Extraction method Number of detected peaks Sample-to-sample variation Speed of the method polar Non-polar polar Non-polar Methanola ++++ Small Fast and easy Biphasic-neutral + + Small Moderate Time consuming Biphasic-acidic +++ ++ Small Moderate Time consuming Biphasic-basic ++ +++ Small Moderate Time consuming a Only one phase derived from methanol extraction solvent system ++++    the highest detected peak numbers (in this experiment) +++      the second high detected peak numbers (in this experiment) ++        the third high detected peak numbers (in this experiment) +          the fourth high detected peak numbers (in this experiment)

Table 3: Summary of solvent extraction systems for intracellular metabolites extraction.