In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Simultaneously , we found that these two lemon essential oil components could inhibit acetylcholinesterase activity in vitro using the Ellman method .
### Output Text:
Simultaneously , we found that these two lemon essential oil components could inhibit acetylcholinesterase activity in vitro using the Ellman method . 