how to define the primary key in xml file

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ASP.NET4

Hi all my query is that i have to save the date in the xml file which i have done successfully but my record is saving without primary key so my question is how can i declare primary key is in XML file my date is saving like

<Comments>
   <Name>a</Name>
   <location>aa</location>
   <Email>abhimanyu@gmail.com</Email>
   <Description>dfedf</Description>
   <Date>10/5/2013 1:17:31 PM</Date>
 </Comments>
 <Comments>
   <Name>a</Name>
   <location>aa</location>
   <Email>abhimanyu@gmail.com</Email>
   <Description>dfedf</Description>
   <Date>10/5/2013 1:17:32 PM</Date>
 </Comments>