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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