adding a menu separator in wordpress main menu?

the problem is not adding the seperator but deleting it from the last menu item, as it shows in all items…

what i use is:

ul.header-nav a.nav-top-link {
  color: #000;
  opacity: 0.5;
  border-right: 1px solid #FFFFFF;
  }
ul.header-nav a.nav-top-link.last {
    border-right: none;

but i dont know why its not working, what the wrong in the code!?

thank you