How to create a WindowsIdentity using a username string such as "JohnDoe" in C#

Question

I have the isAdministrator(String a) as follows:

public static bool IsAdministrator(String aString)
{
    WindowsIdentity identity = new WindowsIdentity(a);
    WindowsPrincipal principal = new WindowsPrincipal(identity);
    return principal.IsInRole(WindowsBuiltInRole.Administrator);
}

Where aString is a username. When I ran this I realized I need a UPN. Is there anyway around this?

Thanks in advance!