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: The fixed dose of carboplatin at levels 1 through 4 was given to achieve an area under the concentration - time curve ( AUC ) of 5 using the Calvert formula .
### Output Text:
The fixed dose of <span class="chemical">carboplatin</span> at levels 1 through 4 was given to achieve an area under the concentration - time curve ( AUC ) of 5 using the Calvert formula . 