key word bro'...."sterility". Keeping things completely sterile is the toughest part. Bacteria grows everywhere. If one of your pre-made syringes get tainted, some doc's gonna have to drain the oil and pus for ya'. Better to just prepare it when you plan to shoot it.
When doing this you need to draw with a separate needel than you will be shooting with. Draw the AS, then switch to the caped shooting needle(which obviously has remained caped) and store in a dark place. Many people do this and as long as your needle is sterile it is no more dangerous having the gear sit in the syring than a vial.