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: However , urinary PGI2 was not different between controls and patients with OAB .
### Output Text:
However , urinary <span class="chemical">PGI2</span> was not different between controls and patients with OAB . 