Peak shape for polar vitamins in LCMS has been shown to be improved when using a 0.1% formic acid additive instead of ammonium acetate. The sample diluent should be 50% DI water/ 50% acetonitrile/ 0.1% formic acid.
If you are using 50% isopropanol or 50% methanol in the aqueous solvent, the carryover could be due to insufficient eluting strength.

Try using DI water/ 0.1% formic acid instead of 50% 2-propanol/ 50% DI water/ 0.1% formic acid.

Cogent Diamond Hydride HPLC Column Ordering Information