Sellme69:
Selorian is correct- most urine based scents have preservatives added to deter the bacteria buildup, whick is natural. They will begin to breakdown and turn in ammonia. Scents without preservatives must be refrigerated to ensure that bacteria proliferation is held to a minimum, keeping the ammonia smell from overtaking the natural urine odor.
We use freshly collected deer scents, simply because we have had more consistent results with fresh urine than the others. There are numerous online sites by deer farms that collect bottle and sell freshly collected deer urine. We get ours from www.southernwhitetailscents.com.