In a GC determination of tetra-ethyl lead (TEP b), an injection which is having 1.0 :g/l of TEP b and 0.40 :g/l of nonane, used as an internal standard, gives a peak area ratio of the 3.6 (TEPb over nonane). The sample is spiked with 1.10 :g/l of nonane and injected, giving a peak ratio of 0.95 (TEPb over nonane). How can determine the TEP b in the sample?