I want to try basic conditions for LCMS in negative mode to try to satisfy the different pKa values of my 5 analytes which gear towards acidic and basic. pH5 is fine but the basic analytes can resolve better if I try to ionise them to see if I can sacrifice the performance of my resolution or acidic analytes. What column material have you tried in HILIC mode chromatography. I wanted to try charged phases but will only do cation or anion exchange. neutral phases not applicable. I am worried if that pH will degrade the material. Such as pH 6.5-8. Thank you for your support.
I read that HILIC columns are not so sensitive to pH change but the production HILIC materials in columns is more difficult to attach the hydrophilic ligands.
J Chromatogr Sci. 2013 Aug;51(7):684-93. doi: 10.1093/chromsci/bmt015. Epub 2013 Mar 13.
Main interactions and influences of the chromatographic parameters in HILIC separations.
Greco G1, Letzel T.
I am also wondering which pH range is most suitable for acidic analyte +ve mode ionisation and which pH range for negative mode ionisation for electrospray ionisation and atmospheric pressure APCI ionisation.
I am analysing small molecules. Using that pH may show improvement in the resolution and the ligands are bonded so beyond pH 6.5 might be problematic. There was not much difference in resolution after running 5-15mM buffer concentrations; after best pH established with no difference from pH 4 to pH 6.5.
I echo Farooq's answer, with a short comment. I have not had much success affecting resolution with pH in the ranges that you are talking about. However, being slightly above the pKb of your compound will radically alter your negative ion responses so from that standpoint it is definitely worth a shot. Silica gets really iffy above a pH of around 8 so be aware that you may trash your column.
From an ESI+ standpoint I prefer working in the formate buffer range (pH around 3); plus, the ammonium ion will sometimes give you a nice ammonia adduct to play with. APCI is not as sensitive, in my experience, to the pH since the ionization mechanism is not dependent upon solution charge state. For ESI- I typically try to work around a pH of 7.5-8.5, but I use RPLC that is stable in that range (like an aqueous-type phase). I do almost exclusively small molecule work, and I have found over time that I get better results using high aqueous phases than I do with HILIC.
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