If a person has contact with a metal object for a sufficient amount of time, metal ions may transfer from the object to the skin (or clothing).  These ions will react with the Trace Metal Detection (TMDT) solution and fluoresce under ultraviolet (UV) light.  The color of the fluorescence can indicate the type of metal contacted, and the pattern observed can indicate the general shape of the metal object.  This technique is most often used in corroboration of an apparent self-inflicted gunshot wound.  Other scenarios may produce contact with metal objects in which this technique may also be useful.

The image above shows the different color reactions produced from different metals contacting the skin followed by treatment with TMDT solution and viewing under UV light.  Below is an example of the reaction that can be obtained after holding a galvanized aluminum pipe.