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: Blood samples for the analysis of plasma potassium , terbutaline , metoprolol ( racemic , R - and S - isomer ) , and alpha - hydroxymetoprolol concentrations were taken at regular time intervals , during 8 hr after metoprolol .
### Output Text:
Blood samples for the analysis of plasma <span class="chemical">potassium</span> , <span class="chemical">terbutaline</span> , <span class="chemical">metoprolol</span> ( racemic , R - and S - isomer ) , and <span class="chemical">alpha - hydroxymetoprolol</span> concentrations were taken at regular time intervals , during 8 hr after <span class="chemical">metoprolol</span> . 