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: The animal model used to produce infarction implies artery ligation but chemical induction can be easily obtained with isoproterenol .
### Output Text:
The animal model used to produce infarction implies artery ligation but chemical induction can be easily obtained with <span class="chemical">isoproterenol</span> . 