- For LC/MS applications: Positive ion mode favors acidic mobile phases and Negative ion mode favors basic mobile phases. However, feel free to experiment using both ionization modes and don't forget about using adducts (e.g. ammonium and sodium) with all types of samples to improve signal response. *Maintain these buffers at or below 10 mM. Adjust the pH of the mobile phase to be 1 to 2 units away from your sample's pKa.
Table 1: Popular examples of useful volatile mobile phase buffering systems.
BUFFERING AGENT USEFUL pH RANGE
- Ammonium formate 2.8 - 4.8; 8.2. - 10.2
- Formic Acid 3.3 - 4.3
- Pyridine/Formic Acid 3.3. 4.3, 4.8 - 5.8
- Trimethylamine/Formic Acid 3.3 - 4.3, 9.3 - 10.3
- Ammonium Acetate 3.8 - 5.8; 8.2 - 10.2
- Acetic Acid 4.3 - 5.3
- Trimethylamine/Acetic Acid 4.3 -5.3, 9.3 - 10.3
- Ammonia/Formic Acid 3.3 - 4.3, 8.8 - 9.8
- Ammonia/Acetic Acid 4.3 - 5.3, 8.8 - 9.8
- Ammonium Bicarbonate 5.9 - 6.9, 8.8 - 9.8
- Ammonium Carbonate 5.9 - 6.9, 8.8 - 9.8
- 1-Methylpiperidene 10.0 - 12.0
*Notes: (1) Formic acid is slightly stronger and more volatile than Acetic acid. Formic is often available in higher purity grades and absorbs less in the UV region for most chromatography applications. It works well in positive mode LC/MS analysis, esp at 0.1%. (2) Trifluoroacetic acid (TFA) is volatile, but we do not recommend its use in LC/MS applications as it can raise the background levels in Negative Mode LC/MS (m/z 113) and be very hard to remove from the source and cause contamination.(3) Triethylamine (TEA) causes similar problems with high background signals when used in Positive Mode LC/MS (m/z 102). Many ion pairing reagents suppress ionization.