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: Serum samples from the first and second days contained 3 . 6 and 1 . 9 mg / l carbachol , respectively .
### Output Text:
Serum samples from the first and second days contained 3 . 6 and 1 . 9 mg / l <span class="chemical">carbachol</span> , respectively . 