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: Serum levels of 7 . 4 + / - 3 . 0 mumol / L ( peak ) and 4 . 0 + / - 2 . 8 mumol / L ( trough ) at the MTD were maintained during 14 days of therapy .
### Output Text:
Serum levels of 7 . 4 + / - 3 . 0 mumol / L ( peak ) and 4 . 0 + / - 2 . 8 mumol / L ( trough ) at the MTD were maintained during 14 days of therapy . 