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: Intraventricular infusion of dopamine sufficient to raise the brain dopamine to the same extent did not prevent the ammonia coma nor affect the blood and brain ammonia concentrations .
### Output Text:
Intraventricular infusion of <span class="chemical">dopamine</span> sufficient to raise the brain <span class="chemical">dopamine</span> to the same extent did not prevent the <span class="chemical">ammonia</span> coma nor affect the blood and brain <span class="chemical">ammonia</span> concentrations . 