Solvents or mobile phase is usually selected based on solubility and compatibility of a compound with various ionization techniques used in LC-MS. The volatility and ability of a solvent to donate a proton are significant in ESI and further atmospheric ionization methods. The special considerations should be taken when selecting a buffer for liquid chromatography-mass spectrometry applications.
The volatile buffers are needed to avoid fouling of the API interface an increase in buffer concentration in both positive and negative ESI may lead to a decrease in the signal of the molecule. However, this effect is dependent on some molecules that only show a small loss of response.Below listed are the compatible buffers (volatile) in LC-MS.
- Trifluoroacetic acid (TFA) (pKa-0.2)
- Formic acid (pKa-3.8)
- Ammonium Formate (pKa-3.8)
- Acetic acid (pKa-4.8)
- Ammonium acetate (pKa-4.8)
- Ammonium Formate (pKa-9.2)
- Ammonium acetate (pKa-9.2)
- Ammonia (NH4OH) (pKa-9.8)
The buffer is more effective when used in ± 1 pH unit of its pKa, however adequate buffering from the pKa can provide the ± 2 pH unit. Always use the highest quality and properly filtered buffer solutions. A buffer concentration of up to 25 mM will be adequate for LC-MS analysis. The accurate concentration of buffer is dependent on the analysis goals, lower concentrations of the buffer will typically increase column lifetime. It is recommended to avoid the use of phosphate buffer in the analysis of LC-MS, as it has low volatility.
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