Quantile Normalization:

This non-parametric technique is motivated by the idea that the distribution of ion abundances is expected to be identical across measurements [37]. Median quantile normalization is performed by first sorting m/z variables in each spectrum by their abundances, then the median value of the highest abundances was substituted for the highest abundance in every spectrum, the median value of the next highest abundances was assigned to the next highest abundance in every spectrum, and so forth. Therefore, quantile normalization as a rank order-based method is thought to be more robust than intensity-based in some respects, but its limitations including discarding potentially informative abundance values and impracticability on tissue with different compositions may decrease its effectiveness [18].