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: It was found that these groups did not differ in their overall frequency of ECG abnormalities ; however , there were significant differences in the frequency of conduction defects .
### Output Text:
It was found that these groups did not differ in their overall frequency of ECG abnormalities ; however , there were significant differences in the frequency of conduction defects . 