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: For this purpose , experimental conditions were created in which NSAIDs had previously been observed to produce effects on phasic and tonic pain by either central or peripheral mechanisms .
### Output Text:
For this purpose , experimental conditions were created in which NSAIDs had previously been observed to produce effects on phasic and tonic pain by either central or peripheral mechanisms . 