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: During culture , no morphological changes were found at effective concentrations of PG - 9 .
### Output Text:
During culture , no morphological changes were found at effective concentrations of <span class="chemical">PG - 9</span> . 