Broadband phase correction of FT-ICR mass spectra via simultaneous excitation and detection

In typical Fourier transform ion cyclotron resonance (FT-ICR) mass spectra, temporally dispersed excitation and the delay between excitation and detection result in continuous variation of signal phase with frequency in the detected time-domain ion signal. The complex frequency-domain spectrum of such a signal is a linear combination of absorption- and dispersion-mode spectral components with corresponding asymmetric peaks. For this reason, magnitude-mode spectral display is usually employed to yield a phase-independent uniform and symmetrical peak shape at the expense of spectral resolution. In this work, we implement simultaneous excitation and detection to enable Fourier deconvolution to recover absorption-mode spectra for both low- and high-field FT-ICR instruments. These spectra yield resolving power improvement factors approaching the maximum theoretical limit of 2.0, as well as reduction in frequency assignment errors relative to conventional magnitude-mode spectra. The Fourier deconvolution procedure has the additional benefit of correcting for spectral variation resulting from nonuniform power distribution over the excitation bandwidth and the potential benefit of providing useful diagnostic information for interpretation of experimental performance.     

Matrix-assisted laser desorption/ionization fourier transform mass spectrometry for the identification of orcokinin neuropeptides in crustaceans using metastable decay and sustained off-resonance irradiation

Vacuum UV matrix-assisted laser desorption/ionization (MALDI) Fourier transform ion cyclotron resonance mass spectrometry (FTMS) has been applied to the direct analysis of crustacean neuronal tissues using in-cell accumulation techniques to improve sensitivity. In an extension of previous work by Li and co-workers (Kutz, K. K.; Schmidt, J. J.; Li, L. Anal. Chem. 2004, 76, 5630-5640), and with a focus on the Maine lobster, Homarus americanus, we report that many peaks appearing in direct tissue spectra from crustaceans result from the metastable decay of aspartate-containing neuropeptides with localized protonation sites. We report on mass spectral characteristics of crustacean neuropeptides under MALDI-FTMS conditions and show how fragments formed by Asp-Xxx cleavages can be used to advantage for the identification of orcokinin peptides, a ubiquitous family of crustacean neuropeptides with a highly conserved N-terminus sequence. We show that predicted fragment ion fingerprints (FIFs) can be used to screen internally calibrated direct tissue spectra to provide high-confidence identification of previously identified orcokinin peptides. We use FIFs, identified based upon characteristic neutral losses, to screen for new members of the orcokinin family. Sustained off-resonance irradiation of y-series fragment ions is used to sequence the variable C-terminus. We apply these techniques to the analysis of CoG tissues from Cancer borealis and Panulirus interruptus and show that orcokinins in P. interruptus were misidentified in a previous MALDI-TOF study.     

Optical excitation of LiF:Mg,Ti following alpha and beta irradiation

It is demonstrated experimentally that optical excitation of irradiated LiF:Mg,Ti (TLD-100) by 4 eV photons has the same effect for both alpha particle (high-ionisation density) irradiation and photon/electron irradiation. In both cases, peak 5a converts to peak 4 causing peak 4 to increase following the bleach. Such an observation is consistent with the major premise of track structure theory that radiation effects following heavy changed particle (HCP)/neutron irradiation are due exclusively to the interaction of the secondary electrons created by the HCP slowing down.     

Combination resonances in the response of the duffing oscillator to a three-frequency excitation

Summary The response of a weakly nonlinear, single-degree-of-freedom system with cubic nonlinearities to multifrequency excitations is studied analytically and numerically. The method of multiple scales is used to obtain uniformly valid, approximate solutions of the governing equation for various combination resonances. The analytical and numerical solutions are in virtually perfect agreement for all cases considered, but difer markedly from the exact solution of the linearized equation of motion. The peak amplitudes in the solutions of the nonlinear equation can be several times those in the solutions of the linearized equation, and they can occur rather more often; moreover, the addition of a static load can affect the natural frequency and, hence, either tune or detune a resonance, producing profound changes in the response. The present results demonstrate that the actual response of a structure can lead to a fatigue life that is much shorter than what is predicted by linear analysis. Hence, the conventional structural engineering practice of considering the structure to be safe from resonant responses when none of the frequencies of the excitation matches the natural frequency is shown to be fraught with danger; a practicing engineer, therefore, cannot afford to be ignorant of nonlinear phenomena.     

Two-dimensional ECD FT-ICR mass spectrometry of peptides and glycopeptides

2D FT-ICR MS allows the correlation between precursor and fragment ions by modulating ion cyclotron radii for fragmentation modes with radius-dependent efficiency in the ICR cell without the need for prior ion isolation. This technique has been successfully applied to ion-molecule reactions, Collision-induced dissociation and infrared multiphoton dissociation. In this study, we used electron capture dissociation for 2D FT-ICR MS for the first time, and we recorded two-dimensional mass spectra of peptides and a mixture of glycopeptides that showed fragments that are characteristic of ECD for each of the precursor ions in the sample. We compare the sequence coverage obtained with 2D ECD FT-ICR MS with the sequence coverage obtained with ECD MS/MS and compare the sensitivities of both techniques. We demonstrate how 2D ECD FT-ICR MS can be implemented to identify peptides and glycopeptides for proteomics analysis.     

Analysis of amino acid isotopomers using FT-ICR MS

Fluxes through known metabolic pathways and the presence of novel metabolic reactions are often determined by feeding isotopically labeled substrate to an organism and then determining the isotopomer distribution in amino acids in proteins. However, commonly used techniques to measure the isotopomer distributions require derivatization prior to analysis (gas chromatography/mass spectrometry (GC/MS)) or large sample sizes (nuclear magnetic resonance (NMR) spectroscopy). Here, we demonstrate the use of Fourier transform-ion cyclotron resonance mass spectrometry with direct infusion via electrospray ionization to rapidly measure the amino acid isotopomer distribution in a biomass hydrolysate of the soil bacterium Desulfovibrio vulgaris Hildenborough. By applying high front-end resolution for the precursor ion selection followed by sustained off-resonance irradiation collision-induced dissociation, it was possible to determine exactly and unambiguously the specific locations of the labeled atoms in the amino acids, which usually requires a combination of 2-D 13C NMR spectroscopy and GC/MS. This method should be generally applicable to all biomass samples and will allow more accurate determination of metabolic fluxes with less work and less sample.     

Interrogation of N-Linked oligosaccharides using infrared multiphoton dissociation in FT-ICR mass spectrometry

The structural elucidation of oligosaccharides remains a major challenge. Mass spectrometry provides a rapid and convenient method for structural elucidation based on tandem mass spectrometry. Ions commonly are selected and subjected to collision-induced dissociation (CID) to obtain structural information. Unfortunately, N-linked oligosaccharides are relatively large compounds and are not readily fragmented using CID. In this report, we illustrate the use of infrared multiphoton dissociation (IRMPD) to obtain structural information for large N-linked oligosaccharides. The IRMPD and CID behavior of oligosaccharides were compared for high-mannose-type oligosaccharides. Fragmentation that could not be obtained through conventional CID in Fourier transform ion cyclotron resonance mass spectrometry was observed with N-linked oligosaccharides. O-Linked and N-linked glycans of similarly large sizes were compared. It was found that internal cross-ring cleavages were observed only for N-linked oligosaccharides. The mannose branch points of N-linked oligosaccharides are apparently more susceptible to cross-ring cleavages.     

The nonlinear response as a function of irradiation, storage and thermal excitation of second order thermoluminescence signal

The nonlinear response of second-order thermoluminescence (TL) glow peak, based on the peak height, has been discussed. The present work takes into account the possibility of competition between trap filling and trap emptying during irradiation and the possibility of trap emptying during storage period after the end of irradiation. By using the peak height as a measure of TL intensity, the glow peak grows supralinearly at low doses. However, an appreciable sublinear response arises at high doses due to recombination during irradiation and during storage period after irradiation. The amount of nonlinearity depends on: the relative magnitudes of the trapping parameters involved, the total doses imparted, the imparting time of the doses and the storage time after irradiation.     

Analysis of phosphate position in hexose monosaccharides using ion-molecule reactions and SORI-CID on an FT-ICR mass spectrometer

Through the use of ion-molecule reactions and SORI-CID, the phosphate position in hexose phosphate monosaccharides has been determined in the negative ion mode. Trimethyl borate was used as a reagent gas and was found to react readily with the phosphorylated hexose monosaccharides. After reaction of the reagent gas with the hexose phosphate, ion activation of the precursor by SORI-CID yielded different MS/MS spectra. Different diagnostic ions were generated for the two isomers, thus enabling differentiation and linkage position determination of the phosphate moiety.     

Characterization of isotopic abundance measurements in high resolution FT-ICR and Orbitrap mass spectra for improved confidence of metabolite identification

Currently there is limited information available on the accuracy and precision of relative isotopic abundance (RIA) measurements using high-resolution direct-infusion mass spectrometry (HR DIMS), and it is unclear if this information can benefit automated peak annotation in metabolomics. Here we characterize the accuracy of RIA measurements on the Thermo LTQ FT Ultra (resolution of 100,000-750,000) and LTQ Orbitrap (R = 100,000) mass spectrometers. This first involved reoptimizing the SIM-stitching method (Southam, A. D. Anal. Chem. 2007, 79, 4595-4602) for the LTQ FT Ultra, which achieved a ca. 3-fold sensitivity increase compared to the original method while maintaining a root-mean-squared mass error of 0.16 ppm. Using this method, we show the quality of RIA measurements is highly dependent on signal-to-noise ratio (SNR), with RIA accuracy increasing with higher SNR. Furthermore, a negative offset between the theoretical and empirically calculated numbers of carbon atoms was observed for both mass spectrometers. Increasing the resolution of the LTQ FT Ultra lowered both the sensitivity and the quality of RIA measurements. Overall, although the errors in the empirically calculated number of carbons can be large (e.g., 10 carbons), we demonstrate that RIA measurements do improve automated peak annotation, increasing the number of single empirical formula assignments by >3-fold compared to using accurate mass alone.     

Ion-molecule reactions for the characterization of polyols and polyol mixtures by ESI/FT-ICR mass spectrometry

A mass spectrometric method is described for the identification and counting of hydroxyl groups in an analyte. Analytes introduced into a FT-ICR mass spectrometer and ionized by positive mode ESI were allowed to react with the neutral reagent diethylmethoxyborane. This results in derivatization of the hydroxyl groups of the analytes by replacement of a proton with a diethylborenium ion. Protonated polyols react by consecutive derivatization reactions, wherein all, or nearly all, of the hydroxyls are derivatized. The polyol derivatization products are separated by 68 mass units in the mass spectrum. This 68 Da mass shift, along with 30 Da mass shifts arising from intramolecular derivatization of the primary derivatization products, makes it easy to count the number of functional groups present in the analyte. The utility of this method for the analysis of polyols as single-component solutions, as mixtures, or in HPLC effluent (LC-MS analysis) is demonstrated.     

Ultraviolet and infrared excitation in chromium forsterite

Abstract

A new ultraviolet excitation route for chromium forsterite lasers is investigated. Enhanced gain with simultaneous 1064 nm infrared and 266 nm ultraviolet pumping leads to a 10–20% increase in the forsterite pulse energy, compared to the 1064 nm only case. A quantum efficiency of 0·6 is inferred for ultraviolet excitation. The results have significant implications for efficiency and practicability of flashlamp-pumped and high-power systems.     

Multiple ion isolation applications in FT-ICR MS: exact-mass MSn internal calibration and purification/interrogation of protein-drug complexes

Two new applications using multiple ion isolations in the cell of a Fourier transform-ion cyclotron resonance mass spectrometer equipped with an electrospray ionization source are described. A procedure that uses multiple ion isolations of an analyte and calibrants for internal calibration at each stage in a MSn experiment, under high-resolution exact-mass conditions, for structural characterization/elucidation of angiotensin I and rapamycin is illustrated. Fragment ion mass accuracies < 1.0 ppm are demonstrated and routinely achieved. Purification of a mixture is illustrated by isolating multiple charge states of a protein-drug complex from residual protein for further MSn studies to elucidate the site of covalent drug bonding using IRMPD for a mixture of epidermal growth factor receptor (EGFr) protein and EGFr-drug complex. The procedure developed for multiple ion isolations is referred to as multi-CHEF, multiple correlated harmonic excitation fields, in which tailored waveforms are used to notch out multiple mass regions of a spectrum with minimal off-resonance excitation.     

Machines for static and sustained loading

Conclusions Testing machines for static loading have common characteristic features which are independent from machine purpose and design. Sheer machine properties can affect the test result. Some of these influence factors which are of special interest for the moment, have been mentioned and possibilities pointed out how to reduce or eliminate the influences. More details have been given on the influence of machine stiffness and its diminution by the controlled hydraulic machine, as well as on the influence of the load measuring device and the necessity of an inertialess load measurement for certain tests. New-style regulating devices and strain measuring instruments have been dealt with; the influence of the load application elements has been shown by the example of compression-, tension- and buckling-specimens, and clamping tools indicated which have been developed under consideration of the same. The problems have been illustrated by building material testing machines for compression and buckling tests, as well as tensile and universal testing machines. Creep and relaxation testers for tests at ambient temperature, under elevated temperature or in a vacuum, as well as for concrete testing, have been described in short.     

Impact of different ionization methods on the molecular assignments of asphaltenes by FT-ICR mass spectrometry

Over the years, ultrahigh resolution mass spectrometry has successfully illustrated the extreme complexity of crude oil and related solubility or polarity based fractions on a molecular level. However, the applied ionization technique greatly influences the outcome and may provide misleading information. In this work, we investigate the atmospheric pressure laser ionization (APLI) technique coupled with Fourier transform ion cyclotron resonance mass spectrometer to analyze the asphaltene fraction of a crude oil. These results were compared to data obtained by using other existing atmospheric pressure ionization methods. Furthermore elemental analysis and solid state NMR were used to obtain the bulk characteristics of the asphaltene sample. The results of the different ionization techniques were compared with the bulk properties in order to describe the potential discrimination effects of the ionization techniques that were observed. The results showed that APLI expands the range of the assigned molecules, while retaining information already observed with the generally used ion sources.     

Isotope-selective excitation of 41Ca isotope in Doppler-free two-photon continuous-wave excitation: a case study

Seven schemes are studied theoretically for Doppler-free two-photon excitation of rare (41)Ca isotope using single-mode continuous-wave lasers. The ionization efficiencies and optical selectivities for all the schemes are calculated for various powers of the excitation and ionization lasers and for various focusing conditions of the two lasers. To maximize the ionization efficiencies and the optical selectivities, wavelength-dependent Stark compensation is used. Certain laser wavelengths of the ionization step termed as magic wavelengths are identified for compensating the Stark shift induced by the excitation laser. The effects of the Stark-shift-induced asymmetry and its reversal by selecting the appropriate magic wavelength for the ionization step for various excitation and ionization laser intensities are investigated. The ionization efficiency and optical selectivity for the best scheme after Stark compensation are found to be 8.4 x 10(-4) and approximately 9 x 10(3), respectively.     

Study of double bond equivalents and the numbers of carbon and oxygen atom distribution of dissolved organic matter with negative-mode FT-ICR MS

A strong linear relationship was observed between the average double bond equivalence (DBE) and the ratio of carbon to oxygen atoms in oxygenated compounds of dissolved organic matter (DOM). Data were acquired by a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS), equipped with a negative-mode electrospray ionization source. The slope and y-intercepts extracted from the linear relationship can be used to compare DOM samples originating from different locations. Significant differences in these parameters were observed between inland riverine and offshore coastal DOM samples. Offshore coastal DOM molecules underwent a change of one DBE for each removal or addition of two oxygen atoms. This suggested the existence of multiple carboxyl groups, each of which contains a double bond and two oxygen atoms. Inland riverine samples exhibited a change of ~1.5 DBE following the addition or removal of two oxygen atoms. This extra change in DBE was attributed to cyclic structures or unsaturated chemical bonds. The DBE value with maximum relative abundance and the minimum DBE value for each class of oxygenated compounds showed that approximately two oxygen atoms contributed to a unity change in DBE. The qualitative analyses given here are in a good agreement with results obtained from analyses using orthogonal analytical techniques. This study demonstrates that DBE and the carbon number distribution, observed by high resolution mass spectrometry, can be valuable in elucidating and comparing structural features of oxygenated molecules of DOM.     

Electron irradiation and microstructure in polypyrrole

Free-standing films of polypyrrole have been electrochemically synthesized from solutions of pyrrole andp-toluene sulphonate in water. The resulting polymer was exposed to a variety of electron-beam conditions in an analytical scanning transmission electron microscope and the interaction between the electron beam and the sample was investigated by elemental X-ray spectroscopy and electron diffraction. In transmission mode, with an 80 kV beam, the films were found to be stable with respect to the production of sulphur X-rays for up to 500 live seconds. Molecular ordering was observed to be destroyed by the action of the beam. Such techniques, together with X-ray scattering, were subsequently utilized to examine spatial variations in molecular trajectory and counter-ion content; these variations were found to be associated with the nodular surface structure that characterizes polypyrrole.     

A semi-classical description of excitation in sputtering and in electron-atom collisions

The application of a simplistic, semi-classical approach for describing excitation in sputtering and in electron-atom collisions is discussed. It is shown that the approach is able to describe relative populations when Si I, Si II and Si III are formed in sputtering as well as relative populations when Rb I, Rb II and Rb III are formed in electron-atom impact. The approach also compares favorably with the Born-approximation calculations of Vainshtein et al. for Rb I, provided it is again used to describe populations in a relative (rather than absolute) sense.