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: Intense vasospasm with threatened gangrene arose in the arm used for the infusion .
### Output Text:
Intense vasospasm with threatened gangrene arose in the arm used for the infusion . 