Classically, peak sharpening agents such as triethylamine (TEA) were added in small amounts to the mobile phase(s). However, with modern bonding and endcapping, this is usually not necessary. For the chemistries you mentioned, especially the BEH Shield RP18, I am surprised to hear such a request.
Tailing can be caused by several things. What type of analyte(s) is/are tailing (e.g., acid, base, etc) and what are the chromatographic conditions (e.g., pH, injection volume, analyte concentration, etc) that you are running? A chromatogram would be helpful as well.
The analyte is an acid (pKa of ~4.4).
My mobile phase is a combination of 40% 20mM chloroacetic acid (i also tried 0.1% TFA) with 60% Acetonitrile. My injection volume is 5uL with a column temp of 40dC on a 50mm column using a concentration of 1.6mg/mL.
Attached is a chromatogram
What is the calculated tailing factor for that peak? Given the size of the peak (~0.85 AUFS) the peak shape does not look horrible. If you take your standard and dilute it 1:100 (in mobile phase) and re-inject, what is the peak shape/tailing factor? I am wondering if you are overloading the column. The peak shape was the same using 0.1% TFA?
From the attachment, it may not be tailing, and it would be sample overload. If you dilute the sample or reduce the inject volume, the problem would be solved.