Determination and removal of impurities in 2-D LC-MS of peptides

Problems occurring during operation of a 2-D LC-MS system for separation and identification of neuropeptides, such as contamination of the used salts and column bleed, are described. When using polysulfoethyl aspartamide, which is widely used as a strong cation exchange stationary phase in the first dimension, interfering peaks were observed in the second-dimension reversed-phase chromatograms. The observed peaks, found to be caused by column bleeding, had abundance above the threshold value and influenced the quality of the analyses. The origin of the peaks was verified and appropriate measures are proposed. Additionally, peaks caused by polyethylene glycols (PEGs), covering approximately 5 min of feasible chromatographic time in every fraction, were observed. The commercial ammonium formate salts used to prepare the first-dimension mobile phase were found to contain PEG impurities, and in subsequent work the salt solutions were prepared from formic acid and ammonia to avoid any additional contaminations.