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: Plasma paclitaxel concentrations were measured at the completion of paclitaxel infusion and at 24 hours in 19 patients .
### Output Text:
Plasma <span class="chemical">paclitaxel</span> concentrations were measured at the completion of <span class="chemical">paclitaxel</span> infusion and at 24 hours in 19 patients . 