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: METHODS : This was a prospective , single - intervention study of consenting adult patients undergoing procedural sedation with propofol between December 28 , 2002 , and October 31 , 2003 .
### Output Text:
METHODS : This was a prospective , single - intervention study of consenting adult patients undergoing procedural sedation with <span class="chemical">propofol</span> between December 28 , 2002 , and October 31 , 2003 . 