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: Thus , the reduction in blood and brain ammonia and the prevention of ammonia coma after L - dopa , can be accounted for by the peripheral effect of dopamine on renal function rather than its central action .
### Output Text:
Thus , the reduction in blood and brain <span class="chemical">ammonia</span> and the prevention of <span class="chemical">ammonia</span> coma after <span class="chemical">L - dopa</span> , can be accounted for by the peripheral effect of <span class="chemical">dopamine</span> on renal function rather than its central action . 