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 mean ( + / - standard deviation ) time of data collection was 16 . 4 minutes ( + / - 7 . 1 ; range 5 to 34 minutes ) .
### Output Text:
The mean ( + / - standard deviation ) time of data collection was 16 . 4 minutes ( + / - 7 . 1 ; range 5 to 34 minutes ) . 