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: Using the same stereotactic space limits as in the above mentioned migraine study no brain stem activation was found in the acute pain state compared to the pain free state .
### Output Text:
Using the same stereotactic space limits as in the above mentioned migraine study no brain stem activation was found in the acute pain state compared to the pain free state . 