AP Invoices without a Purchase Order Qry

If not correct community, please advise as to correct community. More a business application database question than a techinal PL/SQL question.

A rather simple concept, but I am struggling for a simple solution.

I need all A/P Invoices without a Purchase Order. I can work on fine-tuning filters, but I can not find a clean way to select the invoice and left outer join on the PO. Excerpt SQL gets close - but the distribution tables are adding a lot of noise to the information.

Again, all I need is A/P Invoices to PO Header where PO Header does not exist...that's it!