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: The percentage of HIT antibody - positive patients was 0 . 5 % preoperatively , 5 . 6 % on POD 7 , and 5 . 6 % on POD 14 .
### Output Text:
The percentage of HIT antibody - positive patients was 0 . 5 % preoperatively , 5 . 6 % on POD 7 , and 5 . 6 % on POD 14 . 