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: Increased density and / or size and PRL content were indicated for the majority of the PRL cell population in both types of separation protocols .
### Output Text:
Increased density and / or size and PRL content were indicated for the majority of the PRL cell population in both types of separation protocols . 