I think you cant use  KH2PO4 with ELSD.

Dear Dr. Teodoro,

Thanks for your answer. What can be the other best choice?

1. Are your samples in the same matrix? 

2. Are you ensuring you have enough buffering capacity in your mobile phase to avoid pH changes during your runs? (Assuming you're doing RPC).

You can use ammonium acetate

If you want get consistent and precise data. You just follow the following points-

1. mobile should simple and should have evaporative in nature,

2. Do not use in-organic buffers solution.

3. pH plays major role on the peak  elution, i.e. retention time.

4.Use the buffer solutions of TFA, acetate, propionate buffer, ammonium acetate and formic acid. 

Hi, you can not use nonvolatile modifiers in your mobile phase because the detector you used is ESLD. KH2PO4 will leave a significant amount of residue in the system after evaporation which will definitely effect your results. Try other volatile modifiers, such as Trifluoroacetic Acid and Acetic Acid.

Salam, my friend. excuse me are you sure from using ESLD. I thought you used UV detector

In addition to the pH control, I think you should also set a constant temperature of the column using an oven.

Hi , try also to monitor your pressure if it is constant. Mobile phase must always be constant. Try also to run the blank between the samples to see if there is a carry over.

Dear Shaukat,

Please see : http://www.chromacademy.com/images/Ishikawa-diagram-variable-peak-height-area.pdf

You could download the HPLC Troubleshooting Guide, and See : http://www.chromacademy.com/chromatography-troubleshooting-HPLC-variable-peak-height.html

The first factor that should control is leaking sample

valve. Many factors should be checked .

Thank you all for providing helpful knowledge.

ELSD is usually used for high-boiling, non-volatile samples.  When you use KHPO4 as mobile phase modifier, you will get a constant stream of non-volatile salt as the background. Are you sure this is even a suitable condition for piperazine and diethanol amine?  Are you trying to replicate literature condition or develop new conditions?

Sometimes, gradual development of small but significant leak of the eluent (which may not be in the form of physical droplets) in the system may also cause this kind of retention time shift.