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: OBJECTIVE : To describe and increase awareness of a clinical scenario in which the onset or manifestations of heparin - induced thrombocytopenia are delayed .
### Output Text:
OBJECTIVE : To describe and increase awareness of a clinical scenario in which the onset or manifestations of <span class="chemical">heparin</span> - induced thrombocytopenia are delayed . 