I work on a nucleoside with pKa 2.3 for the base and pKa 9.3 for the acid. The mobile phase is 5 mM sodium lauryl sulphate with 0.5% acetic acid (pH about 3) to acetonitrile 92:8. The column is Zorbax SB C18. Flow is 1.2 mL/mim. Detection UV 255 nm. The RT is very stable but I have got tailing peaks (Tailing factor 1.3 or more). Different temperatures were tried and did not give better results. Why the peak tails? Any suggestions and recommendations? Thanks a lot in advance.

Try another column. Once we changed Zorbax to Inertsil and received sufficient improvment of peak shape.

There can be 50 million reasons why peaks tail, from the sample solvent to extra-column bandspreading to detector issues. It can also be the column, but it does not have to be the column. It could be that the peak tails on your particular column because the column is not endcapped. It could be that you inject too much for the ion-pair system. Why do you use ion-pair chromatography anyway? It would not be my first choice....

Isn't the pH of your mobile phase a little too close from the pKa of your analyte ? If I remember correctly, at pH 3 with a pKa of 2.3 you have a 85/15 mixture of protonated/unprotonated species... I think you should try a little higher pH, around 5 maybe...