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: STUDY OBJECTIVE : To determine the effect of metoprolol on dobutamine stress testing with technetium - 99m sestamibi single - photon emission computed tomography imaging and ST - segment monitoring , and to assess the impact of intravenous glucagon on metoprolol ' s effects .
### Output Text:
STUDY OBJECTIVE : To determine the effect of <span class="chemical">metoprolol</span> on <span class="chemical">dobutamine</span> stress testing with <span class="chemical">technetium - 99m sestamibi</span> single - photon emission computed tomography imaging and ST - segment monitoring , and to assess the impact of intravenous glucagon on <span class="chemical">metoprolol</span> ' s effects . 