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: Terbutaline , a beta2 - adrenoceptor agonist used to arrest preterm labor , has been associated with increased concordance for autism in dizygotic twins .
### Output Text:
<span class="chemical">Terbutaline</span> , a beta2 - adrenoceptor agonist used to arrest preterm labor , has been associated with increased concordance for autism in dizygotic twins . 