Chromatography Forum

With the method detailed below we are consistently getting a small peak at around 32min - unfortuneately the same RT as one of our known impurity pks.
MP A: H2O, pentanesulfonic acid, acetic acid 1.7%
MP B : MeOH
40% B for 10min, Gradient to 80% B to 30min then hold 10min, then back to 40%
Have tried: different system, column, cleaning column, no acetic acid in MP A
Peak size does not vary with inj volume
Could it be the sulfonic acid?
Any suggestions gratefully recieved
Mick

What is your detector and if UV at what wavelenght?

Does your peak area and retention time of your ghost peak change with different equilibration times? Do you see the peak if you inject mobile phase A? Do you see the peak if you do the gradient without injection?

The peak is unchanged when 40% MeOH only is injected regardless of volume
System is shimadzu so gradient cant be run w/o injection as far as I know
Detector is D2 lamp 273nm
I have not yet tried diff equilabration time but will

Run a few blanks with varying equilibration or with varying the time of the initial 40% hold. If the peak increases with this hold, it is a mobile phase component, possibly in your mobile phase A. If the peak area remains the same, I would check the injector - wash solvents etc.

mpepler,

The problem can't be due to the sulfonic acid as it does not absorb in the UV. However, sulfonic acids are often contaminated with chromaphoric contaminants. I would try switching to another source of the sulfonic acid (Fluka has some good-quality high purity grades).

Thanks for comments. Peak does seem to vary with equilibration time, I suspect a sulfonic acid contaminant. Case is closed for time being.

Mpepler,

One of the reason I asked you if the area of the peak is varying with equilibration was in order to eliminate such possibilities. However, if it was a sulfonic acid contaminant it should vary with different equilibration times as depending on the equilibration time you would fix a different amount of contaminant in the stationary phase and then elute it during your gradient elution. As the peak area does not vary, it should not be a sulfonic acid impurity...

Sorry Kostas I thought that was what I said. Unfortuneately I am somewhat constrained from further investigation on this issue. Thanks for your help.

Oupps,

Indeed!