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: This study compared various concentrations of CHP to determine if a tolerable concentration could be identified with retention of antimicrobial efficacy .
### Output Text:
This study compared various concentrations of <span class="chemical">CHP</span> to determine if a tolerable concentration could be identified with retention of antimicrobial efficacy . 