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: Mean peak forced expiratory volume in 1 second ( FEV1 ) increases over baseline and the proportion of patients attaining at least a 15 % increase in the FEV1 ( responders ) were 31 % and 90 % , respectively , for ipratropium and 17 % and 50 % , respectively , for theophylline .
### Output Text:
Mean peak forced expiratory volume in 1 second ( FEV1 ) increases over baseline and the proportion of patients attaining at least a 15 % increase in the FEV1 ( responders ) were 31 % and 90 % , respectively , for <span class="chemical">ipratropium</span> and 17 % and 50 % , respectively , for <span class="chemical">theophylline</span> . 