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: Eleven patients ( six male ) with median age 47 years ( range 27 - 73 ) , median disease duration 50 months ( range 9 - 178 ) and median follow - up period of patients 13 . 8 months ( range 5 - 27 ) were enrolled in this study .
### Output Text:
Eleven patients ( six male ) with median age 47 years ( range 27 - 73 ) , median disease duration 50 months ( range 9 - 178 ) and median follow - up period of patients 13 . 8 months ( range 5 - 27 ) were enrolled in this study . 