Cocaine is a tropane alkaloid with the formula C17H21NO4 and a molar mass of 303.36 g/mol. Its structure contains two ester groups, and those esters are its weak point. In the presence of moisture, cocaine is gradually hydrolysed to benzoylecgonine, C16H19NO4, at 289.33 g/mol.
Benzoylecgonine is also the main compound the human body produces when it breaks down cocaine, which is why it is the target in urine drug analysis. On a surface, its presence tells a different story. It shows that cocaine residue has been in place long enough, and in humid enough conditions, to begin breaking down. Measuring both compounds together gives an indication of whether residue is recent or aged.
Surface type has a strong effect on what is recovered. Hard, non-porous surfaces such as glass, stone, tiles, dashboards and toilet cisterns hold powder residue on the surface where a wipe collects it efficiently. Fabrics, carpets, upholstery and filter media trap fine particles within their fibres, which calls for a sampling approach matched to the material and the question being asked.
HVAC filters deserve special mention. Because they draw air from an entire space, filter media can act as a record of what has been circulating in a building, including in rooms that were never sampled directly.