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: OBJECTIVES : To correlate optical density and percent inhibition of a two - step heparin - induced thrombocytopenia ( HIT ) antigen assay with thrombosis ; the assay utilizes reaction inhibition characteristics of a high heparin concentration .
### Output Text:
OBJECTIVES : To correlate optical density and percent inhibition of a two - step <span class="chemical">heparin</span> - induced thrombocytopenia ( HIT ) antigen assay with thrombosis ; the assay utilizes reaction inhibition characteristics of a high <span class="chemical">heparin</span> concentration . 