Particle Combination Analysis or PCA is fundamentally different from traditional trace evidence that is used in police work and that most people are familiar with. In fact, other than that both involve things smaller than people usually examine with a naked eye, they have little in common. There are three differences in particular:
Size - particles are much smaller. Dozens or more would fit on a cross section of traditional trace evidence items such as hairs or fibers.
Number and Variation - Not only are there hundreds or thousands of particles on virtually any object of interest, there are hundreds or thousands of different types of particles. This means there are hundreds of times as many to be found and, they are much more likely to occur on any piece of evidence.
Process for making inferences - Unlike traditional trace evidence analysis which involves a process of searching for matches with single particle types, PCA involves making inferences from combinations of particles. These combinations can either be matched with known sets of combinations, or inferences can be made from the character and make-up of the combination itself.