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: Reversal of ammonia coma in rats by L - dopa : a peripheral effect .
### Output Text:
Reversal of <span class="chemical">ammonia</span> coma in rats by <span class="chemical">L - dopa</span> : a peripheral effect . 