I don't know the structure of of your compound but there are several possibilities based on chemical characteristics of your compound: if your compound is  basic (BPA seems to be a basic compound) and the solvent in which you have prepared your sample has the pH value 1 less than the PKa of your compound then a substantial portion of your compound would in ionised form (let say around 66%) and and 34% will be non-ionised. Therefore, presume that the detection method is other than mass spectrometry, the smaller peak corresponds to the non-ionised portion of your compound.

Alternatively, it could be any contaminant having the same nominal mass of your compound or the second peak could be because of any positional or geometric isomers

Hi,

 I think this can be due to  the purity of the compound you have injected. What do you use to dissolve your compound? The peak shape can be due to the column properties. Did you use the same brand column as in the literature?

Based on alpesh's idea, i measured again with ajusing the pH to 4 and 12, respectively. i got the result below, first one is at pH 4 and second one is at 12. It seems at pH 12 it becomes better, but when i enlarge the peak, we can still see a small change (picture 3) 

 i prepared the sample in three times distilled water. I exactly followed the literature ( analysis method from agilent company)

Can we use the data in the second picture directly. Now i ajusted the pH by using H2SO4 and NaOH without buffer solution. One more question is whether i should use buffer solution to prepare the sample or not 

Thanks very much. Aurea

i have to dissolve the BPA into aqeous phase becasue i use it as a target pollutant to do water treatment. so i will ajust the pH before the analysis. But i have a quesion. the pKa of BPA is around 10. if i control the pH less than 5, the dominant species will be non-ionised BPA. if the pH is more than 10, it will be ionised form.  so for both of these two pHs, there will be only one clean peak. Maybe the position will be a little bit different.  But for mine, i still can see there is a bend in the right hand of the peak  which seems like there is another peak, as U can see in the picture above. 

I am encountering the same problem. I am using 60% water and 40% acetonitrile. Even though there is longer retention time than in your run, there is a small peak after the main peak. How did you overcome this challenge. 

 Wash HPLC with water  for three times before analyzing your sample. If the problem was not solved, increase the detection time of DAD. 

Thanks Aurea, you suggestion is very nice.  i tried several solution such as water, ACN, ethanol, etc to wash the column. Finally, i found that washing with water is a good choice for my analysis.  After i washed the column with water for three times (retention time 15 min), the tailing peak was totally disppeared. i prepared the stock solution first by dissoving 50 mg of BPA in 1 L of water and the solution was stired at 30 degree for 48 h. Then, i dilute the stock solution  to 5 , 10 , 20 ppm for the test. So it is possbile because i used the water as the solvent. 

Dear Aurea

Thanks very much for you valuable suggestion. I will try to flow water slightly acidified before analyzing the samples.