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: Anaesthesia with a propofol infusion and avoidance of serotonin antagonists provided a nausea - free postoperative course without exacerbation of the depression disorder .
### Output Text:
Anaesthesia with a <span class="chemical">propofol</span> infusion and avoidance of <span class="chemical">serotonin</span> antagonists provided a nausea - free postoperative course without exacerbation of the depression disorder . 