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 purpose of this study was to screen for efficacy of a different formulation of adenosine marketed in the US , using both acute noxious stimulation and capsaicin - evoked mechanical hypersensitivity .
### Output Text:
The purpose of this study was to screen for efficacy of a different formulation of <span class="chemical">adenosine</span> marketed in the US , using both acute noxious stimulation and <span class="chemical">capsaicin</span> - evoked mechanical hypersensitivity . 