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: Concentrations of amphotericin B in plasma were not significantly different among the three groups at any time during the study .
### Output Text:
Concentrations of <span class="chemical">amphotericin B</span> in plasma were not significantly different among the three groups at any time during the study . 