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: Plasma propofol levels were measured for 48 h following drug administration and evaluated according to a three - compartment model .
### Output Text:
Plasma <span class="chemical">propofol</span> levels were measured for 48 h following drug administration and evaluated according to a three - compartment model . 