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 , we performed a high - resolution 3D gradient echo , T1 - weighted sequences at a 1 - mm slice thickness .
### Output Text:
In addition , we performed a high - resolution 3D gradient echo , T1 - weighted sequences at a 1 - mm slice thickness . 