akhter says:

string Url = "impprnt.aspx?I_ID = " + I_ID + "";

In the URL you have passed space after I_ID as Query String name.

So you need to retrieve with space or remove space from url.

Change with below code.

string Url = "impprnt.aspx?I_ID= " + I_ID;

Then retrieve with below code.

Request.QueryString["I_ID"]

Or you can use ClientScript.RegisterStartupScript.

Refer below article.

Use target _blank with Response.Redirect in ASP.Net