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: In order to clarify whether the same is true for migraine with aura , in the present study we examined the headache response to intravenous infusion of glyceryl trinitrate ( GTN ) ( 0 . 5 microg / kg / min for 20 min ) in 12 sufferers of migraine with aura .
### Output Text:
In order to clarify whether the same is true for migraine with aura , in the present study we examined the headache response to intravenous infusion of <span class="chemical">glyceryl trinitrate</span> ( <span class="chemical">GTN</span> ) ( 0 . 5 microg / kg / min for 20 min ) in 12 sufferers of migraine with aura . 