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: In 414 patients , contrast volume , presence of diabetes mellitus , use of N - acetylcysteine , mean baseline SCr , and estimated glomerular filtration rate were comparable in the 2 groups .
### Output Text:
In 414 patients , contrast volume , presence of diabetes mellitus , use of <span class="chemical">N - acetylcysteine</span> , mean baseline SCr , and estimated glomerular filtration rate were comparable in the 2 groups . 