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 is probably because PPA has less beta 2 activity than does norepinephrine .
### Output Text:
This is probably because <span class="chemical">PPA</span> has less beta 2 activity than does <span class="chemical">norepinephrine</span> . 