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: Microdialysis samples were collected preischemia , before IT injection , and at 2 , 4 , 8 , 24 , and 48 h of reperfusion ( after IT injection ) .
### Output Text:
Microdialysis samples were collected preischemia , before IT injection , and at 2 , 4 , 8 , 24 , and 48 h of reperfusion ( after IT injection ) . 