BLOB fields in Access tables

I want my Delphi 4 application to be able to use Access and Interbase
databases interchangeably and I need to be able to create tables and/or
modify structures at runtime.  Everything works OK at the moment except that
the native Interbase driver will accept the SQL statement:

ALTER TABLE TABLE1 ADD SOMENOTES BLOB SUB_TYPE 1

but the native Access driver will not though it accepts CHAR(nn) and INTEGER
quite happily.  Can anybody out there help ?