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 concentrations of morphine in these neonates was excessive ( 60 and 90 mg / ml ) .
### Output Text:
Plasma concentrations of <span class="chemical">morphine</span> in these neonates was excessive ( 60 and 90 mg / ml ) . 