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: Sodium retention occurred on days 2 , 3 and 6 after PAN injection .
### Output Text:
<span class="chemical">Sodium</span> retention occurred on days 2 , 3 and 6 after <span class="chemical">PAN</span> injection . 