How to define different purchasing authority levels within a company?

[cobnutweb]

Hi all, is it possible to define "ability levels" within users assigned to a company?  

For example, I want Bob, the company manager, to be able to purchase anything from the store, to any limit.  I want Ted, a senior employee, to be able to buy anything, but only up to a certain budget limit, and I want Janice, the new intern, to only be able to create orders that have to be approved.

I've had a look around, but I can't see anything that achieves this.  The closest I can see are overall limits, which don't go far enough.

Jon