AS Atomic spectrometry CA FAAS GC–FID GC–MS GFAAS Gas chromatography HS ICP-OES Journal Article Metals PCA Research Support, Non-U.S. Gov't Terpene biosynthesis Thymus VOC Volatile Organic Compounds atomic spectrometry cluster analysis flame atomic absorption spectrometry gas chromatography–flame ionization detector gas chromatography–mass spectrometry graphite furnace atomic absorption spectrometry headspace inductively coupled plasma optical emission spectrometry principal component analysis volatile organic compounds