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: EEG recordings were obtained before and 15 minutes following penicillin - G injection , and at 10th minutes following each stimulus for analysis in terms of frequency , amplitude , and power spectrum .
### Output Text:
EEG recordings were obtained before and 15 minutes following <span class="chemical">penicillin - G</span> injection , and at 10th minutes following each stimulus for analysis in terms of frequency , amplitude , and power spectrum . 