We extracted 50 samples of bacterial plasmid using GeneJet plasmid Midi-Prep Kit from Applied BioSystem and we measured their concentrations then stored them at -80℃ freezer. We left them in the -80℃ for while then we measured them and we found that their concentrations has decreased from than the first time. What could be the reason and how can we overcome this problem in the next 50 samples?