Potential analytical applications of interfacing a GC to an FT-ICR MS: fingerprinting complex sample matrixes

Details of interfacing a high-pressure gas chromatograph to the internal ion source of a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) are described. We present our preliminary results and potential analytical applications of GC/FT-ICR for analyzing complex biological and environmental sample matrixes, such as petroleum mixtures. Based on GC/FT-ICR data, rapid characterization of various automobile gasoline samples is possible. Comparison between acquired data from the GC/FT-ICR MS (in broadband mode) and a commercial GC quadrupole mass spectrometer (QMS) (over a wide mass range) indicates that sensitivity of the GC/FT-ICR MS is an order of magnitude lower. High mass resolution and mass measurement accuracy of FT-ICR MS can be utilized for unambiguous molecular formula identification of unknown analytes.

Gas phase hydrogen deuterium exchange reactions of a model peptide: FT-ICR and computational analyses of metal induced conformational mutations

We utilized gas phase hydrogen/deuterium (H/D) exchange reactions and ab initio calculations to investigate the complexation between a model peptide (Arg-Gly-Asp[triple bond]RGD) with various alkali metal ions. The peptide conformation is drastically altered upon alkali metal ion complexation. The associated conformational changes depend on both the number and type of complexing alkali metal ions. Sodium has a smaller ionic diameter and prefers a multidentate interaction that involves all three amino acids of the peptide. Conversely, potassium and cesium form different types of complexes with the RGD. The [RGD + 2Cs - H]+ species exhibit the slowest H/D exchange reactivity (reaction rate constant of approximately 6 x 10(-13) cm3molecule(-1)s(-1) for the fastest exchanging labile hydrogen with ND3). The reaction rate constant of the protonated RGD is two orders of magnitude faster than that of the [RGD + 2Cs - H]+. Addition of the first cesium to the RGD reduces the H/D exchange reaction rate constant (i.e., D0) by a factor of seven whereas sodium reduces this value by a factor of thirty. Conversely, addition of the second alkali metal ions has the opposite effect; the rate of D0 disappearance for all [RGD + 2Met - H]+ species (Met[triple bond]Na, K, and Cs) decreases with the alkali metal ion size.

Reactions of O*- with methyl benzoate: a negative ion chemical ionization and Fourier transform ion cyclotron resonance study

The reactions of O*- with methyl benzoate have been examined by the measurement of negative ion chemical ionization (NICI) mass spectra using a CI source, with confirmatory studies carried out on a Fourier transform ion cyclotron resonance mass spectrometer. Reaction mechanisms have been elucidated using isotopically labeled esters. Nucleophilic attack at the carbonyl carbon and the aromatic ring were important reaction pathways. Nucleophilic attack at the carbonyl carbon was followed by the production of products (C6HsCO2- and CH3OCO2-) characteristic of radical, beta-fragmentation. Using 18O-labeled methyl benzoate, the SN2 reaction was found to account for a smaller percentage, 21(+/-1)%, of the benzoate product. Aromatic ring attack resulted in formation of [M + O - H]- and [M - 2H]*- ions. Although aryl hydrogens accounted for most H2*+ abstracted by O*-, evidence for abstraction of HarylH*+alkyl and HalkylH*+alkyl was also found. Although present at much lower abundance, dehydrobenzoate, dehydrophenoxy, and C7H6*- ([M - 2H - CO2]*-) radical anions were also observed. An Haryl/Halkyl exchange associated with formation of the benzoate anion was attributed to an Halkyl abstraction that occurred within the methanol/dehydrobenzoate ion-dipole complex. The [M - 2H]*-, dehydrobenzoate, dehydrophenoxy, and [M - 2H - CO2]*- ion signals were quenched by reaction with O2. Conditions required for production of O*- spectra under NICI conditions were also examined.

Electrospray ionization and matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry of permethylated oligosaccharides

Mass spectra of fragments of permethylated oligosaccharides are analyzed by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Sustained off-resonance irradiation (SORI) collision-induced dissociation (CID), quadrupolar axialization, multiple stages of isolation and dissociation (MSn), and ion remeasurement are exploited for carbohydrate structural analyses. That SORI CID internal energies are adequate for linkage analysis of a permethylated glucose oligomer is demonstrated by identifying ring-opened fragment ions from MALDI-generated mass-isolated and collisionally activated ions. Ion remeasurement and axialization techniques enhance the sensitivity of ion fragmentation analysis. Multiple stages of isolation and dissociation of ion fragments (MSn) provide for structural analysis of an electrospray-ionized permethylated lacto-N-fucopentaose isomer (LNFP II). Compared to MS2 spectra taken with a triple quadrupole, FT-ICR MSn (n > 2) provides more extensive characterization of the parent molecular structure than is available from a single stage of ion isolation and dissociation (MS2).

Detection, number, and sequence location of sulfur-containing amino acids and disulfide bridges in peptides by ultrahigh-resolution MALDI FTICR mass spectrometry

Here, we present several strategies for determining the number of sulfur atoms and disulfide bridges in selected biologically active peptides, based on MALDI FTICR mass spectrometry at femtomole sample consumption level. First, based on the 2-Da mass increase per disulfide bridge reduction, we show that repeated laser shots on the same sample spot can reduce (and therefore reveal the presence of) the disulfide bridge in oxytocin. Second, we show that the primary sequence positions of the disulfide-bridged cystines can be inferred from the presence/absence of MALDI-induced reduction in cystine-containing fragment ions. Third, we show that the presence and number of sulfur atoms as well as the degree of reduction in a peptide can all be determined directly from isotopic relative abundances of mass-resolved 34S, 13C2, and reduced all-12C species in a single ultrahigh-resolution MALDI FTICR mass spectrum. Methods for achieving such ultrahigh mass resolution of peptide ions of closely spaced m/z (m/delta m50% approximately 950,000 at m/z approximately 650) at modest magnetic field (3 T) are discussed.

High-resolution multistage MS, MS2, and MS3 matrix-assisted laser desorption/ionization FT-ICR mass spectra of peptides from a single laser shot

By combined and repeated use of sustained off-resonance irradiation (SORI) for ion dissociation, stored waveform inverse Fourier transform (SWIFT) waveforms for ion isolation, and ion axialization and remeasurements techniques, we obtain for the first time MS, MS2, and MS3 FT-ICR mass spectra from peptide ions (enzymatic digest products of horse cytochrome c) produced from a single laser shot. The successive fragmentation of gas-phase ions detected from the same initial batch of ions increases the sensitivity of analysis of trace amounts of biological samples in structural mass spectrometry, and fragment identification is facilitated by resolution of carbon-13 isotopic distributions. The method is illustrated by analyses of subfemtomole amounts of crudely purified samples of tryptic digest solutions of horse cytochrome c and bovine cytochrome c. The high-resolution primary ion mass spectrum, along with the collision-induced dissociation (CID) and MSn capabilities of FT-ICR, help to determine the primary amino acid sequence of the fragment ions beyond what is obtained from enzymatic digestion alone, without prior chromatographic separation and purification.

Attomole biomolecule mass analysis by matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance

Significantly improved sensitivity for analysis of biomolecules by MALDI FT-ICR mass spectrometry is achieved by (i) microscope-monitored sample deposition onto a small indentation on the probe tip and (ii) multiple remeasurement of ions from a single laser shot. A simple modification to the solids probe tip allows for microdeposition of a few amols of analyte onto small indentation spots previously aligned with the laser beam. Ion multiple remeasurement of the same ion packet enhances the signal-to-noise ratio and thus extends the achievable FT-ICR MS detection limit. We demonstrate that FT-ICR can be used to detect parent and structurally significant fragment ions of peptides and phospholipids at low amol amounts. Positive ion mass spectra for approximately 90 amol of a mixture of angiotensin II and bradykinin, approximately 40 amol of dipalmitoylglycerophosphatidylcholine, and approximately 8 amol of substance P constitute the lowest reported detection limits to date for FT-ICR mass analysis of MALDI-generated ions.

Collisional relaxation of metastable electronic states of Fe(+)

The overall rate constants for collisional relaxation of metastable excited states of Fe(+) by He, Ar, Kr, H2, (2)H2, CO, N2, NO, CH4, and CH3OH have been studied by using charge-exchange ion-molecule reaction chemistry. The rate constants vary according to the nature of the quenching reagent as well as the energy level and electron configuration of the Fe(+) ions. In general, NO, CH4, and CH3OH are the most efficient quenching reagents with rate constants that approach the Langevin collision rate, whereas the reaction rates for the rare gas atoms are slow and vary depending upon the specific electron configuration of the Fe(+) ion. The mechanism of collisional relaxation is discussed with emphasis on a curve-crossing. mechanism for the rare gas atoms. An electron-transfer mechanism is described for the relaxation of high lying (Fe(+))*.

Detection of high-mass biomolecules in Fourier transform ion cyclotron resonance mass spectrometry: theoretical and experimental investigations

A simple modification to an ion cyclotron resonance (ICR) cell for detection of high-mass ions with large spatial and kinetic energy distributions is described. The modification consists of a copper wire positioned inside the ICR cell running parallel to the magnetic field lines. The wire acts as an ion guide to position the ions at the exact center of the ion cell. Ion trajectories are calculated using SIMION, and the predictions based on the calculations are compared with the experimental results. The results presented in this paper show that the ion-guide ICR cell can be used to detect singly charged biomolecule ions of up to 157,000 Da.

Mass measurement accuracy of matrix-assisted laser desorbed biomolecules: a Fourier-transform ion cyclotron resonance mass spectrometry study

The use of binary matrices and internal calibrants to improve the mass measurement accuracy in matrix-assisted laser desorption ionization (MALDI) with Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry is described. Binary matrices enhance the analyte ion yield and enable a complete MALDI-FTICR mass spectrum to be obtained from a single laser shot. The advantage of single-laser-shot data acquisition is that it eliminates line-broadening due to shot-to-shot frequency variations. It is shown that unresolved product ions, mainly due to loss of H2O and/or NH3, shift the centroid of an unresolved multi-component peak. A mass measurement accuracy of 12 ppm was obtained for the bovine insulin [M+H]+ ion using melittin as an internal calibrant.

Laser desorption studies of high mass biomolecules in Fourier-transform ion cyclotron resonance mass spectrometry

Matrix-assisted laser desorption ionization is used to obtain Fourier-transform ion cyclotron resonance mass spectra of model peptides (e.g., gramicidin S, angiotensin I, renin substrate, melittin, and bovine insulin). Matrix-assisted laser desorption ionization yields ions having appreciable kinetic energies. Two methods for trapping the high kinetic energy ions are described: (i) the ion signal for [M+H]+ ions is shown to increase with increasing trapping voltages, and (ii) collisional relaxation is used for the detection of [M+H]+ ions of bovine insulin.

Detection of femtomole and sub-femtomole levels of peptides by tandem magnetic sector/reflectron time-of-flight mass spectrometry and matrix-assisted laser desorption ionization

This article describes results of low-level (sub-femtomole) detection of peptides by matrix-assisted laser desorption ionization. The matrix-assisted laser desorption ionization method can be used for low-level detection of the parent ion, either [M + H](+) or [M + Na](+), and collision-induced dissociation of the parent ion can be performed at the picomole level. The instrument used for these studies is a novel high-performance magnetic sector (electric(E)/magnetic(B) sector)/reflectron time-of-flight (TOP) tandem mass spectrometer (EB/TOF).