Kent J. Gillig

Peak capacity of ion mobility mass spectrometry: separation of peptides in helium buffer gas.

Advances in the field of proteomics depend upon the development of high-throughput separation methods. Ion mobility-mass spectrometry is a fast separation method (separations on the millisecond time-scale), which has potential for peptide complex mixture analysis. Possible disadvantages of this technique center around the lack of orthogonality between separation based on ion mobility and separation based on mass. In order to examine the utility of ion mobility-mass spectrometry, the peak capacity (phi) of the technique was estimated by subjecting a large dataset of peptides to linear regression analysis to determine an average trend for tryptic peptides. This trend-line, along with the deviation from a linear relationship observed for this dataset, was used to define the separation space for ion mobility-mass spectrometry. Using the maximum deviation found in the dataset (+/-11%) the peak capacity of ion mobility-mass spectrometry is approximately 2600 peptides. These results are discussed in light of other factors that may increase the peak capacity of ion mobility-mass spectrometry (i.e. multiple trends in the data resulting from multiple classes of compounds present in a sample) and current liquid chromatography approaches to complex peptide mixture analysis.

Development of a Fourier-transform ion cyclotron resonance mass spectrometer-ion mobility spectrometer

In an effort to incorporate ion-molecule reaction chemistry with ion mobility measurements we designed and constructed a novel instrument that combines a Fourier-transform ion cyclotron resonance (ICR) mass spectrometer with an ion mobility drift cell and a time-of-flight mass spectrometer. Measured mobilities for Ar+ and CO+ in helium are in excellent agreement with accepted literature values demonstrating that there are no adverse effects from the magnetic field on ion mobility measurements. Drift cell pressure, extracted from the measured mobility of Ar+ in helium, indicate that a pressure of ∼0.25 Torr is achieved in the present configuration. There are significant technological challenges associated with combining ICR and ion mobility that occurred during construction of this instrument, such as differential pumping and aperture alignment are presented.

A study of peptide-peptide interactions using MALDI ion mobility o-TOF and ESI mass spectrometry.

Matrix-assisted laser desorption ionization ion mobility coupled to orthogonal time-of-flight mass spectrometry (MALDI-IM-oTOF MS) is evaluated as a tool for studying non-covalent complex (NCX) formation between peptides. The NCX formed between dynorphin 1–7 and Mini Gastrin I is used as a model system for comparison to previous MALDI experiments (Woods, A. S.; Huestis, M. A. J. Am. Soc. Mass Spectrom.2001, 12, 88–96). The dynorphin 1–7/Mini Gastrin I complex is stable after more than a ms drift time through the He filled mobility cell. Furthermore, the effects of solution pH on NCX ion signal intensity is measured both by MALDI-IM-MS analysis and by nanoelectrospray mass spectrometry. When compared to the previous MALDI study this work shows that all three techniques give similar results. In addition, fragmentation can be observed from of the non-covalent complex parent ion that occurs prior to TOF mass analysis but after mobility separation, thus providing NCX composition information.

Analysis of protein mixtures by matrix-assisted laser desorption ionization-ion mobility-orthogonal-time-of-flight mass spectrometry

Abstract Matrix-assisted laser desorption ionization (MALDI)-ion mobility (IM)-time-of-flight (TOF) mass spectrometry (MS) has been applied to the analysis of enzymatically digested protein mixtures. The IM-TOF MS technique is rapid relative to other approaches to coupling separation methods with mass spectrometry (e.g., LC–MS, CE–MS, etc.), and MALDI-IM-TOF MS retains the advantage of reduced chemical noise which makes chromatography–mass spectrometry such a powerful analytical method. The use of IM separation prior to mass analysis also facilitates the use of internal calibration. MALDI-IM-TOF MS was evaluated by analyzing low picomole amounts of single proteins and mixtures of proteins digested with trypsin, without using time consuming “clean-up” procedures (e.g., lyophilization, dialysis, etc.). In all cases, a larger number of predicted digest fragments and higher amino acid coverages are obtained by MALDI-IM-TOF MS when compared with a conventional MALDI-TOF MS analysis. There also appears to be less signal suppression in high pressure MALDI compared to high vacuum MALDI. For example, the ratio of lysine-to-arginine terminated digest fragments appears to be higher in high-pressure MALDI relative to high-vacuum MALDI.

Investigation of the dynamics of matrix‐assisted laser desorption/ionization ion formation using an electrostatic analyzer/time‐of‐flight mass spectrometer

A hybrid electrostatic analyzer/time-of-flight mass spectrometer was used to examine the matrix-assisted laser desorption/ionization (MALDI) ion kinetic energy distributions for Na + , protonated 7-hydroxy-4-methylcoumarin matrix ions and protonated bradykinin under a variety of source region accelerating electric field conditions. Broad kinetic energy distributions are observed for the three targeted ions with the ions having both positive and negative kinetic energies relative to the full acceleration potential. The positive kinetic energy ions are shown to have flight times in good agreement with flight times calculated from first principles while the flight times of the negative kinetic energy ions diverge positively from the calculated values. Analysis of the flight time shifts for the negative kinetic energy ions allows an entrainment velocity of ∼550 m s -1 for the material to be calculated. A composite picture of the dynamics of MALDI ion formation is presented which combines promptly formed protonated matrix and analyte ions with ions formed/accelerated at delayed times in an expanding, constant velocity plume of laser desorbed material.

Optimization of a matrix-assisted laser desorption ionization-ion mobility-surface-induced dissociation-orthogonal-time-of-flight mass spectrometer: simultaneous acquisition of multiple correlated MS1 and MS2 spectra

Abstract Optimization of a matrix-assisted laser desorption ionization-ion mobility-surface-induced dissociation-o-time-of-flight mass spectrometer for peptide sequencing is discussed. Surface-induced dissociation (SID) spectra obtained by using stainless steel, Au grids, and fluorinated self-assembled monolayers (F-SAM) on Au are compared. The F-SAM surfaces yield similar fragment ions to those obtained using an adventitious hydrocarbon coated stainless steel surface; however, optimum collision energies differ for the two surfaces. The advantage of ion mobility-time-of-flight (TOF) is the ability to simultaneously acquire MS 1 and MS 2 spectra, which greatly facilitates high-throughput sequencing of peptides and mixtures of peptides resulting form the proteolytic digest of proteins. Simultaneous acquisition of ion mobility and TOF spectra introduces a time element to SID experiments that can be used as a probe of ion/surface interactions.

Experimental and Theoretical Studies of (CsI)nCs + Cluster Ions Produced by 355 nm Laser Desorption Ionization

Collision cross-sections of gas-phase (CsI)n = (1-7)Cs(+) cluster ions formed by pulsed-UV laser (355 nm) desorption ionization are measured by ion mobility-mass spectrometry. Experimental collision cross-sections are compared with calculated cross sections of candidate structures generated from a search for the lowest energy structures at the DFT/B3LYP/LACV3P** and MP2/LACVP3P** levels. The relative stabilities of these candidate structures are examined by IM-CID-MS, and the experimental results are compared to theoretical predictions. Analysis of (CsI)n = (1-7)Cs(+) cluster ion dissociation energies shows that the lower fragmentation thresholds are observed for cluster ions with the lower predicted stability.

Ion mobility-mass spectrometer interface for collisional activation of mobility separated ions

An ion mobility-mass spectrometer (IM-MS) interface is described that can be employed to perform collisional activation and/or collision-induced dissociation (CID) with good transmission of mobility separated ions to the MS analyzer. The IM-MS interface consists of a stacked-ring ion guide design, where the field strength and pressure ratio can be operated such that structural rearrangement reactions and/or CID are achieved as a function of the effective ion temperature. The ion dynamics and collisional activation processes in the IM-MS interface are described as a function of the ion-neutral collisions, ion kinetic energies, and effective ion temperature. The applicability of the IM-CID-MS methodology to studies of peptide ion fragmentation is illustrated using a series of model peptides.

Ion motion in a Fourier transform ion cyclotron resonance wire ion guide cell

Abstract Ion motion in a novel Fourier transform ion cyclotron resonance (FT-ICR) cell design is investigated. The cell design consists of a cylindrical electrostatic trap and an inner wire used to create a potential well inside the ICR cell. The ion cell is modeled as a Kingdon trap in an axial magnetic field. A theoretical description is given and advantages to using this cell (wire ion guide cell) design are discussed. Experimental results obtained using both an elongated wire-ion guide (WIG) cell (aspect ratio = 3.3) and a small WIG cell (aspect ratio = 1) to determine parameters affecting trapping performance are presented.

A novel approach to collision-induced dissociation (CID) for ion mobility-mass spectrometry experiments

Collision induced dissociation (CID) combined with matrix assisted laser desorption ionization-ion mobility-mass spectrometry (MALDI-IM-MS) is described. In this approach, peptide ions are separated on the basis of mobility in a 15 cm drift cell. Following mobility separation, the ions exit the drift cell and enter a 5 cm vacuum interface with a high field region (up to 1000 V/cm) to undergo collisional activation. Ion transmission and ion kinetic energies in the interface are theoretically evaluated accounting for the pressure gradient, interface dimensions, and electric fields. Using this CID technique, we have successfully fragmented and sequenced a number of model peptide ions as well as peptide ions obtained by a tryptic digest. This instrument configuration allows for the simultaneous determination of peptide mass, peptide-ion sequence, and collision-cross section of MALDI-generated ions, providing information critical to the identification of unknown components in complex proteomic samples.