Peaks before T0

[Omar]

I am running a screening generic gradient method that among other compounds contains Thiourea, Benzene Sulfonic acid and Pthalic Acid. The system is being run on an XBridge C18 column 100mm x 3.0mm, 3.5um at a flow rate of 0.5mL/min. When using a buffer at pH 2.0 the elution order is the following Thiourea (~1min assumed as T0) followed by benzensulfonic acid and phthalic acid. When running the same gradient and conditions by using a buffer at pH 10.5. Th elution order is the following Pthalic Acid (~0.6min), benzene sulfonic acid (~0.7min), Thiourea (~1min). My questions are the following:

Since thiourea is considered a void marker is it possible that I have peaks before t0?
Is it possible that thiourea is being somewhat retained in the basic conditions?
Is it possible that the same column has different T0 values at different pHs?

Thanks for your help.

[HW Mueller]

You can search the forum for "charge exclusion".

[Uwe Neue]

To follow up on Hans's remark:
Thiourea, which is not ionic, did not change retention.
The other two analytes are negatively charged, which reduces retention by charge repulsion from the silanols, which are also negatively charged at pH 10.