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: In the 7 - hour period after terbutaline administration , plasma samples were taken for determination of plasma potassium levels and drug concentrations .
### Output Text:
In the 7 - hour period after <span class="chemical">terbutaline</span> administration , plasma samples were taken for determination of plasma <span class="chemical">potassium</span> levels and drug concentrations . 