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: A Tween 80 - coated poly - L - lactid acid nanoparticles was used as a model of colloidal drug delivery system , able to trespass the BBB .
### Output Text:
A Tween 80 - coated <span class="chemical">poly - L - lactid acid</span> nanoparticles was used as a model of colloidal drug delivery system , able to trespass the BBB . 