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: To test this hypothesis we performed an experimental pain study in seven healthy volunteers , using the same positioning in the PET scanner as in the migraine patients .
### Output Text:
To test this hypothesis we performed an experimental pain study in seven healthy volunteers , using the same positioning in the PET scanner as in the migraine patients . 