Described herein are computer-readable storage mediums, methods and systems useful for analyzing samples via mass spectrometry. Aspects described herein include methods for normalizing mass spectrometry data that include providing a reference set of mass spectrometry data obtained from a first external standard sample having one or more isotopic standards, wherein the reference set of mass spectrometry data comprises one or more m/z intensity ratios. Methods described herein are useful for reducing errors based on instrument response and ionization efficiencies and improve reproducibility of data from instrument to instrument and from day to day.