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: ( 8 of 12 compared to only 9 of 22 cases with prolactin between 20 and 35 ng . / ml . ) .
### Output Text:
( 8 of 12 compared to only 9 of 22 cases with prolactin between 20 and 35 ng . / ml . ) . 