I don't know what you mean by purifying per se--there is a difference between chelating and clarifying shampoos...
clarify: works on the surface of the hair, no deeper than the cuticle. It works to remove debris from the surface of the hair.(great for removing product build-up)
chelate: works to remove anything that has managed to "bond" to the cortex of the hair, the core of the hair. (for example chlorine damage or chemical damage can be improved by using a chelating shampoo)