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: Blood samples were examined the day before contrast - enhanced computed tomography ( CT ) and serially for 3 days thereafter .
### Output Text:
Blood samples were examined the day before <span class="chemical">contrast</span> - enhanced computed tomography ( CT ) and serially for 3 days thereafter . 