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: After a minimum of 1 week of treatment , serum levetiracetam concentrations were measured before and 2 , 4 , and 6 hours after drug administration , and maximum and minimum serum concentrations and elimination half - life were calculated .
### Output Text:
After a minimum of 1 week of treatment , serum <span class="chemical">levetiracetam</span> concentrations were measured before and 2 , 4 , and 6 hours after drug administration , and maximum and minimum serum concentrations and elimination half - life were calculated . 