That's a definition issue. You need to reduce your overall body fat, and the separation will come. Continue to develop all your muscles. I like dumbbell presses, (over the head), and in from military barbell presses, for strength and size. Biceps, it doesn't matter. Work your back, and your bi's will develop and also hit your bi's and tri's. Your tri's are probably even more important for good definition in the arms.
Once I got my bf% below 10% I had pretty good definition. Now, it's even lower, and they look great.