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: The interpeduncular nucleus regulates nicotine ' s effects on free - field activity .
### Output Text:
The interpeduncular nucleus regulates <span class="chemical">nicotine</span> ' s effects on free - field activity . 