In fact, it is an identification of shooting residue. Shooting residue, also known as shooting object or shooting residue, is smoke composed of gunpowder particles and metal powder. When the gun fires, the bullet shoots from the muzzle or escapes from the end of the barrel. They are usually left around the inner edge and bullet hole, on the back of the shooter's hand, between his fingers, on the surface of the warhead, on the inner wall of the barrel and on the surface of related parts. The composition of shooting residue is mainly unburned and incompletely burned powder particles (containing nitrate nitrite groups) and metal powders (such as Cu, Pb, Fe, Zn, etc.). ) friction between the warhead and the barrel, and combustion products (such as Ba, Sb, etc. ) emitting powder. Generally speaking, there are two methods to detect shooting residue: physical method and chemical method. The advantage of physical evidence inspection is that it does not destroy the extracted physical evidence. The main methods are atomic absorption spectrometry, fluorescence analysis, scanning electron microscopy imaging and molecular activation analysis. There are also several chemical inspection methods. For the inspection of human hand residues, diphenylamine or diphenylbenzidine is mainly used to test nitrate. The method is to use 1% sulfuric acid solution of diphenylamine or diphenylbenzidine to drop on filter paper or paraffin film. If there is a blue reaction, it proves that there is nitrate. This is the so-called "smoke reaction"