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: Incidence of heparin - induced thrombocytopenia type II and postoperative recovery of platelet count in liver graft recipients : a retrospective cohort analysis .
### Output Text:
Incidence of <span class="chemical">heparin</span> - induced thrombocytopenia type II and postoperative recovery of platelet count in liver graft recipients : a retrospective cohort analysis . 