AutoNumber Feature in MS Access

In Access there is an autonumber feature that will pull up the next
sequenced 
number for use in a database.  What I am trying to do is get Access to
pullup 
a sequenced number that is Alphanumeric.  I am creating invoices and need
the 
system to pull up a new alphanumeric number to crate an individual invoice

with no duplication of number.  Is this possible in Access or does it only
do 
AutoNumber with just numbers?

Would appreciate any help possible.

Thank you.