Does anyone have a good reference or can share their expert view on effect of pH on HPLC instruments? I know there are lot of resources available online on effect of pH of the mobile phase on retention, peak shape, sensitivity and column stability. I would like to know about how neutral to basic pH can affect HPLC instruments’ operation and functionability.
PS: For HILIC chromatography*
In the 'olde' days an alkaline pH was known to kill a C18 column by cleaving the ODS linkages. Nowadays, they have speciality columns that can tolerate a pH up to 12. But, I would still be careful and gently increase the pH via using different types of buffers (phosphate, borate...). Other HPLC components (like PDA cells which can be etched) may be less pH tolerant.
In addition to Bruce Neagle 's reply, the modifier can affect the detector. The effects of base on flow cell materials was mentioned. ELSD and mass spectrometers need a volatile modifier, ruling out inorganic salts as buffers such as phosphates. The problem isn't pH per se, but rather that such modifiers either cause the detector to become dirty or interfere with detection.
Another minor issue with inorganic modifiers is that they can cause increased pump seal wear. As the piston moves out of the cylinder, it is coated with a thin film of mobile phase which evaporates leaves micro-crystals in inorganic salts are used. The crystals wear the pump seal a little faster. Some HPLC manufacturers offer an option that washes the pump cylinder to eliminate this issue altogether.
One needs to be careful they don't precipitate salts when using them as buffers. It is also best to wash the system at the end of the day (including the column, no matter which modifier is used) with solvent containing no modifier. I once forgot I used a buffer in a system and almost crashed the pump switching to normal phase when the sodium phosphate precipitated.
I use inorganic buffers as needed, even for prep HPLC (I need a desalting step for this) with no issues.
Polymeric reverse phase columns work with a wide range of pH, but are only compatible with aqueous reverse phase solvents. If running normal phase or HILIC, alumina is also tolerant of a wide pH range.
The number of volatile buffers that are alkaline are very limited (MS capable). In addition many of these salts are insoluble as the percent composition of an organic mobile phase increases.
Remember, pH is an aqueous phenomenon! The equivalent theory in organic media involves electron transport.
Thank you so much everyone for your expert views. I have working experience with reverse phase chromatography coupled with MS. Recently a project came up, where we need to detect Dichloroacetic acid (DCA), since its very polar small molecule, HILIC chromatography seems to be a good fit. But we do not have the right HILIC type of column for the analysis. I am thinking about using an ion-pairing reagent, wither Triethylamine or Dimethylhexylamine. Both have pKa in between 10-11. What would be a correct pH of the mobile phase given that their pKa are in the range of 10-11. Do I expect to see better results with this mobile phase with C18 column or a HILIC type of column. Sorry if I am making some grave mistakes. HILIC chromatography is very new to me and I found references where amino column has been reported for analyzing DCA. But we do not have the funding to buy that column or to go down a chemical derivatization route at this moment.
I sincerely thank you all for your advises.
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