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: Blood was collected from the antecubital vein four times : 60 min before and after the nitroglycerin application , and 60 and 120 min after the beginning of the migraine attack ( mean 344 and 404 min ; 12 subjects ) .
### Output Text:
Blood was collected from the antecubital vein four times : 60 min before and after the <span class="chemical">nitroglycerin</span> application , and 60 and 120 min after the beginning of the migraine attack ( mean 344 and 404 min ; 12 subjects ) . 