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: ( + / - ) - PG - 9 antinociception peaked 15 min after injection and then slowly diminished .
### Output Text:
( + / - ) - <span class="chemical">PG - 9</span> antinociception peaked 15 min after injection and then slowly diminished . 