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: The electrocardiograms ( ECG ) of 99 cocaine - abusing patients were compared with the ECGs of 50 schizophrenic controls .
### Output Text:
The electrocardiograms ( ECG ) of 99 <span class="chemical">cocaine</span> - abusing patients were compared with the ECGs of 50 schizophrenic controls . 