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: Areas of secondary hyperalgesia and allodynia were quantified immediately after injection and after 15 , 30 and 60 min .
### Output Text:
Areas of secondary hyperalgesia and allodynia were quantified immediately after injection and after 15 , 30 and 60 min . 