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: Using a positron emission tomography ( PET ) study it was shown recently that in migraine without aura certain areas in the brain stem were activated during the headache state , but not in the headache free interval .
### Output Text:
Using a positron emission tomography ( PET ) study it was shown recently that in migraine without aura certain areas in the brain stem were activated during the headache state , but not in the headache free interval . 