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: Twenty common migraine patients received a one sided frontotemporal application of nitroglycerin ( 10 patients ) or placebo ointment ( 10 patients ) in a double blind study .
### Output Text:
Twenty common migraine patients received a one sided frontotemporal application of <span class="chemical">nitroglycerin</span> ( 10 patients ) or placebo ointment ( 10 patients ) in a double blind study . 