During use of HPLC the retention time was shifted for fluorene (15.83 min) and pyrene (17.527) compound. I used the Acetonitrile :water (70:30) as the mobile phase .The obtained result is feasible or not.
the retention time can surely shift by altering the composition of the mobile phase. There are also other factors than can cause a retention time shift. I suppose you use reversed phase mode? got the retention times higher or lower? over what timeframe or across how many runs did the shift occur?