A: The "smoke reaction" you mentioned is actually a kind of identification of shooting residues. Shooting residue, also known as shooting object or shooting residue, is soot composed of gunpowder particles and metal powder that are ejected from the muzzle or escaped from the end of the barrel when the gun is fired. They are usually left at the inner edge and around the 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 and nitrite groups), metal powder (such as Cu, Pb, Fe, Zn, etc.) rubbed between warhead and barrel, and combustion products of firing powder (such as Ba, Sb, etc.).

The extraction methods of shooting residue are different according to the different media attached to it. For the residue on the surface of the shooting target and around the bullet hole, the main method mentioned is copying. Copying methods are divided into wet copying and dry copying. Wet copying is to spray some organic solvents, such as ethanol, acetone, carbon tetrachloride, etc., on the filter paper, then cover the surface of the residue, then cover it with a thin paper, and iron it with electricity for a few minutes, thus lifting the shooting residue onto the filter paper. Dry copying method is mainly used to extract residues from soft, thin and smooth objects such as clothes, leather, paper and textiles. The method is to put the filter paper on a clean desktop, cover the suspected residue on clothes and other places on the filter paper, and then put a piece of paper on the clothes and other places and heat them with an electric iron for a few minutes.

There are two methods to extract the residue on the back of the suspect's hand. One is dissolution method, that is, a cotton swab is soaked in 5% nitric acid solution, and then the suspect's hand is wiped to extract the residue on the cotton swab. The other is the paraffin film method, that is, the molten hot paraffin at about 60℃ is evenly covered on the back of the suspect's hand and other parts, and the paraffin film is removed after the paraffin is solidified, so that the residue is extracted to the surface of the paraffin film. This is actually the "impression test" that Poirot said in the Nile Massacre.

For the residue in the barrel, the method mentioned is to roll the filter paper into a paper tube and insert it into the barrel, then flow in 10% ammonia water, and then take it out after the ammonia water completely soaks the filter paper, so as to lift the shooting residue in the barrel onto the filter paper.

Generally speaking, there are two methods to test the shooting residue: physical and chemical methods. The advantage of physical examination is that it does not destroy the extracted material evidence. The main methods are atomic absorption spectrometry, fluorescence analysis, scanning electron microscopy and molecular activation analysis. There are also several chemical inspection methods. For the inspection of the residue on human hands, the nitrate radical is mainly detected by diphenylamine or diphenylbenzidine. The method is to drop the sulfuric acid solution of diphenylamine or diphenylbenzidine of 1% on the filter paper or paraffin film. If there is a blue reaction, it is proved that there is nitrate radical. This is the so-called "smoke reaction".

Smoke reaction has high sensitivity, but its specificity is not strong. If it is negative, bullet residue can generally be ruled out, but if it is positive, it is not certain that there must be bullet residue. Because some cigarette ash, fertilizer, some chemical reagents, legumes, urine and other substances can also make benzidine reagent positive. In view of this, the identification of bullet residues is now generally carried out by neutron activation technology and flameless spectrophotometric analysis.