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: The maximal NGF contents obtained by PG - 9 were 17 . 6 - fold of the control value .
### Output Text:
The maximal NGF contents obtained by <span class="chemical">PG - 9</span> were 17 . 6 - fold of the control value . 