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: STUDY DESIGN AND METHODS : Plasma samples from before and after CPB were analyzed postoperatively for argatroban concentration using a modified ecarin clotting time ( ECT ) assay .
### Output Text:
STUDY DESIGN AND METHODS : Plasma samples from before and after CPB were analyzed postoperatively for <span class="chemical">argatroban</span> concentration using a modified ecarin clotting time ( ECT ) assay . 