life of clean pee

Hi. I am planning on substituting for a drug test (as per N2's instruction). I have been practicing to see how long I can keep the clean pee at an acceptable temperature, and I think I pretty well got it. I was just wondering though, if anyone knows if it is possible for me to collect the clean urine more than a few hours beforehand. Can I keep it in the fridge; if so, for how long before it starts to lose its natural chemical properties? Will re-warming the pee in warm water affect the composition of it in a detectable way. This drug test will be for a job that is a major career move for me- it is really important that I pass it because failure does not just mean not getting the job-there will be other reprocussions. No, quitting is not an option, it is a last resort. I tried it a few times. Sucked majorly. Take my word for it and take another hit. All help is greatly appreciated.
p.s. I am probably being overly paranoid over how closely my urine will be examined. Just trying to CMA as well as possible.