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: NGF , PGE2 , PGF2alpha and PGI2 were measured using enzyme - linked immunosorbent assay and compared between the groups .
### Output Text:
NGF , <span class="chemical">PGE2</span> , <span class="chemical">PGF2alpha</span> and <span class="chemical">PGI2</span> were measured using enzyme - linked immunosorbent assay and compared between the groups . 