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 prospective , randomized , double - blind , placebo - controlled trial was performed to determine the effect - site concentration ( Ce ) of remifentanil to prevent the pain without producing complications .
### Output Text:
A prospective , randomized , double - blind , placebo - controlled trial was performed to determine the effect - site concentration ( Ce ) of <span class="chemical">remifentanil</span> to prevent the pain without producing complications . 