Abstract
Liquid matrix-assisted laser desorption/ionization (MALDI) allows the generation of predominantly multiply charged ions in atmospheric pressure (AP) MALDI ion sources for mass spectrometry (MS) analysis. The charge state distribution of the generated ions and the efficiency of the ion source in generating such ions crucially depend on the desolvation regime of the MALDI plume after desorption in the AP-to-vacuum inlet. Both high temperature and a flow regime with increased residence time of the desorbed plume in the desolvation region promote the generation of multiply charged ions. Without such measures the application of an electric ion extraction field significantly increases the ion signal intensity of singly charged species while the detection of multiply charged species is less dependent on the extraction field. In general, optimization of high temperature application facilitates the predominant formation and detection of multiply charged compared to singly charged ion species. In this study an experimental set-up and optimization strategy is described for liquid AP-MALDI MS which improves the ionization efficiency of selected ion species up to 14 times. In combination with ion mobility separation, the method allows the detection of multiply charged peptide and protein ions for analyte solution concentrations as low as 2fmol/μL (0.5μL, i.e. 1fmol, deposited on the target) with very low sample consumption in the low nL-range.
Highlights
Matrix-assisted laser desorption/ionization (MALDI) [1,2] and electrospray ionization (ESI) [3] are key ionization techniques for the analysis of biomolecules by mass spectrometry (MS)
Given that the standard commercial atmospheric pressure (AP)-ESI ionization interface sufficiently desolvates and declusters electrospray plumes, initial testing was undertaken for the detection of multiply charged analytes using a liquid AP-MALDI target assembly without the ion transfer tube attached to the modified cone assembly
This study shows that MS ion source parameters play a significant role for the efficient generation and detection of multiply protonated analytes in liquid AP-MALDI MS
Summary
Matrix-assisted laser desorption/ionization (MALDI) [1,2] and electrospray ionization (ESI) [3] are key ionization techniques for the analysis of biomolecules by mass spectrometry (MS). The pulsed nature of MALDI makes it more amenable to coupling with axial time-of-flight (TOF) mass analyzers with the ionization event occurring in a high-vacuum region. ESI was developed at atmospheric pressure (AP) and like many other AP sources is coupled to mass analyzers designed for analysis of continuous ion beams, including tandem quadrupole and orthogonal TOF-based systems [3,4,5]. Ion generation at ambient pressure requires efficient ion transportation to the high-vacuum regions of the mass analyzer, the ion source operation at elevated pressures provides more facile coupling of powerful analytical separation techniques such as liquid chromatography, and within the mass spectrometer, gas-phase techniques such as tandem mass spectrometry (MS/MS) [6] and ion mobility [7] separation. It was discovered that, under certain conditions, AP-MALDI ion sources generate predominantly multiply charged ions [18,19,20], with similar abundances to those typically generated by ESI
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