Chromatography Forum

Hello

I would like to try a buffer in the pH range of 4-5 and use it in a HPLC-ELSD system.
I could use ammonium formate or ammonium acetate...

My questions:
Which of the 2 buffers could be more suitable for the ELSD? Any advantages for 1 of the 2?

What could be the ideal concentration?
Normally we are using concentrations between 10 - 50 mMol for HPLC-UV systems, but I read an article that suggested much lower concentrations around 0.1 mMol for ELSD?!

Thank you in advance for your help!

If you're shooting for pH 4-5, acetate, with a pKa of 4.7 would be the better choice.

As far as the detector is concerned, the lower the buffer concentration, the better. However, you need sufficient buffer to control ionization of your sample and any residual active silanols on your column (I'm assuming you're using a silica-based bonded-phase column here). Too little buffer and you will see peak shape problems and variable retention times. "The devil is in the details", so try a series of experiments starting at say, 25 mM and cutting the concentration in half each time (make sure you allow plenty of equilibration time between changes).

Something to be aware of is that ELSDs are not truly linear, so you may need to run additional standards and if you use gradient chromatorgaphy they are not linear with modifier concentration (efficiency of nebulisation to blame - some newer models have a temperature controlled nebuliser for this reason).