Sorting issue in wpf grid

Question

I need to create a grid with a list of objects as the datasource -- very basic. I also need to add 2 columns that are always fixed. So when the user sorts on one of the columns from the list of objects these 2 columns never sort. Similar to a Sort in Excel after selecting the sort range.

Example:

A B C D

L 1 16.1 12/2/2012

L 2 17.4 3/14/2011

R 3 8.2 5/15/2012

R 4 9.0 10/19/2012

Not sure how well the above grid will show up, but I need columns A and B to always remain in that order while the user sorts columns C and D. Columns C and D will sort together