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: Prolactin exceeded 20 ng . / ml . in 5 men and was normal in 2 at repeat determination .
### Output Text:
Prolactin exceeded 20 ng . / ml . in 5 men and was normal in 2 at repeat determination . 