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: We hypothesized that propofol infusion pain might be prevented by infusing remifentanil before starting the propofol infusion in a clinical setting where target - controlled infusions ( TCI ) of both drugs were used .
### Output Text:
We hypothesized that <span class="chemical">propofol</span> infusion pain might be prevented by infusing <span class="chemical">remifentanil</span> before starting the <span class="chemical">propofol</span> infusion in a clinical setting where target - controlled infusions ( TCI ) of both drugs were used . 