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: Maximal CS - mediated protection was observed in experimental animals pretreated 24 h prior to the toxic insult .
### Output Text:
Maximal <span class="chemical">CS</span> - mediated protection was observed in experimental animals pretreated 24 h prior to the toxic insult . 