Imaging of biological tissues by desorption electrospray ionization mass spectrometry

Mass spectrometry imaging (MSI) provides untargeted molecular information with the highest specificity and spatial resolution for investigating biological tissues at the hundreds to tens of microns scale. When performed under ambient conditions, sample pre-treatment becomes unnecessary, thus simplifying the protocol while maintaining the high quality of information obtained. Desorption electrospray ionization (DESI) is a spray-based ambient MSI technique that allows for the direct sampling of surfaces in the open air, even in vivo. When used with a software-controlled sample stage, the sample is rastered underneath the DESI ionization probe, and through the time domain, m/z information is correlated with the chemical species' spatial distribution. The fidelity of the DESI-MSI output depends on the source orientation and positioning with respect to the sample surface and mass spectrometer inlet. Herein, we review how to prepare tissue sections for DESI imaging and additional experimental conditions that directly affect image quality. Specifically, we describe the protocol for the imaging of rat brain tissue sections by DESI-MSI.

omniSpect: an open MATLAB-based tool for visualization and analysis of matrix-assisted laser desorption/ionization and desorption electrospray ionization mass spectrometry images

We present omniSpect, an open source web- and MATLAB-based software tool for both desorption electrospray ionization (DESI) and matrix-assisted laser desorption ionization (MALDI) mass spectrometry imaging (MSI) that performs computationally intensive functions on a remote server. These functions include converting data from a variety of file formats into a common format easily manipulated in MATLAB, transforming time-series mass spectra into mass spectrometry images based on a probe spatial raster path, and multivariate analysis. OmniSpect provides an extensible suite of tools to meet the computational requirements needed for visualizing open and proprietary format MSI data.

Damping factor links periodic focusing and uniform field ion mobility for accurate determination of collision cross sections

The methodology for obtaining accurate ion-neutral collision cross section (Ω) values for peptides and proteins using periodic focusing ion mobility spectrometry (PF IMS) is presented. A mobility dampening factor (represented by the term α) is introduced to account for the relative increase in ion-neutral collisions in PF IMS compared to uniform field ion mobility spectrometry (UF IMS) for equivalent operating conditions. The results show that α may be easily quantified both theoretically and empirically for a specific PF IMS design operating at a given pressure based upon the charge state of the analyte. By simply incorporating an α term into traditional UF IMS expressions, accurate Ω values were obtained with excellent agreement (≤4% difference) compared to UF IMS measurements found in the current literature.

Kinetics of surface-induced dissociation of N(CH3)4(+) and N(CD3)4(+) using silicon nanoparticle assisted laser desorption/ionization and laser desorption/ionization

The implementation of surface-induced dissociation (SID) to study the fast dissociation kinetics (sub-microsecond dissociation) of peptides in a MALDI TOF instrument has been reported previously. Silicon nanoparticle assisted laser desorption/ionization (SPALDI) now allows the study of small molecule dissociation kinetics for ions formed with low initial source internal energy and without MALDI matrix interference. The dissociation kinetics of N(CH(3))(4)(+) and N(CD(3))(4)(+) were chosen for investigation because the dissociation mechanisms of N(CH(3))(4)(+) have been studied extensively, providing well-characterized systems to investigate by collision with a surface. With changes in laboratory collision energy, changes in fragmentation timescale and dominant fragment ions were observed, verifying that these ions dissociate via unimolecular decay. At lower collision energies, methyl radical (CH(3)) loss with a sub-microsecond dissociation rate is dominant, but consecutive H loss after CH(3) loss becomes dominant at higher collision energies. These observations are consistent with the known dissociation pathways. The dissociation rate of CH(3) loss from N(CH(3))(4)(+) formed by SPALDI and dissociated by an SID lab collision energy of 15 eV corresponds to log k = 8.1, a value achieved by laser desorption ionization (LDI) and SID at 5 eV. The results obtained with SPALDI SID and LDI SID confirm that (1) the dissociation follows unimolecular decay as predicted by RRKM calculations; (2) the SPALDI process deposits less initial energy than LDI, which has advantages for kinetics studies; and (3) fluorinated self-assembled monolayers convert about 18% of laboratory collision energy into internal energy. SID TOF experiments combined with SPALDI and peak shape analysis enable the measurement of dissociation rates for fast dissociation of small molecules.

Comparative performance of tripole and quadrupole ion guides at elevated pressure

This study presents the first practical demonstration of an operational tripole ion guide. The transmission was measured for both the tripole and quadrupole ion guides at 1 Torr pressure. It was found that the quadrupole provides 2.5-3 times better ion transmission efficiency. Two different electric schemes for driving the tripole were tested. Similar transmission characteristics were obtained in both cases. A brief analysis of the tripole performance and ways to improve it is presented.

Atmospheric pressure ionization permanent magnet fourier transform ion cyclotron resonance mass spectrometry

A new Fourier transform ion cyclotron resonance mass spectrometer based on a permanent magnet with an atmospheric pressure ionization source was designed and constructed. A mass resolving power (full-width-at-half-maximum) of up to 80,000 in the electron ionization mode and 25,000 in the electrospray mode was obtained. Also, a mass measurement accuracy at low-ppm level has been demonstrated for peptide mixtures in a mass range of up to 1200 m/z in the isotopically resolved mass spectra.

Submicrosecond surface-induced dissociation of peptide ions in a MALDI TOF MS

Surface-induced dissociation (SID) has been implemented in a matrix-assisted laser desorption/ionization time-of-flight mass spectrometer (MALDI TOF MS), allowing production of tandem mass spectrometric information for peptide ions (MALDI TOF SID TOF). The instrument retains the standard operational modes such as the reflectron monitoring of the MALDI-generated intact ions and postsource decay. We show through ion trajectory simulations and experimental results that implementing SID in a commercial MALDI TOF spectrometer is feasible and that the SID products in this instrument fall in an observation time frame that allows the specific detection of fast-fragmentation channels. The instrument design, pulse timing sequence, and high-voltage electronics together with SID spectra of MALDI-generated peptide ions are presented. Standard peptides such as YGGFLR, angiotensin III, fibrinopeptide A, and des-Arg1-bradykinin were dissociated by means of hyperthermal collisions with a gold surface coated with a self-assembled monolayer of 2-(perfluorodecyl)ethanethiol. With the extraction fields and the short observation times used, the spectra obtained show intense low-mass ion signals such as immonium, b2, b3, and y2 ions. TOF data analysis involved matching simulated and experimental flight times and indicates that the observed fragments are produced at approximately 250 ns after the precursor ion collides with the surface. This submicrosecond gas-phase fragmentation time frame is complementary to the observation time frame of existing SID spectrometers, which are on the order of 10 micros for tandem quadrupoles and are larger than a few milliseconds for SID implemented in Fourier transform ion cyclotron resonance spectrometers.