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 another series of measurements , reproducibility and reference values of the tremor was assessed in 65 healthy subjects in three sessions , at 9 a . m . , 4 p . m . and 9 a . m . , respectively , 1 week later .
### Output Text:
In another series of measurements , reproducibility and reference values of the tremor was assessed in 65 healthy subjects in three sessions , at 9 a . m . , 4 p . m . and 9 a . m . , respectively , 1 week later . 