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: These results show that ipratropium is a more potent bronchodilator than oral theophylline in patients with chronic airflow obstruction .
### Output Text:
These results show that <span class="chemical">ipratropium</span> is a more potent bronchodilator than oral <span class="chemical">theophylline</span> in patients with chronic airflow obstruction . 