The first example of a matrix-assisted laser desorption/ionization (MALDI) process producing multiply charged mass spectra nearly identical to those observed with electrospray ionization (ESI) is presented. MALDI is noted for its ability to produce singly charged ions, but in the experiments described here multiply charged ions are produced by laser ablation of analyte incorporated into a common MALDI matrix, 2,5-dihydroxybenzoic acid, using standard solvent-based sample preparation protocols. Laser ablation is known to produce matrix clusters in MALDI provided a threshold energy is achieved. We propose that these clusters (liquid droplets) are highly charged, and under conditions that produce sufficient matrix evaporation, ions are field-evaporated from the droplets similarly to ESI. Because of the multiple charging, advanced mass spectrometers with limited mass-to-charge range can be used for protein characterization. Thus, using an Orbitrap mass spectrometer, low femtomole quantities of proteins produce full-range mass spectra at 100,000 mass resolution with <5-ppm mass accuracy and with 1-s acquisition. Furthermore, the first example of protein fragmentation using electron transfer dissociation with MALDI is presented.

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) is noted for its ability to produce primarily singly charged ions. This is an attribute when using direct ionization for complex mixtures such as protein digests or synthetic polymers. However, the ability to produce multiply charged ions, as with electrospray ionization (ESI), has advantages such as extending the mass range on mass spectrometers with limited mass-to-charge (m/z) range and enhancing fragmentation for structural characterization. We designed and fabricated a novel field free transmission geometry atmopsheric pressure (AP) MALDI source mounted to a high-mass resolution Orbitrap Exactive mass spectrometer. We report the ability to produce at will either singly charged ions or highly charged ions using a MALDI process by simply changing the matrix or the matrix preparation conditions. Mass spectra with multiply charged ions very similar to those obtained with ESI of proteins such as cytochrome c and ubiquitin are obtained with low femtomole amounts applied to the MALDI target plate and for peptides such as angiotensin I and II with application of attomole amounts. Single scan acquisitions produce sufficient ion current even from proteins.

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We report the first experimental observation of the dependence of strong-field ionization rate on the sign of the magnetic quantum number. We measure the strong-field sequential double ionization yield of argon by two time-delayed near-circularly polarized laser pulses. It is found that double-ionization yield is enhanced more than 3 times if two lasers have the opposite helicities. Analysis shows that the single ionization of both the neutral and ion prefer the same sign of the magnetic quantum number. A qualitative and intuitive model is proposed to help understand this phenomenon.