2.2 Odorants preparation
This experiment used three odorants, namely tetrahydrothiophene, 2-methylpyrazine, and trimethylindole, purchased from Aladdin Company (Shanghai, China). Appropriate amounts of the above three odorants were added to a felt-tip pen (Burgart Messtechnik, Wedel, Germany). Regarding the safety of the odorants: first, after adding an appropriate amount of the three odorants, the pen can be used for 6 months without adding odorants again during the period. Also, the amounts of tetrahydrothiophene, trimethylindole, and 2-methylpyrazine used in this experiment were 1 mg, 2 mg, and 300 mg, respectively. A-fter animal experiments and literature review, the damage of these three odorants to the human body can be ignored19-21.