Hello Narasimha

Can you give an idea of the instrument in question, GC or LC for example and a good check is to change your vial cap. This may be a source of contaminants.

If its a GC your talking about look at your wash solvents and your GC septa.. They can introduce compounds to samples.

If you are running with a mass spec you get the masses then do a search to identify the what the compound is then you can look at what steps are needed

All in all without the instrument its difficult to give good comments.

G

I agree with Garry but one option might be that you should flush the column only with the strongest solvent in case of HPLC to get rid of the impurities of previous injections as previous injections may also leave some impurities.

Agree with the previous 2 answers. It could be "ghost peaks" from material bound to the solid phase of the column from previous injections. Another possibility is a change in method, e.g. with either a steep gradient or different pH conditions (if water is one of the mobile phases), can cause the alkyl chains to break off from its binding to silanol groups in ODS/C8/C18 type solid phase packing of a reverse phase column. It can be caused by fluid shear from mixing of 2 mobile phases or getting close the extremes of operational pH (pH 4-8) of reverse phase columns. The answer I provided is relevant to HPLC. For GC, it could be other reasons.