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: Subsequently 20 migraine patients , who developed an early onset attack with frontotemporal nitroglycerin , received the drug in a second induction test at other body areas .
### Output Text:
Subsequently 20 migraine patients , who developed an early onset attack with frontotemporal <span class="chemical">nitroglycerin</span> , received the drug in a second induction test at other body areas . 