COMPARISON OF ELECTROSPRAY IONIZATION AND PLASMA DESORPTION MASS-SPECTRA OF PEPTIDES AND PROTEINS

The electrospray ionization (ESI) and plasma desorption (PD) mass spectra of over 20 peptides and proteins, with molecular weights (Mr) ranging between 1182 and 143000, have been directly compared. Both techniques produced molecular ions for the majority of materials studied; however, neither approach proved to be universally applicable. PD failed for a number of proteins that were successfully analyzed by ESI, including some of very high Mr. On the other hand, ESI failed for proteins that apparently could not acquire a sufficient number of positive charges to allow transmission through the quadrupole mass filter. A non‐covalently bound adduct, ribonuclease S, did not survive either method intact and a simple glycoprotein, ribonuclease B, did not yield the expected molecular ion with either approach. The mass measurement accuracy of quadrupole ESI is five to tenfold better than obtained with a commercial time‐of‐flight PD mass spectrometer. Furthermore, ESI's superior mass resolution (with quadrupole mass spectrometers) will prove to be particularly helpful for the characterization of mixtures of closely related materials. Sensitivity was only compared qualitatively but is highly compound dependent with both techniques. In favorable cases, ESI spectra can be obtained on low femtomolar quantities of proteins while PD typically requires several hundred femtomoles to high picomoles, depending on a number of factors including Mr. Copyright © 1990 John Wiley & Sons, Ltd.