pass querystring value of linkbnbutton placed in gridview to iframe source

Question

I need to pass the querystring value, of linkbutton to iframe source here is the Code

 <asp:GridView ID="GridView1" runat="server" CellPadding="4" ForeColor="#333333" 
        Width="100%" AutoGenerateColumns="False" ShowHeaderWhenEmpty="True" DataKeyNames="ItemCode">
        <AlternatingRowStyle BackColor="White" />
        <EditRowStyle BackColor="#2461BF" />
        <FooterStyle BackColor="#507CD1" Font-Bold="True" ForeColor="White" />
        <HeaderStyle BackColor="#507CD1" Font-Bold="True" ForeColor="White" />
        <PagerStyle BackColor="#2461BF" ForeColor="White" HorizontalAlign="Center" />
        <RowStyle BackColor="#EFF3FB" />
        <SelectedRowStyle BackColor="#D1DDF1" Font-Bold="True" ForeColor="#333333" />
        <SortedAscendingCellStyle BackColor="#F5F7FB" />
        <SortedAscendingHeaderStyle BackColor="#6D95E1" />
        <SortedDescendingCellStyle BackColor="#E9EBEF" />
        <SortedDescendingHeaderStyle BackColor="#4870BE" />
        <Columns>
          <asp:BoundField DataField="State" HeaderText="State"/>
          <asp:BoundField DataField="City" HeaderText="City"/>
          <asp:BoundField DataField="LandlineNo1" HeaderText="Contact"/>
          <asp:BoundField DataField="Item_name" HeaderText="Name"/>
          <asp:BoundField DataField="Qty" HeaderText="qty"/>
          <asp:BoundField DataField="rate" HeaderText="Amount"/>
          <asp:BoundField DataField="DeliveryDate" HeaderText="Deliverydate"/>
          <asp:TemplateField>
            <ItemTemplate>

                <asp:LinkButton ID="lnkDetails" runat="server" Text="Release" PostBackUrl='<%# "~/popup.aspx?RowIndex=" + Container.DataItemIndex %>'></asp:LinkButton>
                  <cc1:modalpopupextender ID="mp1" runat="server" PopupControlID="Panl1" TargetControlID="lnkDetails"
    CancelControlID="Button2" BackgroundCssClass="Background">
</cc1:modalpopupextender>
            </ItemTemplate>

          </asp:TemplateField>
        </Columns>

    </asp:GridView> 
     <asp:Panel ID="Panl1" runat="server" CssClass="Popup" align="center" style = "display:none">
    <iframe style=" width: 600px; height: 400px;" id="irm1" src="popup.aspx?id=<% =request.Querystring(" id=")%>" runat="server"></iframe>
   <br/>
    <asp:Button ID="Button2" runat="server" Text="Close" />
</asp:Panel>

how to pass the value of querystring to popup.aspx page for now its going null