Time domain nuclear magnetic resonance: a key complementary technique for the forensic differentiation of foam traces

Polymer foams have significant potential as contact traces for reconstructing the dynamics of felonies or for proving the presence of individuals at crime scenes. Discrimination of such items may be carried out using techniques, such as infrared spectroscopy or microscopy, which are commonly employed in the forensic examination of trace evidence. However, in the case of foams made of polyether urethane, the discriminating power attained by this approach is not satisfying. Time domain NMR (TD-NMR) allows the investigation of these materials in terms of total proton content (H%), rigid phase fraction and mobility of the soft phase (solid echo T2). Proton content and rigid fraction are partially correlated, in agreement with the chemistry of the material. The H% and T2 values are instead practically independent, and their simultaneous measurement allows discrimination of a specimen with high precision, comparable to TGA, without damaging the sample and using less than 50 mg of the material.