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: Spectral analyses were conducted by dividing the whole spectrum into different frequency bands including delta , theta ( slow and fast ) , alpha - sigma , beta ( 1 and 2 ) and gamma ( 1 and 2 ) bands .
### Output Text:
Spectral analyses were conducted by dividing the whole spectrum into different frequency bands including delta , theta ( slow and fast ) , alpha - sigma , beta ( 1 and 2 ) and gamma ( 1 and 2 ) bands . 