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: BACKGROUND : In 2 separate centers , we observed a notable increase in the incidence of postoperative convulsive seizures from 1 . 3 % to 3 . 8 % in patients having undergone major cardiac surgical procedures .
### Output Text:
BACKGROUND : In 2 separate centers , we observed a notable increase in the incidence of postoperative convulsive seizures from 1 . 3 % to 3 . 8 % in patients having undergone major cardiac surgical procedures . 