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: Our results show that the most affected frequency bands were delta , theta , beta - 2 and gamma - 2 bands during the epileptiform activity and there were marked differences in terms of spectral densities between three investigated episodes ( basal activity , latent period and epileptiform activity ) .
### Output Text:
Our results show that the most affected frequency bands were delta , theta , beta - 2 and gamma - 2 bands during the epileptiform activity and there were marked differences in terms of spectral densities between three investigated episodes ( basal activity , latent period and epileptiform activity ) . 