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: As a contribution to this issue we decided to research the possible passage of mannitol into the brain after administration to 21 brain tumor patients .
### Output Text:
As a contribution to this issue we decided to research the possible passage of <span class="chemical">mannitol</span> into the brain after administration to 21 brain tumor patients . 