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: METHODOLOGY : Serum levels of sFas ( soluble Fas ) were measured by ELISA in 24 patients with acute liver failure and 10 normal control subjects .
### Output Text:
METHODOLOGY : Serum levels of sFas ( soluble Fas ) were measured by ELISA in 24 patients with acute liver failure and 10 normal control subjects . 