How do I get a xml-named datagrid-column

Im trying to select a value from a specific column in a datagrid. That column is defined in a xml-file
firstColumn=dgXML.@firstColumn; // Column7

When I click an item in the datagrid, I want to select the content of that column.
e.target.selectedItem.firstColumn;
but it does not work even though everything works perfectedly, when referring the correct column like this.
e.target.selectedItem.Column7;
firstColumn has been set as String and is correctly traced just above
trace(firstColumn); // Column7
e.target.selectedItem.firstColumn; // undefined

What could be wrong