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: BACKGROUND : Fibrin sealants ( FS ) derived from human plasma are frequently used in neurosurgery .
### Output Text:
BACKGROUND : Fibrin sealants ( FS ) derived from human plasma are frequently used in neurosurgery . 