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: Analyses of a blood sample revealed unusually high plasma concentrations of metoprolol ( greater than 3000 ng / ml ) and diltiazem ( 526 ng / ml ) .
### Output Text:
Analyses of a blood sample revealed unusually high plasma concentrations of <span class="chemical">metoprolol</span> ( greater than 3000 ng / ml ) and <span class="chemical">diltiazem</span> ( 526 ng / ml ) . 