Chromatography Forum

The GUO, Scrinivasan, Gaiki paper in Chromatographia, 66, 223 is a nice study…as the first study by Guo and Gaiki (J. Chromatogr. A, 1074 (2005) 71-80).
In my opinion these papers supports my previous statement. The ZIC®-HILIC and the polymer based ZIC®-pHILIC phases are less dependent on ionic strength in the mobile phase compared to an amino or bare silica column.

I’ve attached two hyperlinks for separation of small organic acids (fumaric, oxalic and citric acid) using the same experimental conditions (pH 6.8) on both polymer and silica based zwitterionic columns.
http://www.sequant.com/sn/p_notes.php?id=7
http://www.sequant.com/sn/p_notes.php?id=8
Under these conditions silanol effects should be prominent and repel negatively charged analytes compared to the polymer based column, yet similar results are obtained.

So I am in the fray:
Here a quote from the Guo.... article: "Of course, the retention on the silica and sulfabetaine phases could also be reduced by electrostastic repulsion of the negatively charged acids from the negatively charged silica and sulfabetaine phases." Etc., etc.

I have mentioned evidence in this forum which shows that there can be a severe effect due to pH and ionic strength (the ZIC looks very close to the Atlantis) on elemental ion retention. Maybe your pH of 6.8 was measured before adding organic? Who knows what it is? I am sure that if you hike up the pH and/or lower the ionic strength you will get exclusion. Furthermore, polymeric phases are apparently not free of ionic effects themselves (if necessary I will dig out the ref.).
(It should be remembered that one can obviate the SiO- effect, not only via pH, but also by swamping with salts).