the % recovery is not calculated as concentrations. The procedure is simply to spike a known amount of the analyte on a piece of the same material to be sampled (“coupon”), swab it, throw the swab in a known amount of an appropriate solvent, determine the concentration of the analyte in that solution (by HPLC, TOC or whichever analytical method) and, knowing the amount of solvent used, calculate the absolute quantity of the analyte in the solution and hence in the swab. The % recovery is the :
% recovery = 100 * (quantity of analyte in the swab/quantity of analyte spiked on the coupon).
The concentration of the analyte in the solvent after dipping the swab in it (actually, the swab is not dipped because it remains in the solvent) does not change because the amount of solvent is known and does not change.