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: During the 14 - day run - in and during washout periods , inhaled beta - agonists were withheld and ipratropium bromide was substituted for rescue purposes .
### Output Text:
During the 14 - day run - in and during washout periods , inhaled beta - agonists were withheld and <span class="chemical">ipratropium bromide</span> was substituted for rescue purposes . 