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: The purpose of this investigation was to determine whether the neurotoxicity of theophylline is altered in advanced pregnancy .
### Output Text:
The purpose of this investigation was to determine whether the neurotoxicity of <span class="chemical">theophylline</span> is altered in advanced pregnancy . 