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: Consecutive patients received either immediate coronary CTA in the ED ( without serial markers ) or underwent coronary CTA after a brief observation period with serial cardiac marker measurements .
### Output Text:
Consecutive patients received either immediate coronary CTA in the ED ( without serial markers ) or underwent coronary CTA after a brief observation period with serial cardiac marker measurements . 