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: Of 425 studies identified , 20 were included in the meta - analysis ( 840 patients ) .
### Output Text:
Of 425 studies identified , 20 were included in the meta - analysis ( 840 patients ) . 