Narelle, the problem is that an Input Mask limits the number of Letter or Digits that you can enter. If all your values were 5 letters and then the comma and then an initial, it would be fairly easy but I assume that that isn't the case. You might want to look at Validation Text to see if that could help. Ultimately, it is probably not something you can force in just one field. You might be able to concatentate the values by using a LastName Field and then a First Initial field that limits you to 1 Character. You could do that with an Input Mask. Then put them together
LastName & "," & FirstInitial
Paul