Analytical methods for detecting and quantifying caffeine include GC-MS-SIM (gas chromatography d mass spectrometry d selected-ion monitoring), GC-MS-MS-MRM (multi-reaction monitoring), SPE-LCeMS/MS (solid phase extraction coupled to liquid chromatography and tandem mass spectrometry), and their detection levels range from 1.0 to 20.0 ng/l in final solution