A novel workflow control system for Fourier transform ion cyclotron resonance mass spectrometry allows for unique on-the-fly data-dependent decisions

Experimental investigation of the 2D ion beam profile generated by an ESI octopole-QMS system

In this paper, we have employed an ion imaging approach to investigate the behavior of ions exiting from a quadrupole mass spectrometer (QMS) system that employs a radio frequency octopole ion guide before the QMS. An in-vacuum active pixel detector (Timepix) is employed at the exit of the QMS to image the ion patterns. The detector assembly simultaneously records the ion impact position and number of ions per pixel in every measurement frame. The transmission characteristics of the ion beam exiting the QMS are studied using this imaging detector under different operating conditions. Experimental results confirm that the ion spatial distribution exiting the QMS is heavily influenced by ion injection conditions. Furthermore, ion images from Timepix measurements of protein standards demonstrate the capability to enhance the quality of the mass spectral information and provide a detailed insight in the spatial distribution of different charge states (and hence different m/z) ions exiting the QMS.

Absorption-mode: the next generation of Fourier transform mass spectra

The Fourier transform spectrum can be presented in the absorption-mode (commonly used in FT-NMR), magnitude-mode (FT-ICR), and power-mode (engineering applications). As is routinely used in FT-NMR, it is well-known that the absorption-mode display gives a much narrower peak shape which greatly improves the spectrum; recently, the successful solution of the phase equation allowed broadband phase correction which makes it possible to apply the absorption-mode routinely in FT-ICR. With the empirical evidence provided herein, it has been confirmed that in addition to the improvement on resolving power, compared to the conventional magnitude-mode, the new absorption-mode improves the signal-to-noise ratio (S/N) of a spectrum by 1.4-fold and can improve the mass accuracy up to 2-fold with no extra cost in instrumentation. Therefore, it is worthwhile to apply and promote absorption-mode in routine FT-ICR experiments.

Vacuum compatible sample positioning device for matrix assisted laser desorption∕ionization Fourier transform ion cyclotron resonance mass spectrometry imaging

The high mass accuracy and resolving power of Fourier transform ion cyclotron resonance mass spectrometers (FT-ICR MS) make them ideal mass detectors for mass spectrometry imaging (MSI), promising to provide unmatched molecular resolution capabilities. The intrinsic low tolerance of FT-ICR MS to RF interference, however, along with typically vertical positioning of the sample, and MSI acquisition speed requirements present numerous engineering challenges in creating robotics capable of achieving the spatial resolution to match. This work discusses a two-dimensional positioning stage designed to address these issues. The stage is capable of operating in ∼1 × 10(-8) mbar vacuum. The range of motion is set to 100 mm × 100 mm to accommodate large samples, while the positioning accuracy is demonstrated to be less than 0.4 micron in both directions under vertical load over the entire range. This device was integrated into three different matrix assisted laser desorption∕ionization (MALDI) FT-ICR instruments and showed no detectable RF noise. The "oversampling" MALDI-MSI experiments, under which the sample is completely ablated at each position, followed by the target movement of the distance smaller than the laser beam, conducted on the custom-built 7T FT-ICR MS demonstrate the stability and positional accuracy of the stage robotics which delivers high spatial resolution mass spectral images at a fraction of the laser spot diameter.

Competition between covalent and noncovalent bond cleavages in dissociation of phosphopeptide-amine complexes

Interactions between quaternary amino or guanidino groups with anions are ubiquitous in nature and have been extensively studied phenomenologically. However, little is known about the binding energies in non-covalent complexes containing these functional groups. Here, we present a first study focused on quantifying such interactions using complexes of phosphorylated A(3)pXA(3)-NH(2) (X = S, T, Y) peptides with decamethonium (DCM) or diaguanidinodecane (DGD) ligands as model systems. Time- and collision energy-resolved surface-induced dissociation (SID) of the singly charged complexes was examined using a specially configured Fourier transform ion cyclotron resonance mass spectrometer (FTICR-MS). Dissociation thresholds and activation energies were obtained from RRKM modeling of the experimental data that has been described and carefully characterized in our previous studies. For systems examined in this study, covalent bond cleavages resulting in phosphate abstraction by the cationic ligand are characterized by low dissociation thresholds and relatively tight transition states. In contrast, high dissociation barriers and large positive activation entropies were obtained for cleavages of non-covalent bonds. Dissociation parameters obtained from the modeling of the experimental data are in excellent agreement with the results of density functional theory (DFT) calculations. Comparison between the experimental data and theoretical calculations indicate that phosphate abstraction by the ligand is rather localized and mainly affected by the identity of the phosphorylated side chain. The hydrogen bonding in the peptide and ligand properties play a minor role in determining the energetics and dynamics of the phosphate abstraction channel.

An external matrix-assisted laser desorption ionization source for flexible FT-ICR Mass spectrometry imaging with internal calibration on adjacent samples

We describe the construction and application of a new MALDI source for FT-ICR mass spectrometry imaging. The source includes a translational X-Y positioning stage with a 10×10 cm range of motion for analysis of large sample areas, a quadrupole for mass selection, and an external octopole ion trap with electrodes for the application of an axial potential gradient for controlled ion ejection. An off-line LC MALDI MS/MS run demonstrates the utility of the new source for data- and position-dependent experiments. A FT-ICR MS imaging experiment of a coronal rat brain section yields ∼200 unique peaks from m/z 400-1100 with corresponding mass-selected images. Mass spectra from every pixel are internally calibrated with respect to polymer calibrants collected from an adjacent slide.

Thermal activation of the co-chaperonins GroES and gp31 probed by mass spectrometry

Many biological active proteins are assembled in protein complexes. Understanding the (dis)assembly of such complexes is therefore of major interest. Here we use mass spectrometry to monitor the disassembly induced by thermal activation of the heptameric co-chaperonins GroES and gp31. We use native electrospray ionization mass spectrometry (ESI-MS) on a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer to monitor the stoichiometry of the chaperonins. A thermally controlled electrospray setup was employed to analyze conformational and stoichiometric changes of the chaperonins at varying temperature. The native ESI-MS data agreed well with data obtained from fluorescence spectroscopy as the measured thermal dissociation temperatures of the complexes were in good agreement. Furthermore, we observed that thermal denaturing of GroES and gp31 proceeds via intermediate steps of all oligomeric forms, with no evidence of a transiently stable unfolded heptamer. We also evaluated the thermal dissociation of the chaperonins in the gas phase using infrared multiphoton dissociation (IRMPD) for thermal activation. Using gas-phase activation the smaller (2-4) oligomers were not detected, only down to the pentamer, whereafter the complex seemed to dissociate completely. These results demonstrate clearly that conformational changes of GroES and gp31 due to heating in solution and in the gas phase are significantly different.