Surface-induced dissociation by Fourier transform mass spectrometry

A detailed procedure for performing surface-induced dissociation (SID) of ions in a dual-cell Fourier transform mass spectrometer is described. It is shown that the technique is applicable to both electron ionization and laser desorption measurements. SID spectra of perfluorotri-n-butylamine, anthracene, (5,10,15,20-tetraphenyl-21H,23H-prophinato)-iron(III) chloride, and [5,10,15,20-tetrakis(2,6-dibromo-phenyl)-21H,23H-prophina to]iron(III) chloride are presented. Conversion efficiencies of molecular ions between 1% and 30% are obtained. It is concluded the method holds promise for dissociation of high mass laser-desorbed ions.     

Attomole peptide analysis by high-pressure matrix-assisted laser desorption/ionization Fourier transform mass spectrometry

A new high-pressure matrix-assisted laser desorption/ionization (HP-MALDI) source for FTMS has recently been described (O'Connor et al. J. Am. Soc. Mass Spectrom., in press). Improvements to the source design, including the incorporation of a new high-pressure gas channel plate, resulted in ions devoid of metastable fragmentation and also in increased sensitivity compared to the HP-MALDI prototype source design. The focus of this contribution is the evaluation of the current HP-MALDI FTMS configuration. The use of nonconductive sample surfaces, such as Parafilm and Teflon, was explored, and spectra from 30 amol of peptide applied to these surfaces were routinely obtained. In addition, the current limit of detection for this configuration is demonstrated to be 300 zmol for the phosphopeptide RRREEE(pS)EEEAA using multishot accumulation of the ions from 15 laser shots in the hexapole and 1 scan. In addition, the performance of the new HP-MALDI FTMS configuration and its potential application for high-throughput proteomics analyses are discussed.     

Accurate mass measurements for peptide and protein mixtures by using matrix-assisted laser desorption/ionization Fourier transform mass spectrometry

A new analytical scheme based on a combination of scanning FTMS, multiple-ion filling, and potential ramping methods has been developed for accurate molecular mass measurement of peptide and protein mixtures using broadband MALDI-FTMS. The scanning FTMS method alleviates the problems of time-of-flight effect for FTMS with an external MALDI ion source and provides a systematic means of sampling ions of different mass-to-charge ratios. The multiple-ion filling method is an effective way of trapping and retaining ions from successive ion generation/accumulation events. The potential ramping method allows the use of high trapping potentials for effective trapping of ions of high kinetic energies and the use of low trapping potentials for high-resolution detection of the trapped ions. With this analytical scheme, high-resolution broadband MALDI mass spectra covering a wide mass range of 1000-5700 Da were obtained. For peptide mixtures of mass range 1000-3500 Da, calibration errors of low part-per-millions were demonstrated using a parabolic calibration equation f2 = ML1/m2 + ML2/m + ML3, where f is the measured cyclotron frequency and ML1, ML2, and ML3 are calibration constants.     

Microchannel plate for surface-induced dissociation in mass spectrometry

A new method for surface-induced dissociation of molecular ions, applied to tandem mass spectrometry, is achieved by collisions at a grazing angle on the inside channel surfaces of a microchannel plate. This technique, termed microchannel SID, is demonstrated by using both positive and negative parent ions in the energy range of 500-2000 eV. Fragmentation spectra of the pentapeptide leucine-enkephalin (555 daltons) at 500 eV show good sequence information with a net fragmentation efficiency of 14%. High mass fragmentation is demonstrated on (CsI)23Cs+ (6113 daltons), with the resultant spectrum showing all cluster fragments from n = 0 to 23.     

Identification and structural elucidation of lectin-binding oligosaccharides by bioaffinity matrix-assisted laser desorption/ionization Fourier transform mass spectrometry

Cortical granule lectin (CGL) is released by the egg of the South African toad Xenopus laevis upon fertilization. The lectin binds to oligosaccharides in the extracellular matrix of the egg to form a physical block to prevent additional sperm penetration or polyspermy. To identify the oligosaccharides that bind to CGL, the lectin was immobilized on the surface of a matrix-assisted laser desorption/ionization probe. This bioaffinity probe was used to determine oligosaccharides that bind preferentially to CGL. Structural analyses based on collision-induced dissociation was used to determine that oligosaccharides with the sulfate esters at the nonreducing ends preferentially bind to the lectin.     

Microscopic laser desorption/postionization fourier transform mass spectrometry

A microscopic laser desorption/postionization Fourier transform mass spectrometer (LD/FTMS) is described. The lateral resolution can be <1 μm with the inherent FTMS high mass resolution intact. Laser postionization allows a certain selectivity and an increase in sensitivity. This capability should allow materials characterization in a wide variety of cases. We demonstrate microscopic desorption and postionization for atoms and molecules.     

Infrared laser isolation of ions in Fourier transform mass spectrometry

A new method for isolating ions for tandem mass spectrometry analyses in Fourier transform mass spectrometry is illustrated. The method employs an infrared laser to dissociate completely the undesired ions. The selected ions are excited to an orbit away from the degradative portion of the laser beam. Ion isolation was accomplished and tandem mass spectrometry experiments were performed on model oligosaccharides and compounds from biological samples.     

Analysis and quantitation of fructooligosaccharides using matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry

Inulin is a class of fructooligosaccharide (FOS) derived from plants, which is often used as a natural food ingredient. Inulin is currently used as an additive in baked goods, dairy products, infant formula, and dietary supplements as a result of its purported health-promoting properties. The growth of health-promoting lactobacilli and bifidobacteria is supported by FOS, giving it the classification of a prebiotic; however, its ability to selectivity stimulate only beneficial bacteria has not been demonstrated. In order to better understand the role of inulin and FOS as prebiotics, matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry has been used for qualitative and quantitative analysis on bacterial growth. A method using an internal standard has been developed to quantify the consumption of FOS by Bifidobacterium longum bv. infantis using a calibration curve. Due to the differential consumption of FOS, the calibration curve was modified to include intensity components for each polymer unit in order to achieve more accurate quantitation. The method described was designed to be more rapid, precise, and robust for quantitative analysis when compared to existing methods.     

Advantages of external accumulation for electron capture dissociation in Fourier transform mass spectrometry

A combination of external accumulation (XA) with electron capture dissociation (ECD) improves the electron capture efficiency, shortens the analysis time, and allows for rapid integration of multiple scans in Fourier transform mass spectrometry. This improves the signal-to-noise ratio and increases the number of detected products, including structurally important MS3 fragments. With XA-ECD, the range of the labile species amenable to ECD is significantly extended. Examples include the first-time determination of the positions of six GalNAc groups in a 60-residue peptide, five sialic acid and six O-linked GalNAc groups in a 25-residue peptide, and the sulfate group position in a 11-residue peptide. Even weakly bound supramolecular aggregates, including nonspecific peptide complexes, can be analyzed with XA-ECD. Preliminary results are reported on high-rate XA-ECD that uses an indirectly heated dispenser cathode as an electron source. This shortens the irradiation time to > or = 1 ms and increases the acquisition rate to 3 scans/s, an improvement by a factor of 10-100.     

Characterization of pyrogenic black carbon by desorption atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry

We present a new method for molecular characterization of intact biochar directly, without sample preparation or pretreatment, on the basis of desorption atmospheric pressure photoionization (DAPPI) coupled to Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. Conventional ionization methods (e.g., electrospray or atmospheric pressure photoionization) for characterization of natural organic matter have limited utility for the characterization of chars due to incomplete solubility in common solvents. Therefore, direct ionization techniques that do not require sample dissolution prior to analysis are ideal. Here, we apply DAPPI FTICR mass spectrometry to enable the first molecular characterization of uncharred parent oak biomass and after combustion (250 °C) or pyrolysis (400 °C). Parent oak is primarily composed of cellulose-, lignin-, and resin-like compounds. Oak combusted at 250 °C contains condensed aromatic compounds with low H/C and O/C ratios while retaining compounds with high H/C and O/C ratios. The bimodal distribution of aromatic and aliphatic compounds observed in the combusted oak sample is attributed to incomplete thermal degradation of lignin and hemicellulose. Pyrolyzed oak constituents exhibit lower H/C and O/C ratios: approximately three-quarters of the identified species are aromatic. DAPPI FTICR MS results agree with bulk elemental composition as well as functional group distributions determined by elemental analysis and solid state (13)C NMR spectroscopy. Complete molecular characterization of biomass upon thermal transformation may provide insight into the biogeochemical cycles of biochar and future renewable energy sources, particularly for samples currently limited by solubility, separation, and sample preparation.     

Applications of electron-ion dissociation reactions for analysis of polycationic chitooligosaccharides in Fourier transform mass spectrometry

Singly protonated, doubly protonated, and sodiated pentaglucosamide (GlcNAc)(5), oligoglucosamines (GlcN)(m)(), and (GlcN)(3)GlcN(3OH14:0) were analyzed in an FTICR mass spectrometer by electron-ion dissociation reactions and compared to collision activation. The general fragmentation mode was found as the asymmetrical sequence fragments (B(n)() and minor C(n)() ion series) with full sequence coverage. Molecular mass information of each glucosamide or glucosamine residue can be readily obtained from the ion series. Fragmentation by electron capture dissociation revealed additional fragmentation of the N-acetyl moiety compared to sustained off-resonance irradiation collision-activated dissociation (SORI-CAD) and electron-induced dissociation (EID). Sodiated GlcNAc(5) molecular adduct ions were analyzed by EID and compared to CAD. Both techniques provided full sequence coverage. EID was more effective, but CAD resulted in the cross-ring ion products (0,2)A(n)() and (2,4)A(n)() for all relevant glucosamide residues.     

Determination of molecular weight distributions of tert-octylphenol ethoxylate surfactant polymers by laser desorption Fourier transform ion cyclotron resonance mass spectrometry and high-performance liquid chromatography

Triton polymers are commercial surfactants whose molecular weight distributions are conventionally determined by means of high-performance liquid chromatography (HPLC). However, in the case of the important octylphenol ethoxylates [p-C8H17-C6H4-O-(CH2CH2O)n-H], HPLC cannot resolve individual oligomers of high molecular weight Triton surfactants (e.g., greater than 2000 u or so; u = unified atomic mass unit). In this paper, we show that laser desorption Fourier transform ion cyclotron resonance mass spectrometry (LD/FT/ICR/MS) provides a simple and accurate measure of such Triton surfactant molecular weight distributions up to at least 3500 u, based on a single-shot laser pulse measurement of a few seconds duration. Comparison of LD/FT/ICR/MS and HPLC molecular weight distributions of low molecular weight surfactants shows that laser desorption/ionization produces minimal fragmentation and thus offers an accurate measure of the relative abundances of the neutral oligomers, without the need for prior chromatographic separation of the components. Moreover, for all Triton polymer molecular weight distributions (700-3000 u), LD/FT/ICR/MS provides much more highly resolved profiles of oligomer relative abundances. Finally, LD/FT/ICR/MS reveals the presence of poly(ethylene oxide) side products of the polymerization process, which are not observed by HPLC with conventional ultraviolet absorption detection.     

Automated liquid injection field desorption/ionization for Fourier transform ion cyclotron resonance mass spectrometry

We describe automation of liquid injection field desorption/ionization (LIFDI) for reproducible sample application, improved spectral quality, and high-throughput analyses. A commercial autosampler provides reproducible and unattended sample application. A custom-built field desorption (FD) controller allows data station or front panel control of source parameters including high-voltage limit/ramp rate, emitter heating current limit/ramp rate, and feedback control of emitter heating current based on ion current measurement. Automated LIFDI facilitates ensemble averaging of hundreds of Fourier transform ion cyclotron resonance mass spectra for increased dynamic range, mass accuracy, and S/N ratio relative to single-application FD experiments, as shown here for a South American crude oil. This configuration can be adapted to any mass spectrometer with an LIFDI probe.