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: CONCLUSION : In a PA retention paradigm , the injection of NTG immediately after learning produced a significant impairment of long - term associative memory in mice , whereas delayed induced hypotension had no effect .
### Output Text:
CONCLUSION : In a PA retention paradigm , the injection of <span class="chemical">NTG</span> immediately after learning produced a significant impairment of long - term associative memory in mice , whereas delayed induced hypotension had no effect . 