gradient broad hump- think it is water

[cbeeb]

Running gradient method Meoh line A: buffer ph 5 line B As the gradient shifts from 45% A to 75%aA a broad hump appears. We do not always see it, but recently nothing will shift it or make it smaller. I have ran the gradient with no injection and it is still present. Water used is from Elga unit( TOC =45ppb) I would like to know if it is possible to get rid of it and how to do it

[tom jupille]

Check at a couple of different wavelengths. If the hump looks worse at longer wavelength, then it is probably due to the change in refractive index as a function of composition. This kind of problem is aggravated if the flow cell is not well aligned with the detector optics, so I would try to compare with another detector of the same model. If your detector looks worse, it may be time to get the detector serviced.

[cbeeb]

Ran on 2 identical systems hump still present, have tried different columns, different grade solvets. As I said we have never seen this until now, and we were able to get rid of it for a few days. Any other ideas?

[KarenJ]

Maybe you need to add a rinse step at the end of your gradient. The hump could be stuff that is accumulating on your column and slowly coming off during your run. If you ramp up to 95-100% organic for 5 min. at the end of each injection, you might rinse the extra garbage off. You may need a little extra equilibration time to get back to your gradient starting point if you add this step.