Hi Mike H.,
Okay, this stationary phase is based on Eclipse Plus...with a solid core. This phase is of intermediate density as you say above, and endcapped. The analyte of particular interest is a di-ketone, just to make sure.
I am not sure that you are prohibited from using TFA altogether...I'm not saying either that TFA is "the solution" to what is going on, at this moment. This is worth reading, anyway:
http://blog.sepscience.com/separationsc ... reparationYou can always add TFA to water to afford a final pH of the eluent that is > 2 if you want. Probably less than 0.1% (v/v) level.
200mM ammonium formate/ACN in a gradient? 200mM seems to me like a high concentration--is this 20 mM and a typo?
Anyway, to me, this isn't a problem with lack of an ion-pairing agent given the info we have. If this is a di-ketone, then the presence of Lewis bases (Formate ion, for example) will encourage the formation of the keto form (tautomerism) of the di-ketone via internal hydrogen bonding, as you note above.
This reference may help a lot:
http://www.sciencedirect.com/science/ar ... 7310000890Journal of Chromatography A, Volume 1217, Issue 12, 19 March 2010, Pages 1912-1915.
"β-Diketones: Peak shape challenges and the use of mixed-mode high-performance liquid chromatography methodology to obtain fast, high-resolution chromatography", by M.J.Rance and S.Wilson
Abstract
Historically, indirect methods have been used for the HPLC analysis of β-diketone compounds because of the very poor peak shapes and resolution obtained on conventional HPLC stationary phases. In this paper we demonstrate that it is possible to obtain good peak shapes for underivatised β-diketone compounds, in a simulated reaction mixture, using only conventional mobile phases with mixed-mode stationary phase HPLC columns. Optimum conditions were obtained using the mixed-mode reversed-phase strong anion exchange column Primesep B, supplied by SIELC Technologies, with a 0.1% aq. TFA/MeOH gradient method and a column temperature of 55 °C.