Negative ion mode results. Compared with results from the positive ion mode, metabolites' profiles of cells from Groups 1, 2, and 3 obtained in the negative ion mode seem to have lower degrees of overlap (Figures 4B), likely due to different detection sensitivities of molecules in the negative ion mode compared with the positive ion modes. However, the same trend was observed from Random Forest analysis: the classification error obtained from Group 4 (0.03) is lower than those from Groups 1 (0.25), 2 (0.29), and 3 (0.17) (Table ). Thus, results from both ion modes indicate that quenching is indispensable to preserve cellular metabolomic integrity, even for cells to be stored under low a temperature such as -80 °C.