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: We investigated if the latter also applies to the beta - 2 adrenoceptor antagonism by metoprolol .
### Output Text:
We investigated if the latter also applies to the beta - 2 adrenoceptor antagonism by <span class="chemical">metoprolol</span> . 