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: Mannitol concentrations were measured in plasma and white matter by a modified version of the enzyme assay of Blonquist et al .
### Output Text:
<span class="chemical">Mannitol</span> concentrations were measured in plasma and white matter by a modified version of the enzyme assay of Blonquist et al . 