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: Theophylline concentrations at this endpoint in serum ( total ) and CSF were similar but serum ( free ) and brain concentrations were slightly different in pregnant rats .
### Output Text:
<span class="chemical">Theophylline</span> concentrations at this endpoint in serum ( total ) and CSF were similar but serum ( free ) and brain concentrations were slightly different in pregnant rats . 