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: There was a direct relationship between CHP concentration and patients ' ratings of pain on an analogue scale .
### Output Text:
There was a direct relationship between <span class="chemical">CHP</span> concentration and patients ' ratings of pain on an analogue scale . 