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: OBJECTIVE : We wanted to test whether assessment of both a central pain - related signal ( chemo - somatosensory evoked potential , CSSEP ) and a concomitantly recorded peripheral signal ( negative mucosal potential , NMP ) allows for separation of central and peripheral effects of NSAIDs .
### Output Text:
OBJECTIVE : We wanted to test whether assessment of both a central pain - related signal ( chemo - somatosensory evoked potential , CSSEP ) and a concomitantly recorded peripheral signal ( negative mucosal potential , NMP ) allows for separation of central and peripheral effects of NSAIDs . 