Access 2007 - Creating Primary Keys

Hi,

Using Access 2007/Windows 7 and looking to create my first working database.

Having spoken to Anthony during my intermediate course, it feels as though the Primary Key in my initial contacts Table will be a unique identifier code relating to individuals - something like 2-4 letters at the start followed (upto say 4) digits. E.g. NOTT01 or OFF0150.

So my first question is:

Should I create the unique identifier code as a single field to contain all the necessary text and number, or would it be better to split the unique identifier into its two elementsin two fields? E.g:

CATEGORY of Person (E.g. NOTT or OFF) and NUMBER within the category

Also, if it were split into two fields, is there a way of making the number a separate AutoNumber for each different category of Person or is this a hope too far do you think?

Many thanks

Martin Parker