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: After extraction , a microdialysis probe was placed at the surgical site for PGE2 and thromboxane B2 ( TXB2 ) measurements .
### Output Text:
After extraction , a microdialysis probe was placed at the surgical site for <span class="chemical">PGE2</span> and <span class="chemical">thromboxane B2</span> ( <span class="chemical">TXB2</span> ) measurements . 