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: PURPOSE : To compare the susceptibility to nephrotoxic effect of CM in patients undergoing PTRA with that of patients submitted to percutaneous coronary intervention ( PCI ) .
### Output Text:
PURPOSE : To compare the susceptibility to nephrotoxic effect of <span class="chemical">CM</span> in patients undergoing PTRA with that of patients submitted to percutaneous coronary intervention ( PCI ) . 