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 : 14 patients seen over a 3 - year period in whom heparin - induced thrombocytopenia became apparent on delayed presentation with thromboembolic complications .
### Output Text:
PATIENTS : 14 patients seen over a 3 - year period in whom <span class="chemical">heparin</span> - induced thrombocytopenia became apparent on delayed presentation with thromboembolic complications . 