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: METHODS : We conducted a retrospective review of 70 consecutive microvascular decompression operations and studied those patients who received topical papaverine for vasospasm .
### Output Text:
METHODS : We conducted a retrospective review of 70 consecutive microvascular decompression operations and studied those patients who received topical <span class="chemical">papaverine</span> for vasospasm . 