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 aim of this study was to investigate , whether there is an association between adherence to guidelines ( standard operating procedures ( SOP ) ) for potentially nephrotoxic antibiotics and the occurrence of AKI .
### Output Text:
The aim of this study was to investigate , whether there is an association between adherence to guidelines ( standard operating procedures ( SOP ) ) for potentially nephrotoxic antibiotics and the occurrence of AKI . 