The first thing bomb specialists do is collect the chemical residue from the explosion site.
In the case of a large explosion, like the one at the World Trade Center, the residue can be
found all over the blast site. More often, it is only on the pieces of the bomb itself.
Next, the residue is passed through a special material. The material attracts some molecules more
strongly than other molecules. The molecules that are the least attracted separate from the
material the fastest; those that are the most attracted take the longest to separate.
In the end, all like molecules line up together. This separation process is known as adsorption.
Adsorption: An Analogy
Let's say you mix together sawdust and tiny pieces of metal, and then pour them into a pile on a
table. If you take a magnet and slowly wave it back and forth over the pile, the metal pieces will
be attracted to the magnet and will stick to it. After a few passes the pile will only contain
sawdust, and the magnet will hold all the metal. The reason this would happen is that sawdust
is not attracted by a magnet, but metal is. This gives you some idea of how two different
substances can be separated using a special material that attracts one of the substances more
strongly than the other.