Laserspray Ionization, a New Atmospheric Pressure MALDI Method for Producing Highly Charged Gas-phase Ions of Peptides and Proteins Directly from Solid Solutions

Abstract

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.

Laserspray ionization (LSI) ion mobility spectrometry (IMS) mass spectrometry

“…The unique feature of LSI to produce highly charged ions using direct laser ablation of a solid surface enhances fragmentation as was demonstrated by obtaining nearly complete protein sequence coverage of ubiquitin using electron-transfer dissociation (ETD) technology on a LTQ mass spectrometer [3]. Initial laserspray applications demonstrated the ability to produce singly charged lipid ions from mouse brain tissue under ambient conditions [1].…”

“…Multiply charged ions are especially beneficial for providing the ability to ionize by laser ablation larger molecules such as proteins and synthetic polymers on high-performance but mass range limited mass spectrometers such as the Orbitrap Exactive [2,3]. Full range mass spectra of bovine insulin were obtained on as little as 40 fmol when applying ion transfer capillary temperatures of ϳ350°C [3].The unique feature of LSI to produce highly charged ions using direct laser ablation of a solid surface enhances fragmentation as was demonstrated by obtaining nearly complete protein sequence coverage of ubiquitin using electron-transfer dissociation (ETD) technology on a LTQ mass spectrometer [3]. Initial laserspray applications demonstrated the ability to produce singly charged lipid ions from mouse brain tissue under ambient conditions [1].…”

Laserspray ionization using an atmospheric solids analysis probe for sample introduction

“…Previously, transmission geometry AP-MALDI experiments reported only singly charged ions [5][6][7][8]. It has been shown not to be necessary for the laser beam to pass through the glass to obtain multiply charged ions of peptides [1,9].…”

“…Mass spectra were obtained at high-resolution and using ion mobility spectrometry/MS. (J Am Soc Mass Spectrom 2010, 21, 1889 -1892) © 2010 Published by Elsevier Inc. on behalf of American Society for Mass Spectrometry T he recently introduced laserspray ionization (LSI) mass spectrometry (MS) technique has advantages similar to ESI MS in mass range extension and improved fragmentation related to multiple charging, as well as the advantages of fast direct analysis and laser ablated spatial resolution associated with matrixassisted laser desorption/ionization (MALDI) [1,2]. The LSI method has been demonstrated on atmospheric pressure ionization (API) instruments from two MS manufacturers [1][2][3][4].…”

“…The LSI method has been demonstrated on atmospheric pressure ionization (API) instruments from two MS manufacturers [1][2][3][4]. Transmission geometry was used to obtain data for the original LSI publications in the absence of any applied voltage [1][2][3]. A nitrogen laser beam strikes the backside of the matrix/analyte deposited on the opposite side of a glass microscope slide relative to the laser and near the ion entrance orifice of the mass spectrometer.…”