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: Vital signs were : blood pressure 120 / 67 mm Hg , heart rate 127 / min , respiratory rate 28 / min , and temperature 97 F .
### Output Text:
Vital signs were : blood pressure 120 / 67 mm Hg , heart rate 127 / min , respiratory rate 28 / min , and temperature 97 F . 