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: Serum CPK activity for E - I animals was significantly ( p less than 0 . 05 ) greater than for animals in the S - I and EC groups twenty hours following isoproterenol injection .
### Output Text:
Serum CPK activity for E - I animals was significantly ( p less than 0 . 05 ) greater than for animals in the S - I and EC groups twenty hours following <span class="chemical">isoproterenol</span> injection . 