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: Using a single - dose therapy , peak concentration determination is not necessary , only trough samples should be monitored to ensure levels below 2 microg / ml .
### Output Text:
Using a single - dose therapy , peak concentration determination is not necessary , only trough samples should be monitored to ensure levels below 2 microg / ml . 