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: We report 19 patients with VPA - associated encephalopathy in Germany from the years 1994 to 2003 , none of whom had been published previously .
### Output Text:
We report 19 patients with <span class="chemical">VPA</span> - associated encephalopathy in Germany from the years 1994 to 2003 , none of whom had been published previously . 