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: This difference was not related to either a different clinical risk profile or to the volume of CM administered .
### Output Text:
This difference was not related to either a different clinical risk profile or to the volume of <span class="chemical">CM</span> administered . 