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: In meningioma and metastases patients plasma concentrations of mannitol were higher than white matter concentrations except in three cases with infiltration by neoplastic cells .
### Output Text:
In meningioma and metastases patients plasma concentrations of <span class="chemical">mannitol</span> were higher than white matter concentrations except in three cases with infiltration by neoplastic cells . 