What do you do when the person working under you brings home the same pay?

A woman was bought into my office recently to work under me who has worked at our institution for over 20 yrs and is about 3 or 4 yrs away from retirement. She had a question regarding her paystub and showed it to me. I noticed that she brings home not only the same money I do but a few dollars more. I work in the research field and handle about 80-100 studies at a time (by myself might I add) and she has about 3. All she does is makes copies for me all day while I'm here busting my A**. Everyone here depends on me for everything and I always get it done. I'm what you would call the go to girl- If they need it done right away they bring it to me- I knew I was being underpaid but this new knowledge has me absolutely livid and I want my raise. How do I approach my superiors about this?