hi:

to a 9ml sample containing acetone , 1 ml of 10000ppm acetone is spiked into sample to give a total sample volume of 10ml.

5uL of the unspiked sample is injected into gc-fid and gives area A1 and spiked sample gives area A2. A2>A1 due to spiked acetone.

can somebody tell me how to calculate recovery for acetone?

thanks

If you inject directly, recovery is 100%.
Seriously, this is not a homework forum. Look in your analytical chemistry textbook under method of standard addtion.