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: At the same time , the mean period within each class of amplitudes shortened by 10 - - 20 ms , whereas the mean periods calculated from all oscillations together did not change significantly .
### Output Text:
At the same time , the mean period within each class of amplitudes shortened by 10 - - 20 ms , whereas the mean periods calculated from all oscillations together did not change significantly . 