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: Three of the four patients without significant disease who had cTnI elevations met CK - MB criteria for MI , and the other had a peak CK - MB level of 13 ng / mL .
### Output Text:
Three of the four patients without significant disease who had cTnI elevations met CK - MB criteria for MI , and the other had a peak CK - MB level of 13 ng / mL . 