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: A comparison of glyceryl trinitrate with diclofenac for the treatment of primary dysmenorrhea : an open , randomized , cross - over trial .
### Output Text:
A comparison of <span class="chemical">glyceryl trinitrate</span> with <span class="chemical">diclofenac</span> for the treatment of primary dysmenorrhea : an open , randomized , cross - over trial . 