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: Eight healthy volunteers inhaled nicotine in darkness during a functional magnetic resonance imaging ( fMRI ) experiment ; eye movements were registered using video - oculography .
### Output Text:
Eight healthy volunteers inhaled <span class="chemical">nicotine</span> in darkness during a functional magnetic resonance imaging ( fMRI ) experiment ; eye movements were registered using video - oculography . 