I was thinking about how they have cans of compressed nitrogen for filling paint cans, and also that they sometimes will blow nitrogen in to food containers before sealing them, and that got me interested in extracting nitrogen for a nitrogen filled refrigerator. I read up on it and it appears the main way that nitrogen is extracted from the air now is via liquidification of the air and then relying on the different boiling points. I was curious why they didn't use something like a centrifuge to separate the nitrogen out. I'm guessing from what little I know of centrifuges use in nuclear refinement is that it's not energy efficient since the weights are not different enough to cause easy separation. But I was curious if that was the real reason, or if there were other forces that prevented a centrifuge from being used in this kind of gas separation.
I am not sure it would be wise to use centrifuges to separate oxygen from nitrogen - my impression this is a method of last resort - you need something like that to separate isotopes, which have virtually identical properties. You can use the difference in properties of oxygen and nitrogen to separate them, for example, in the pressure swing adsorption method (http://en.wikipedia.org/wiki/Pressure_swing_adsorption ).
To answer Your question: yes, it is possible to extract nitrogen from the air via a centrifuge, or rather - it is possible to increase the concentration of nitrogen in a gas mixture, there are always going to be some impurities. Whether anybody does it? Probably not, since it's not the most efficient way, as it was noted. In nuclear applications it is used because the requirement is to separate two isotopes of the same chemical element, and since they have exactly the same chemical properties, the only way to separate them is to make use of the mass difference between them. Nitrogen and other air components differ in chemical properties, so probably it's cheaper to use chemical methods here.