Want to save text field as XMl in list(sharepoint)

Question

HI.. I have set for Settings Field (SPField) as "Text".In the below code I have used InnerText.So its saving node values in List.But when I use "InnerXml".It shows it cannot write it as Xml.. I want node values as xml in list.

SPListItem spItem = splistItemCollection[0]; 
spItem["Settings"] = toolSettingsDoc.InnerText.ToString(); 

 sprootweb.AllowUnsafeUpdates = true; 
 spItem.SystemUpdate();
 sprootweb.AllowUnsafeUpdates = false;