How can i call a server side method when browser gets closed [Visual Web Part]

I have created a SharePoint 2010 visual web part. I have create a method CheckPref in usercontrol.ascx.cs. How can i call this method when the browser window is closed? If i useScriptManager PageMethods, an error "PageMethods is undefined" is thrown.

function HandleClose() {
           PageMethods.CheckPref();
    }