Can someone help me to figure out why result of surrogate concentration i am getting is not the expected concentration. Sometimes it is doubled, trippled or sometimes is half of what is suppossed to be. All volumes are followed according to the procedure t

Do you mean the analyte concentration? And have you set up a calibration curve for the analyte(s)?

For better advise we need to know the procedure, injection volume and type of injector, sample concentration, sample solvent, detector, temperature program, etc.

What method are you doing? Are you using internal standards?