We have externalized some composition analysis of a thin silk polymer membrane that we produce in the lab. To solubilize the polymer we use a solution based on formic acid; Since formic acid is a very volatile molecule, once the membrane solidifies, I imagined there were only residual traces of it. The first analysis with proton NMR instead revealed that the formic acid forms an ester with the polymer or other components of the material, remaining partly inside the membrane. The method quantified the different analytes normalizing them on the data of a known standard (dymethil sulfone). The concentration of the analytes was expressed in wt% (using the weight of the initial membrane analyzed as a reference). Initial results indicate that the formic acid in the membrane should be approximately 35 wt%. This value seems enormous, considering that, if so, on a membrane weighing 100 mg the formic acid would weigh 35. The membrane is completely dry (moisture content