hi yes you may notice there are several tiny waves before it reached the peak , and those we usually definited it as impurities,and you can also check the form , at the time 18.813s /19.551/20.894/21.100 23.042 /23.885, there are tiny waves, but when it reach the peak ,the proportion is the largest,which is the content purity of the sample ,i am not very good at math , but there is a formula you can calulate the HPLC purity result, you may can search about it if you are interested in this part.thanks all !