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The present invention relates to a method of monitoring performance of a liquid chromatography-mass spectrometry (LC-MS) system, comprising the steps of:(a) performing LC with a first buffer, said first buffer comprising a defined concentration of a first compound, said performing comprising a step of eluting;(b) subjecting the eluate of said LC to electro-spray ionization; and (c) determining the amount of said first compound in said eluate by means of MS, thereby monitoring said performance; wherein said first compound (i) has either no affinity or negligible affinity to the chromatographic matrix; and (ii) is detectable by MS.