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: METHODS : The study group comprised 40 patients undergoing Sestamibi - SPECT / dobutamine stress test .
### Output Text:
METHODS : The study group comprised 40 patients undergoing <span class="chemical">Sestamibi</span> - SPECT / <span class="chemical">dobutamine</span> stress test . 