(a) THz absorption spectral on the basis of the different purities of moxa wool. Vertical arrows indicate absorption peaks of moxa wool. Error bars have been labeled on each data. (b) PLS prediction model of moxa wool in different purities based on terahertz absorption spectrum. Characteristic absorption peak intensities on different purities of moxa wool at (c) 4.22 THz, (d) 4.57 THz, (e) 5.09 THz, and (f) 6.28 THz.