The problem with talling peaks can occur due to following reasons:
a) Sample reacting with active sites. Solution: First check column performance with standards. If results for test mix are good, add salts or competing base or acid modifier. b) Wrong column type. Solution: Try another column type (e.g. endcapped column for basic compounds). c) Wrong mobile phase pH. Solution: Adjust pH. For basic compounds, higher pH usually provides more symmetric peaks. d) Wrong injection solvent. Solution: Peaks can tail when sample is injected in stronger solvent than mobile phase. If possible, dissolve sample in mobile phase. e) Small void at column inlet. Solution: Repack top of column with particles of same bonded phase functionality. Continue using column in reversed flow direction.