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 duration of the examination and the mean ejection fraction ( EF ) were 16 . 4 + / - 6 . 1 minutes and 60 + / - 9 % , respectively .
### Output Text:
The duration of the examination and the mean ejection fraction ( EF ) were 16 . 4 + / - 6 . 1 minutes and 60 + / - 9 % , respectively . 