Sulfuric acid as mobile phase ionic modifier in HPLC

Sulfuric acid in small concentrations is an ideal ionic modifier for HPLC applications.

Most common materials used in HPLC instruments are fully inert toward diluted sulfuric acid.

Such materials as PTFE, PEEK, Ceramic, SS316 show no interaction with this type of acid in the mobile phase (MP).

Fontana, M. G., “Corrosion Series,” Ind. Eng. Chem, 44, March 1952, p 89A. 

Based on this study, the surface corrosion rate of the SS316 type is less than 5 µm per year in boiling 1% sulfuric acid.  If concentration is lower and temperature below boiling then the corrosion does not have any practical effect.

• Sulfuric acid provides a good UV transparency down to 190 nm.
• Concentrated sulfuric acid can be stored without decomposition for a long time in a closed container.
• It is available in very high purity in semiconductor grade (99.999%) which is fully suitable for HPLC applications.
• Sulfuric acid in small concentrations produces stable and a strongly acidic MP.  No buffer is usually needed if sulfuric acid is used as an acidic modifier.
• The mobile phase with sulfuric acid in 0.1 – 0.5% range can be stored for a long time without forming bacteria in the liquid. 
• It is not volatile, so the concentration of sulfuric acid in the MP does not change over time.
• Most amino compounds are fully protonated in the presence of sulfuric acid in the MP.
• Most carboxylic acids become non-ionized and can be analyzed as neutral compounds. 
• Silanol groups of the silica based columns are significantly deactivated in most cases and do not cause a secondary interaction.