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: Therefore , PG - 9 could represent a potential useful drug able to improve the function of impaired cognitive processes .
### Output Text:
Therefore , <span class="chemical">PG - 9</span> could represent a potential useful drug able to improve the function of impaired cognitive processes . 