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: cTnI was measured with Access ( R ) ( ng / ml ) and a research immunoassay ( pg / ml ) , and compared with cTnT , CK - MB mass and CK .
### Output Text:
cTnI was measured with Access ( R ) ( ng / ml ) and a research immunoassay ( pg / ml ) , and compared with cTnT , CK - MB mass and CK . 