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: Cells were then treated with 30 ng / ml ritonavir or vehicle in the presence of aggregated LDL for 24 h .
### Output Text:
Cells were then treated with 30 ng / ml <span class="chemical">ritonavir</span> or vehicle in the presence of aggregated LDL for 24 h . 