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: An electroencephalogram showed continuous , generalized irregular slowing with admixed periodic triphasic waves indicating symptomatic encephalopathy .
### Output Text:
An electroencephalogram showed continuous , generalized irregular slowing with admixed periodic triphasic waves indicating symptomatic encephalopathy . 