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: Higher plasma ammonium levels and more rapid onset of hyperammonemia were seen in 18 patients with bacterial infections ( p = 0 . 003 and 0 . 0006 , respectively ) and in nine patients receiving high daily doses ( 2600 or 1800 mg / m2 ) of 5 - FU ( p = 0 . 0001 and < 0 . 0001 , respectively ) .
### Output Text:
Higher plasma <span class="chemical">ammonium</span> levels and more rapid onset of hyperammonemia were seen in 18 patients with bacterial infections ( p = 0 . 003 and 0 . 0006 , respectively ) and in nine patients receiving high daily doses ( 2600 or 1800 mg / m2 ) of <span class="chemical">5 - FU</span> ( p = 0 . 0001 and < 0 . 0001 , respectively ) . 