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: Each rat was then injected with alfentanil ( ALF , 0 . 5 mg / kg sc ) .
### Output Text:
Each rat was then injected with <span class="chemical">alfentanil</span> ( <span class="chemical">ALF</span> , 0 . 5 mg / kg sc ) . 