I'm having problems obtaining a blank on a system set up using a C18 column and gradient run. The mobile phase used consists of a buffer which contains octane sulphonic acid, TEA and phosphoric acid in a solvent mix of methanol and acetonitrile.
I've tried using different brand of reagents,different columns, different instruments, new glassware and 10 days later there is no change in the blank. Any suggestions on how to alleviate
this problem?????

Yours,

A frustrated analyst

When you refer to a "blank" do you mean a stable baseline? Often, in gradient applications, you will see a rise in the baseline. However, in your particulat mobile phase, it may not be suitable to try a gradient run when using ion-pairing reagents. The conditions of the column are constantly changing. I will need more details.
Also, this question is for anyone. What is the reason for using TEA in a mobile phase that contains C8 sulphonic acid? Doesn't using them together defeat their purpose?

For several months, I have been following a gradient analytical method with 5 mM tetrabutylammonium hydrogensulfate (pH 5.9) as ion-pair reagent and acetonitrile as organic solvent, on a C18 5µm column. pH adjustment seems to be crucial for separating two closely eluting impurities. At the end of a day's work these 2 impurities have come closer, apparently due to a slight buffer pH variation: pH becomes more basic (possibly due to dissolution of atmospheric CO2 in the mobile phase?), but the resolution is still good enough.
However, I have been recently experiencing some trouble with this method, with sometimes more than a week without obtaining separation, even with a new column and freshly prepared buffer and acetonitrile.
Can someone give me a hint? Could Helium sparging be of any help?
Please note that I can´t change any analytical parameters.

Sometime TEA can improve peak tailing when you're using alkylsulfonates as ion-pairing agent. Why you say you can't use them together?.
I think that if the system works you can use them. But the point is that there is a sutil equilibrium between TEA, pH and AlkylSO3-, thus, in my experience, phase mobile conditions must be carefully adjusted. And also in my opinion, there are too many variables to keep in mind for a gradient method.