
Conclusion
Negative peaks in electrospray ionization mass spectrometry can arise due to charge suppression, solvent effects, adduct formation, and instrumental settings. By carefully considering these factors and troubleshooting steps, it is possible to identify and resolve the cause of negative peaks in a mass spectrum. This will ensure accurate and reliable results.
Relevant sources
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