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: Treatment effect was not dependent on patient ' s age , duration or localization of PHN ( trigeminal involvement was excluded ) , sensory disturbance or pain character .
### Output Text:
Treatment effect was not dependent on patient ' s age , duration or localization of PHN ( trigeminal involvement was excluded ) , sensory disturbance or pain character . 