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: The marked vasodilator and negative inotropic effects of propofol are disadvantages in frail elderly patients .
### Output Text:
The marked vasodilator and negative inotropic effects of <span class="chemical">propofol</span> are disadvantages in frail elderly patients . 