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: A small amount of capsaicin was administered subcutaneously in the right forehead to evoke a burning painful sensation in the first division of the trigeminal nerve .
### Output Text:
A small amount of <span class="chemical">capsaicin</span> was administered subcutaneously in the right forehead to evoke a burning painful sensation in the first division of the trigeminal nerve . 