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: CONCLUSION : The use of light propofol sedation decreased the prevalence of postoperative delirium by 50 % compared with deep sedation .
### Output Text:
CONCLUSION : The use of light <span class="chemical">propofol</span> sedation decreased the prevalence of postoperative delirium by 50 % compared with deep sedation . 