Concentration and in Situ Detection of Peptides Using Liquid Matrix-Assisted Laser Desorption Ionization Matrixes

A ternary component system composed of (alpha-cyano-4-hydroxycinnamic acid/3-aminoquinoline/quinoline (CHCA/3-AQ/Q; at a weight ratio of 1:4:4) was used as an extraction solvent as well as a liquid matrix for matrix-assisted laser desorption ionization (MALDI) mass spectrometry analysis. Peptides in aqueous solutions were extracted, concentrated, and prepared for MALDI analysis in one step. Extracting peptides in aqueous solutions was analogous to dispersive liquid-liquid microextraction and completed in less than 2 min because CHCA/3-AQ/Q was dispersed rapidly into the aqueous phase by ultrasonication during extraction. The detection limit for peptides in aqueous solutions was as low as 1.25 nM for angiotensin I. Protein digests obtained from conventional MALDI analysis and the proposed method were compared with respect to sequence coverage. The new approach was applied to sample cleanup, preconcentration, and in situ analysis of protein digests in signal suppressing agents such as Tris buffer and urea.