Peak Sholder appears frequently

[fatma]

Iam working with MP ACN:H2O:TEA (20:80:0.4) pH=3 on RP-C18 column with flow rate 1.00 ml/min. I use this system in quantification of a drug containing amino group in plasma samples
first, the peak is good but with tailing. The next day i found a peak shoulder in all samples even i made enough conditioning for the column and this shoulder hinders me in getting good linear results...
if TEA is the cause of this shoulder?...is it preciptating on the column... I need a help to improve my peak shape and to eliminate the shoulder,...I think that the diluent is not the reason, because the peak appears well once when it is prepared in water or ACN alone...
so I need your advice....

[enashendy]

In the next day, I found the peak shape becomes good with little tailing...
i.e. the column is working well.. but why a shoulder appears in other times...I want to know a possible cause...
I appreciate your help.......

[Uwe Neue]

If a peak shape changes significantly from day to day, it is likely that there is a problem with the sample make-up, and that there has been a difference between different days.
Another possibility is column equilibration with the mobile phase.