Direct characterization of intact polypeptides by matrix‐assisted laser desorption electrospray ionization quadrupole Fourier transform ion cyclotron resonance mass spectrometry

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Abstract
We report the characterization of a recently introduced hybrid ionization source, matrix‐assisted laser desorption electrospray ionization (MALDESI), coupled to a quadrupole Fourier transform ion cyclotron resonance mass spectrometry (QFT‐ICR‐MS) system. We first demonstrate the ability of MALDESI‐QFT‐ICR MS to directly analyze and provide high mass measurement accuracy (∼1 part‐per‐million) of a polypeptide using internal calibration. Second, we show the potential of MALDESI‐QFT‐ICR MS for the top‐down characterization of multiply charged polypeptide cations. Finally, we demonstrate sub‐femtomole detection limits in MALDESI‐QFT‐ICR MS using a combination of naturally occurring peptides and their respective stable isotope labeled forms. The results presented herein demonstrate the feasibility of several potential applications for MALDESI‐QFT‐ICR MS for the direct analysis of intact biological molecules. Copyright © 2007 John Wiley & Sons, Ltd.