What is HMF? Aside of chemical reasons and column interaction, peak tailing can be caused by hardware such as excessively long tubing lengths between the HPLC components or poor connections. The latter is the easiest and fastest to check and mitigate. Assuming short tubing of uniform ID and zero-dead volume connections on the system, column interactions can be explored. Typically, multiple modes of interaction of the compound with stationary phase result in peak tailing. Good luck

Thank you so much madam.

5-HMF is 5-hydroxymethyl furfural

Tailing of peak of molecull like 5-HMF usually should not appear, since no nitrogen atoms are present in structure, which would result in tailing as result of secondary interactions of analyte with stationary phase.

This molecul has faraway pKa which will result in ionisation.

I suppose that more probable is problem with physical destruction, or blockage, clogg of column inlet surface or filter.

Try other C18 column,(5-HMF is highly polar and ODS is better choice for proper retention and separation) with better manufacturing characteristics, base deactivated for reducing quantities of metal cations in silica, and endcapped. These will highly reduce tailing. If tailing persists, then check for extracolumn volumes in capilaries, injectors

Since you indicated the mobile phase composition, I assume you are applying HILIC mode isocratic elution. Try to switch the chromatographic mode to reversed-phase by using endcapped c18 columns for HMF. If you cannot change the mode try to add some buffering agents such as ammonium formate to decrease tailing. It is very common to use AF in the HILIC phase to improve peak shape. But I strongly suggest using more robust RP conditions as mentioned above.