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 mean duration of action was 3 . 8 hours with ipratropium and 2 . 4 hours with theophylline .
### Output Text:
The mean duration of action was 3 . 8 hours with <span class="chemical">ipratropium</span> and 2 . 4 hours with <span class="chemical">theophylline</span> . 