Abstract: A method for detecting the presence of a target material comprising a source of a beam of terahertz radiation comprising illuminating a suspected subject with THz radiation having been determined to provide sufficiently clustered PCA classification to distinguish the target materials from non-target materials. The method further provides for determining the PCA classification for target materials as being sufficiently clustered and differently placed in n-space coordinates as to permit differentiating target materials from non-target materials. Then the weighting factors along with the n-spaced coordinates can be used for subjects under interrogation.