Impersonate tag in Web.Config

I'm using impersonate tag in my web.config in Asp.net 4.0 website.

Below is my Web.Config code:

<system.web>
    <authentication mode="Windows">
        <identity impersonate="true"                 
            userName="Administrator" 
            password="LALLA$26526"/>
     </authentication>
</system.web>

When I run app in Visual Studio I get this error:

Parser Error Message: Unrecognized element 'identity'.

Source Error:

Line 50:    <system.web>
Line 51:        <authentication mode="Windows">
Line 52:            <identity impersonate="true"             
Line 53:                 userName="Administrator"
Line 54:                 password="LALLA$26526"/>

Where am i going wrong?