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: The serum vecuronium level measured in 1 patient 14 days after the drug had been discontinued was 172 ng / mL .
### Output Text:
The serum <span class="chemical">vecuronium</span> level measured in 1 patient 14 days after the drug had been discontinued was 172 ng / mL . 