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: Bilateral nephrectomy eliminated the beneficial effect of L - dopa on blood and brain ammonia and the ammonia coma was not prevented .
### Output Text:
Bilateral nephrectomy eliminated the beneficial effect of <span class="chemical">L - dopa</span> on blood and brain <span class="chemical">ammonia</span> and the <span class="chemical">ammonia</span> coma was not prevented . 