Principal component analysis based on terahertz spectra. (a) Principal component scores for 50 samples. Black circles, Panax quinquefolium samples; blue squares, ginseng from the family Araliaceae; red triangles, non-Araliaceae herbs; gray rhombi, other substances. (b) Principal component scores for P. quinquefolium from different growing regions. Red pentagons, seven batches of P. quinquefolium produced in North America; blue pentacles, seven batches of P. quinquefolium produced in China (Jilin).