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: When the treatment last only 2 weeks , the presence of PB did not change significantly the last parameters .
### Output Text:
When the treatment last only 2 weeks , the presence of <span class="chemical">PB</span> did not change significantly the last parameters . 