peak in baseline

[x_harp]

I am trying to develop ion-pair method using a water:acn gradient on a waters atlantis dC18 column. The gradient starts 5 minutes into the separation and goes from 90:10 water:acn to 10:90 water:acn over a 30 minute period. At ~13 minutes into a blank injection a rather large~ 0.18 AU peak is observed (uv-vis detection, the solutes under study require detection at < 220 nm). Further more, this peak cannot be attributed to the blank (diluent) or the ion-pairing agent itself (the peak was even observed on the column when performing the water:acn gradient without the ion-pair in the mobile phase). The same peak was observed on a second Atlantis column. The ion-pairing agent used was tetrabutylammonium bisulfate with MilliQ water and Optima grade acetonitrile. This peak is rather large, and may interfere with possible impurities in the method, thus we want to remove it. Does anyone have any idea as to the source of this peak, or how to minimize it's effect? Thanks.

[Mark Tracy]

This is an example of why one should avoid gradients with ion-pairing agents whenever possible. There are times when you have no other good options, so I will assume this is your case.

The TBA accumulates on the column at low solvent conditions, then sometime during the gradient it elutes in this lump. Unfortunately, this is a feature of the system that you will have to live with. The timing of the lump is affected by the slope of the gradient, so you may be able to move it away from your peaks of interest. When I have needed to do ion-pairing with a gradient, one trick I have used is to use a step gradient to put the lump in between my peaks of interest.

One other thing about this is that the re-equilibration is not complete in a convenient amount of time. You will be injecting before the column is fully at equilibrium, but as long as you are consistent, all is well. Always use an autosampler. You may need to run a couple of throwaway runs before the retention times and baseline are really stable.