I'm trying to develop a method to assay three different compounds simultaneously: methylparaben, propylparaben and a basic compound.
I have used the approach described in "PRACTICAL HPLC METHOD DEVELOPMENT" to obtain the following chromatographic conditions: column 150*4.6mm C18 5um; flow 2ml/min; 40ºC; mobile phase= 34% ACN + 66% buffer pH=6. The method separates the 3 compounds, but the peak associated with the basic compound has a tailing factor above 2. What would be the best approach to improve peak shape?
Thanks in advance for any suggestions.

P.S. : Changing the pH to 3, resulted in a non-selective method with capacity factor values below 1 .

First, what kind of buffer are you using? Only the citrate and carbonate buffer are good at pH 6. Second, try to use "End Capped" columns at this pH. Note: at low pH, the end capped agent is prone to hydrolysis so alway use your end capped column at mid range pH. Third, if the peak shape is still unsatisfactory, you have two choices: 1. Increase the concentration of your buffer. or 2. Add some (~10mM) TEA (triethylamine) but make sure you adjust the pH accordingly. I personally like to try the first option because the latter smells very bad. Good luck.

Thanks for your suggestions genhouson.I will try the citrate buffer as soon as possible.Please let me know which "end-capped " columns do you usually use and which ones do you like best -brand,etc.

P.S:I forgot to mention that the mobile phase also contained TEA(c=0,2%).

My favorite column is Cosmosil 5C18 150x4.6 mm. Others like Agilent Zorbax Eclipse series, and Discovery columns from Supelco should also satisfy your needs.