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: OBJECTIVES : To assess patients ' ability to repeat and recall words presented to them while undergoing procedural sedation with propofol , and correlate their recall with their level of awareness as measured by bispectral index ( BIS ) monitoring .
### Output Text:
OBJECTIVES : To assess patients ' ability to repeat and recall words presented to them while undergoing procedural sedation with <span class="chemical">propofol</span> , and correlate their recall with their level of awareness as measured by bispectral index ( BIS ) monitoring . 