The stability of ATP is very good unless the sample contains ATP degrading enzymes as you can see in the attached paper: Arne Lundin and Annelie Elväng (2005), Overview of new analytical tools for bioluminescent biomass estimation, in Bioluminescence & Chemiluminescence (Akio Tsuji, Masakatsu Matsumoto, Masako Maeda, Larry Kricka, Philip Stanley, Eds.), World Scientific, New Yersey, pp.425-428. If you need any technical support, please don't hesitate to contact me at [email protected].