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: RESULTS : In most glioma patients , mannitol concentrations in white matter were 2 to 6 times higher than in plasma ( mean 3 . 5 times ) .
### Output Text:
RESULTS : In most glioma patients , <span class="chemical">mannitol</span> concentrations in white matter were 2 to 6 times higher than in plasma ( mean 3 . 5 times ) . 