Positive ion mode results. A general trend can be observed: cells in Groups 1, 2, and 3 possess similar profiles of metabolites, whereas those in Group 4 largely distinguish them from the rest three groups. Two major conclusions can be drawn from these results. First, quenching and low temperature storage largely preserved metabolites in dried cells. As illustrated in Figure , the overall metabolites' profiles between Group 1 (cells were freshly quenched and dried) and Group 3 (cells were quenched, dried, and stored at -80 °C for 48 h) are nearly indistinguishable. Storage at -80 °C is an effective way to preserve metabolites in quenched, dried cells. Second, freshly dried cells generally retained cell metabolites. The overall metabolomic profiles of cells in Group 1 and Group 2 (cells were dried without quenching) are largely indistinguishable. These results indicate that rapid vacuum drying at room temperature generally preserved cell metabolites when cells were analyzed soon (e.g., within 30 min) after drying. However, metabolites in unquenched cells changed after storage. Obvious difference of overall profiles of cell metabolites can be observed when comparing the results between Group 2 and Group 4 (unquenched cells, dried and stored at -80 °C). Similarly, significantly different metabolomics profiles can be observed between Group 3 and Group 4. Trends observed in PCA plots are also reflected from results obtained from Random Forest analysis (a higher classification error indicates a lower degree of distinguishment among groups). The classification error obtained from Group 4 (0.16) is lower than those from Groups 1 (0.27), 2 (0.44), and 3 (0.33), indicating that metabolites' profile in Group 4 is more different from the other three groups sharing more similarities (Table ).