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: CONCLUSIONS : When limited quantities of mannitol are available , it should preferentially be given to patients at particularly high risk of nephrotoxicity .
### Output Text:
CONCLUSIONS : When limited quantities of <span class="chemical">mannitol</span> are available , it should preferentially be given to patients at particularly high risk of nephrotoxicity . 