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: Patients were monitored for 23 + / - 4 hours before 5 - FU infusion , and 98 + / - 9 hours during 5 - FU infusion .
### Output Text:
Patients were monitored for 23 + / - 4 hours before <span class="chemical">5 - FU</span> infusion , and 98 + / - 9 hours during <span class="chemical">5 - FU</span> infusion . 