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: These data are lower than those we found during the last 2 decades ( overall prolactin greater than 20 ng . / ml . in 1 . 86 % of 1 , 821 patients , prolactinomas in 7 , 0 . 38 % ) .
### Output Text:
These data are lower than those we found during the last 2 decades ( overall prolactin greater than 20 ng . / ml . in 1 . 86 % of 1 , 821 patients , prolactinomas in 7 , 0 . 38 % ) . 