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: Plasma ammonium level ranged from 248 to 2387 microg % ( mean : 626 + / - 431 microg % ) .
### Output Text:
Plasma <span class="chemical">ammonium</span> level ranged from 248 to 2387 microg % ( mean : 626 + / - 431 microg % ) . 