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 bronchodilator effects of a single dose of ipratropium bromide aerosol ( 36 micrograms ) and short - acting theophylline tablets ( dose titrated to produce serum levels of 10 - 20 micrograms / mL ) were compared in a double - blind , placebo - controlled crossover study in 21 patients with stable , chronic obstructive pulmonary disease .
### Output Text:
The bronchodilator effects of a single dose of <span class="chemical">ipratropium bromide</span> aerosol ( 36 micrograms ) and short - acting <span class="chemical">theophylline</span> tablets ( dose titrated to produce serum levels of 10 - 20 micrograms / mL ) were compared in a double - blind , placebo - controlled crossover study in 21 patients with stable , chronic obstructive pulmonary disease . 