by
crong » Fri Jul 20, 2018 12:02 am
mattmullaney wrote:
Hi Crong,
Maybe the next things to think about are how you are cleaning the LC solvent reservoirs and how the pH is being measured. In the AccQTag method (the old one using NovaPak columns) a stock aqueous eluent was prepared (stored in a refrigerator, warmed to room temperature) and diluted 1:11 prior to use.
This could help with pH consistency between reparation runs.
That's pretty similar to how I prepare it. I make a 10x concentrate, filter through 0.2 um and store at room temp (initially tried to store in fridge but that caused precipitation). I adjust pH after diluting 10x on the day of analysis. The inconsistency is because the method says to adjust pH with fuming HCl and it's impossible to get it to 7.8 exactly. But if there was any Rt shift due to pH, it would have been insignificant compared to the forward Rt shift that accompanies every run, prior to yesterday. For cleaning the solvent bottle, I always use the same one for this mobile phase and just clean by rinsing with water several times then rinse once with a small amount of mobile phase.
I was thinking of trying a mobile phase that's lower in pH (say pH 7.5) just to see what happens. Do you think that would be worthwhile? Also I'll try doing the final pH adjustment with 1:1 HCl:H2O so there's more control.