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: In addition 3D gradient echo data sets from 10 healthy male and 10 healthy female age - matched volunteers were used to compare cerebellar volumes .
### Output Text:
In addition 3D gradient echo data sets from 10 healthy male and 10 healthy female age - matched volunteers were used to compare cerebellar volumes . 