Relative quantification of tau-related peptides using guanidino-labeling derivatization (GLaD) with online-LC on a hybrid ion trap (IT) time-of-flight (ToF) mass spectrometer.

Development of a quantification method based on isotopic variants of O-methyl isourea (OMIU) in conjunction with reversed-phase (RP) liquid chromatography (LC) electrospray mass spectrometry is described for determining the relative quantification of tau-related peptides Ac-VQIVXK-NH2. Extracted ion chromatograms of the mass spectrometric data derived from online microcapillary LC separation identifies the retention times of the isotopically derivatized peptides together with their ion abundances. Data-dependent MSMS analysis of both derivatized variants of the same peptide provides a complementary method for identification and resolution between isobaric species. In addition, with respect to offline LC MALDI a larger number of analogues are detected and formation of amyloid is also observed for the aspartic acid and histidine-containing peptides.