I am doing plasma sample analysis by HPLC.
Some times it happens that same sample giving higher area when it is injected after certain interval. What may be the result?

Could this be an autosampler problem? i.e. sometimes your injection volume is larger (or smaller) than in the previous injection(s). I have also see this when analysing whole blood extracts but because I have an internal standard, the analyte/int std peak area ratio remains constant and hence, this doesn't cause too much of a problem. Of course, there are other possible reasons for this but it might be worth checking the precision of the autosampler.

Best wishes

Maybe a needle wash problem: your sample was drying into the needle after certain interval, dissolved in the next sample, so concentration will increase.