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 2 g / dL decrease in hemoglobin concentrations in patients with anemia was observed at week 2 after the start of treatment .
### Output Text:
A 2 g / dL decrease in hemoglobin concentrations in patients with anemia was observed at week 2 after the start of treatment . 