Remeasurement of ions using quadrupolar excitation Fourier transform ion cyclotron resonance spectrometry

Collision cross section (CCS) measurement by ion cyclotron resonance mass spectrometry with short-time Fourier transform

RATIONALE

The collision cross section (CCS) is an important shape parameter which is often used in molecular structure investigation. In Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS), the CCS affects the ion signal damping shape due to the effect of ion-neutral collisions. It is potential to obtain ion CCS values from FTICR-MS with the help of a proper ion-collision model.

METHODS

We have developed a rapid method to obtain the ion damping profile and CCS for mixtures by only one FTICR-MS measurement. The method utilizes short-time Fourier transform (STFT) to process FTICR-MS time domain signals. The STFT-processed result is a three-dimensional (3D) spectrum which has an additional time axis in addition to the conventional mass-to-charge ratio and intensity domains. The damping profile of each ion can be recognized from the 3D spectrum.

RESULTS

After extracting the decay profile of a specified ion, all the three ion-neutral collision models were tested in curve fitting. The hard-sphere model was proven to be suitable for our experimental setup. A linear relationship was observed between the CCS value and hard-sphere model parameters. Therefore, the CCS values of all the peaks were obtained through the addition of internal model compounds and linear calibration.

CONCLUSIONS

The proposed method was successfully applied to determine the CCSs of fatty acids and polyalanines in a petroleum gas oil matrix. This technique can be used for simultaneous measurement of cross sections for many ions in congested spectra.

Parallel detection in a single ICR cell: Spectral averaging and improved S/N without increased acquisition time

Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) is well-renowned for its ultrahigh resolving power and mass measurement accuracy. As with other types of analytical instrumentation, achievable signal-to-noise ratio (S/N) is an important analytical figure of merit with FTICR-MS. S/N can be improved with higher magnetic fields and longer time-domain signal acquisition periods. However, serial signal averaging of spectra or time-domain signals acquired with multiple ion populations is most commonly used to improve S/N. On the other hand, serial acquisition and averaging of multiple scans significantly increases required data acquisition time and is often incompatible with on-line chromatographic separations. In this study, we investigated the potential for increased S/N by averaging 4 spectra that were acquired in parallel with a single ICR cell with 4 pairs of dipole detection electrodes, each with an independent pre-amplifier. This spectral averaging was achieved with no need for multiple ion accumulation events nor multiple, serial excitation and detection events. These efforts demonstrated that parallel signal acquisition with 4 detector electrode pairs produces S/N 1.76-fold higher than that from a single detection electrode pair. With parallel detection, improved S/N was achieved with no observable loss in resolving power (100,000) as compared with that from a single detection electrode pair. These results demonstrate that parallel detection of multiple induced image current signals with multiple preamplifiers exists as a viable option for future instrumentation to increase achievable S/N and sensitivity. This approach may have general utility especially where conventional serial signal averaging is impractical.

Collision cross section measurements for biomolecules within a high-resolution FT-ICR cell: theory

An energetic hard-sphere collision model for modern high-resolution FT-ICR.

Does the 2,6-didehydropyridinium cation exist?

Reactive intermediates are key species involved in many chemical and biochemical processes. For example, carbon-centered aromatic σ,σ-biradicals formed in biological systems from naturally occurring enediyne antitumor antibiotics are responsible for the irreversible cleavage of double-stranded DNA caused by these prodrugs. However, because of their high reactivity, it is very difficult or impossible to isolate and investigate these biradicals. The aromatic σ,σ-biradical, 2,6-didehydropyridine, has been speculated for many years to be formed in certain organic reactions; however, no definitive proof of its generation has been obtained. We report here the successful generation of protonated 2,6-didehydropyridine and the examination of its chemical properties in the gas phase by using a Fourier transform ion cyclotron resonance mass spectrometer. The results suggest that a mixture of singlet (ground) state and triplet (excited) state 2,6-didehydropyridinium cations was generated. The two different states show qualitatively different reactivity, with the triplet state showing greater Brønsted acidity than that of the singlet state. The triplet state also shows much greater radical reactivity than that of the singlet state, as expected because of the coupling of the nonbonding electrons in the singlet state.

Definitions of terms relating to mass spectrometry (IUPAC Recommendations 2013)

This document contains recommendations for terminology in mass spectrometry. Development of standard terms dates back to 1974 when the IUPAC Commission on Analytical Nomenclature issued recommendations on mass spectrometry terms and definitions. In 1978, the IUPAC Commission on Molecular Structure and Spectroscopy updated and extended the recommendations and made further recommendations regarding symbols, acronyms, and abbreviations. The IUPAC Physical Chemistry Division Commission on Molecular Structure and Spectroscopy’s Subcommittee on Mass Spectroscopy revised the recommended terms in 1991 and appended terms relating to vacuum technology. Some additional terms related to tandem mass spectrometry were added in 1993 and accelerator mass spectrometry in 1994. Owing to the rapid expansion of the field in the intervening years, particularly in mass spectrometry of biomolecules, a further revision of the recommendations has become necessary. This document contains a comprehensive revision of mass spectrometry terminology that represents the current consensus of the mass spectrometry community.

Multistage accurate mass spectrometry: a "basket in a basket" approach for structure elucidation and its application to a compound from combinatorial synthesis

A "basket in a basket" method based on a multistage accurate mass spectrometric (MAMS) technique was developed and demonstrated by obtaining a unique elemental composition of a compound (with a molecular weight of 517) from combinatorial synthesis. The accurate masses for the parent and the fragment ions were obtained with up to five stages of MAMS using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). This approach requires only input of elements used in the synthetic processes and some constraints about unusual light elements, such as fluorine, while the compositions of the parent ions and their fragments are obtained for structure elucidation. Conversely, accuracy of better than 0.02 ppm (assuming elements C, H, N, O, S, and F are involved) would be required in order to define a unique composition for the same mass using a direct accurate mass measurement because the number of possible elemental compositions increases sharply as the mass increases. Similarly, due to the uncertainty in determining elemental compositions of fragments and complexity of possible internal fragmentation, tandem mass spectrometry may not provide enough information for structure elucidation of unknown compounds, especially of the organic molecules in the mass range of 300-1000 Da, typically encountered in combinatorial lead generation. The application of MAMS to combinatorial drug discovery is particularly advantageous since the built-in chemical information from the synthesis can be used as constraints. The implementation of a nanoelectrospray ionization technique makes this approach practical for characterization of small quantities of compounds typically available from lead generation processes.

Substituent effects on the nonradical reactivity of 4-dehydropyridinium cation

Recent studies have shown that the reactivity of the 4-dehydropyridinium cation significantly differs from the reactivities of its isomers toward tetrahydrofuran. While only hydrogen atom abstraction was observed for the 2- and 3-dehydropyridinium cations, nonradical reactions were observed for the 4-isomer. In order to learn more about these reactions, the gas-phase reactivities of the 4-dehydropyridinium cation and several of its derivatives toward tetrahydrofuran were investigated in a Fourier transform ion electron resonance mass spectrometer. Both radical and nonradical reactions were observed for most of these positively charged radicals. The major parameter determining whether nonradical reactions occur was found to be the electron affinity of the radicals--only those with relatively high electron affinities underwent nonradical reactions. The reactivities of the monoradicals are also affected by hydrogen bonding and steric effects.

The spontaneous loss of coherence catastrophe in Fourier transform ion cyclotron resonance mass spectrometry

The spontaneous loss of coherence catastrophe (SLCC) is a frequently observed, yet poorly studied, space-charge related effect in Fourier-transform ion cyclotron resonance mass spectrometry (FTICR-MS). This manuscript presents an application of the filter diagonalization method (FDM) in the analysis of this phenomenon. The temporal frequency behavior reproduced by frequency shift analysis using the FDM shows the complex nature of the SLCC, which can be explained by a combination of factors occurring concurrently, governed by electrostatics and ion packet trajectories inside the ICR cell.

Unintended parametric ejection of ions from an ion cyclotron resonance trap by two- electrode axialization

Azimuthal quadrupolar excitation is a commonly used technique in the field of ion cyclotron resonance mass spectrometry, in particular in combination with buffer-gas collisions to achieve axialization of the stored ions. If the quadrupolar excitation is applied with only one phase to a set of two opposing ring segments (rather than the "regular" method where two sets of electrodes are addressed with opposite polarities), parametric resonance effects at the frequencies 2nu(z) and nu(p) = nu(+) - nu(-) can lead to unintended ejection of ions from the trap. These parametric resonances have been revisited both theoretically and experimentally: multipole components of different azimuthal excitation schemes are derived by a simple vector representation of the excitation signal applied to the ring segments. Thus, parametric contributions can be easily identified, as demonstrated for the two-electrode and the four-electrode quadrupolar excitation schemes as well as further examples. In addition, the effect of the single-phase two-electrode quadrupolar excitation is demonstrated for storage and axialization of cluster ions.

Restrained ion population transfer: a novel ion transfer method for mass spectrometry

Fourier transform ion cyclotron resonance (FTICR) mass spectrometers function such that the ion accumulation event takes place in a region of higher pressure outside the magnetic field which allows ions to be thermally cooled before being accelerated toward the ICR cell where they are decelerated and re-trapped. This transfer process suffers from mass discrimination due to time-of-flight effects. Also, trapping ions with substantial axial kinetic energy can decrease the performance of the FTICR instrument compared with the analysis of thermally cooled ions located at the trap center. Therefore, it is desirable to limit the energy imparted to the ions which results in lower applied trap plate potentials and reduces the spread in axial kinetic energy. The approach presented here for ion transfer, called restrained ion population transfer or RIPT, is designed to provide complete axial and radial containment of an ion population throughout the entire transfer process from the accumulation region to the ICR cell, eliminating mass discrimination associated with time-of-flight separation. This was accomplished by use of a number of quadrupole segments arranged in series with independent control of the direct current (DC) bias voltage applied to each segment of the quadrupole ion guide. The DC bias voltage is applied in such a way as to minimize the energy imparted to the ions allowing transfer of ions with low kinetic energy from the ion accumulation region to the ICR cell. Initial FTICR mass spectral data are presented that illustrate the feasibility of RIPT. A larger m/z range for a mixture of peptides is demonstrated compared with gated trapping. The increase in ion transfer time (3 ms to 130 ms) resulted in an approximately 11% decrease in the duty cycle; however this can be improved by simultaneously transferring multiple ion populations with RIPT. The technique was also modeled with SIMION 7.0 and simulation results that support our feasibility studies of the ion transfer process are presented.

Combining low and high mass ion accumulation for enhancing shotgun proteome analysis by accurate mass measurement

A simple procedure is described that increases sensitivity and dynamic range for the analysis of a proteome batch digest by FT-ICR mass spectrometry. Ions at the low and high mass ranges are preferentially collected using two different sets of tuning conditions. By combing data collected using tuning conditions that favor low mass (m/z < 2000) and high mass (m/z > 2000) ions, 277 proteins are identified for a whole cell lysate of Methanococcus maripaludis in a single HPLC-MALDI FT-ICR mass spectrometry experiment, a 70% improvement compared with previous analyses using a wide mass range acquisition. This procedure improves the detection of low abundance ions and thereby increases the range of proteins that are observed. Because the observed mass range is effectively narrower for each spectrum, mass calibration is more accurate than for the standard method that provides a wide range of masses. The trap plate potential on the analyzer cell may be set to a higher value than used for wide mass range measurements, increasing the ion capacity of the analyzer cell and extending the dynamic range, while still maintaining mass accuracy.

Tandem Mass Spectrometry in Quadrupole Ion Trap and Ion Cyclotron Resonance Mass Spectrometers

Instruments that trap ions in a magnetic and/or electric field play a very important role in the analysis of biomolecules. The two predominant instruments in the category of trapping instrument are the quadrupole ion trap mass spectrometer (QIT-MS) and the ion cyclotron resonance (ICR) MS. The latter is also commonly called Fourier transform MS (FT-MS). The QIT is an inexpensive, simple, and rugged MS used for various routine applications. The ICR-MS is an expensive, high-performance instrument with figures of merit for resolution and mass accuracy surpassing all other mass spectrometers. This chapter covers the basic principles of operation of these instruments, including the trapping/manipulation/detection of ions and various approaches used to activate ions to perform tandem mass spectrometry experiments.

Mass analysis at the advent of the 21st century

There have been many new and exciting developments in mass spectrometer systems in recent years. Many of these developments are being driven by challenges presented by molecular biology. The activity is fueled by resources being devoted to drug development, for example, and other medically and biologically related activities. Progress in these applications will be accelerated by improved sensitivity, specificity, and speed. In mass spectrometry, this translates to greater mass resolving power, mass accuracy, mass-to-charge range, efficiency, and speed. It is safe to say that the demands resulting from current analytical needs are likely to be met to varying degrees but probably not by a single analyzer technology or hybrid instrument. On-line and/or off-line separations and manipulations combined with mass spectrometry will also play increasingly important roles. For any analyzer, or combination of analyzers, to become widely used it must have an important application for which its figures of merit are best suited, relative to competing approaches. The relative cost of competing technologies is also an important factor. The mass filter has seen so much use in the past 30 years because its characteristics best fit a wide range of applications. As an example, biological applications, which are currently driving many instrument development activities in mass spectrometry, demand more information, of higher quality, from less material, faster, and at lower cost. Which technologies will dominate biological applications in the coming years is open to speculation. However, in considering the relative merits of today's dominant mass analyzers, areas of opportunity for improvement are apparent. Furthermore, new and more demanding measurement needs are constantly being recognized that will continue to exercise the creativity of the mass spectrometry community.

Broad-band ion accumulation with an internal source MALDI-FTICR-MS

A new method is described which allows a broad mass range of ions from multiple MALDI events to be accumulated in an FIICR analyzer cell prior to detection. Signal intensity and signal-to-noise ratio are observed to grow in direct proportion to the number of laser shots, providing a substantial improvement to the analysis of low-level samples compared to more commonly used signal-averaging methods. In addition, calibrant ions desorbed from a separate sample spot can be added to the analyzer cell after an analyte population has been accumulated, providing an internal mass standard without the necessity of mixing calibrant and analyte. This method provides excellent mass accuracy by eliminating mass errors due to space charge effects.

Application of multishot acquisition in Fourier transform mass spectrometry

A new method of ion injection and trapping is discussed wherein ions are accumulated over several laser shots in the FT-ICR cell prior to detection. This allows accumulation of ion signal without accumulating noise so that the signal/noise ratio is much improved provided that the "space-charge" limit of the total number of ions in the cell is not exceeded. "In-cell" ion accumulation allows selected ion accumulation by simply sweeping unwanted ions out of the cell prior to subsequent ion trapping events and also allows shifted ion accumulations to correct for time-of-flight distortions in the ion abundance distributions.

Instrumentation and methodology for simultaneous excitation/detection of ions in an FTICR mass spectrometer

A method for direct and continuous detection of ion motion during different perturbation events of the fourier transform ion cyclotron resonance (FTICR) experiment is demonstrated. The modifications necessary to convert an ordinary FTICR cell into one capable of performing simultaneous excitation/detection (SED) using a capacitive network are outlined. With these modifications, a 200-fold reduction in the detection of the coupled excitation signal is achieved. This allows the unique ability not only to observe the response to the perturbation but to observe the perturbation event itself. SED is used successfully to monitor the ion cyclotron transient during single-frequency excitation, remeasurement and exciter-excite experiments.

A novel high-performance fourier transform ion cyclotron resonance cell for improved biopolymer characterization

A new trapped ion cell design for use with Fourier transform ion cyclotron resonance mass spectrometry is described. The design employs 15 cylindrical ring electrodes to generate trapping potential wells and 32 separately assignable rod electrodes for excitation and detection. The rod electrodes are positioned internal to the ring electrodes and provide excitation fields that are thereby linearized along the magnetic field over the entire trapped ion volume. The new design also affords flexibility in the shaping of the trapping field using the 15 ring electrodes. Many different trapping well shapes can be generated by applying different voltages to the individual ring electrodes, ranging from quadratic to linearly ramped along the magnetic field axis, to a shape that is nearly flat over the entire trap volume, but rises very steeply near the ends of the trap. This feature should be useful for trapping larger ion populations and extension of the useful range of ion manipulation and dissociation experiments since the number of stages of ion manipulation or dissociation is limited in practice by the initial trapped ion population size. Predicted trapping well shapes for two different ring electrode configurations are presented, and these and several other possible configurations are discussed, as are the predicted excitation fields based on the use of rod electrodes internal to the trapping ring electrodes. Initial results are presented from an implementation of the design using a 3.5 T superconducting magnet. It was found that ions can be successfully trapped and detected with this cell design and that selected ion accumulation can be performed with the utilization of four rods for quadrupolar excitation. The initial results presented here illustrate the feasibility of this cell design and demonstrate differences in observed performance based upon different trapping well shapes.

Differential non-destructive image current detection in a fourier transform quadrupole ion trap

Dual-detector differential non-destructive Fourier transform detection in a quadrupole ion trap is shown to improve signal intensity and reduce noise compared with spectra recorded using a single detector. A larger area detector in each end-cap electrode is machined to fit its hyperbolic shape and so minimize field imperfections on the z-axis. Argon, acetophenone and bromobenzene spectra were recorded to allow a comparison between single- and dual-detector (differential) modes of detection and to demonstrate the improvement achieved with differential detection. Copyright 1999 John Wiley & Sons, Ltd.

Investigation of low-voltage on-resonance ion selection for Fourier transform mass spectrometry

Low-voltage on-resonance ion selection (LOIS) was recently introduced as an alternative technique for ion selection and storage. Under high pressure conditions and similar to the technique of quadrupolar axialization, unwanted (unselected) trapped ions are eliminated from the analysis cell through collisions with cell plates following orbital expansion. The ions remaining after tens of seconds of mass selection can be detected with better coherence, leading to improvements in ion detection and sensitivity. Here, experiments designed to test ion remeasurement and ion transfer capabilities are presented. Simulations of ion motion give insight into the possible mechanism of ion cooling, which does appear to be the same as that of the axialization process. Because of its ease of use, lack of need for additional hardware devices, and comparable ion selection results, LOIS is an attractive alternative for trapped ion experiments.

Creatine kinase: essential arginine residues at the nucleotide binding site identified by chemical modification and high-resolution tandem mass spectrometry

Phenylglyoxal is an arginine-specific reagent that inactivates creatine kinase (CK). Previous results suggest that modification of the dimeric enzyme at a single arginine residue per subunit causes complete inactivation accompanied by the loss of nucleotide binding; the actual site of modification was not identified. Here, high-resolution tandem mass spectrometry (MS/MS) was used to identify three phenylglyoxal-modified Arg residues in monomeric rabbit muscle CK. Electrospray ionizaton Fourier-transform MS of the phenylglyoxal-modified CK that had lost approximately 80% activity identified three species: unmodified, once-modified (+116 Da), and twice-modified (+232 Da) enzyme in a ratio of approximately 1:4:1. MS/MS restricts the derivatized sites to P122-P212 and P283-V332, whereas MS of Lys-C digestions revealed two modified peptides, A266-K297 and G116-K137. The only Arg in A266-K297 is Arg-291 (invariant), whereas MS/MS of modified G116-K137 shows that two of the three sites Arg-129, Arg-131, or Arg-134 (all invariant) can contain the modification. The recently reported x-ray crystal structure for the octameric chicken mitochondrial CK indicates that its nucleotide triphosphate-binding site indeed contains the equivalent of R291, R129, and R131 reported here to be at the active site of rabbit muscle CK.

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).

Fourier transform ion cyclotron resonance mass spectrometry: a primer

This review offers an introduction to the principles and generic applications of FT-ICR mass spectrometry, directed to readers with no prior experience with the technique. We are able to explain the fundamental FT-ICR phenomena from a simplified theoretical treatment of ion behavior in idealized magnetic and electric fields. The effects of trapping voltage, trap size and shape, and other nonidealities are manifested mainly as perturbations that preserve the idealized ion behavior modified by appropriate numerical correction factors. Topics include: effect of ion mass, charge, magnetic field, and trapping voltage on ion cyclotron frequency; excitation and detection of ICR signals; mass calibration; mass resolving power and mass accuracy; upper mass limit(s); dynamic range; detection limit, strategies for mass and energy selection for MSn; ion axialization, cooling, and remeasurement; and means for guiding externally formed ions into the ion trap. The relation of FT-ICR MS to other types of Fourier transform spectroscopy and to the Paul (quadrupole) ion trap is described. The article concludes with selected applications, an appendix listing accurate fundamental constants needed for ultrahigh-precision analysis, and an annotated list of selected reviews and primary source publications that describe in further detail various FT-ICR MS techniques and applications.

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.

Two-way conversation with a mass spectrometer: nondestructive interactive mass spectrometry

Most mass spectrometers employ destructive detection, so that it is necessary to repeat an experiment in order to vary even one parameter. In contrast, Fourier transform ion cyclotron resonance mass spectrometry offers nondestructive detection, so that ions remain available for further manipulation and redetection. Here, we show for the first time how to perform mass spectrometry interactively. Following each elementary experimental stage, such as ion generation, isolation, dissociation, or detection, the operator is free to choose and tailor the next stage without creating a fresh supply of ions. For example, we can test the effect of varying one parameter over several values without having to repeat the entire experimental event sequence each time, much like varying one letter or word in a sentence without having to rewrite the whole sentence. Such interactive control promises to speed development of complex experimental event sequences, as for optimizing the sequencing and structural analysis of tiny amounts (e.g., femtomoles or less) of biomacromolecules (peptides, nucleic acids, oligosaccharides).

The interface of capillary electrophoresis with high performance Fourier transform ion cyclotron resonance mass spectrometry for biomolecule characterization

The interfacing of capillary electrophoresis (CE) with Fourier transform ion cyclotron resonance-mass spectrometry (FTICR-MS) and the factors which dictate obtainable performance (i.e., sensitivity, mass resolution, scan rate and duty cycle) are described. We demonstrate the current status of the technique with examples of capillary zone electrophoresis (CZE) and capillary isotachophoresis (CITP) with FTICR analyses of proteins and oligonucleotides, and describe current limitations on sensitivity and scan speed. The first on-line interfacing of capillary isoelectric focusing (CIEF) with FTICR is also demonstrated and shown to be effective for separating minor components of protein mixtures for on-line mass spectral analysis. Finally, the potential for greatly improved performance based upon recent advances in FTICR instrumentation and methods is briefly described.

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.

In-cell matrix-assisted laser desorption-ionization fourier transform ion cyclotron resonance mass spectrometry

A new internal matrix-assisted laser desorption-ionization (MALDI) Fourier transform ion cyclotron resonance-mass spectrometry (FTICR-MS) method is introduced. The target is directly positioned at one trapping electrode of a single cylindrical ion cyclotron resonance (ICR) cell and becomes a part of it. The ionization occurs inside the ICR cell in contrast to external or near-cell MALDI-FTICR-MS techniques. Very efficient trapping and mass resolving power better than unit resolution of singly charged peptides and proteins ions up to 2000 u is possible by using only basic FTICR-MS techniques. The sole application of a pulsed retarding potential increases the mass range to 6000 u. No collisional cooling and quadrupolar excitation was done. Sensitivities below 1 fmol, and ion storage times of more than 15 s are shown. High resolving powers of 16,000 and 56,000 are obtained on bovine insulin (5.7 ku) and gramicidin D (1.9 ku), respectively.

A high performance low magnetic field internal electrospray ionization-fourier transform ion cyclotron resonance mass spectrometer

A new in-magnetic field electrospray ionization (ESI) and Fourier transform ion cyclotron resonance mass spectrometer has been constructed and evaluated. This system is characterized by the use of multiple concentric cryopanels to achieve ultrahigh vacuum in the ion cyclotron resonance cell region, a probe-mounted internal ESI source, and a novel in-field shutter. Initial experiments demonstrate high resolution mass measurement capability at a field strength of 1 T. Mass resolution of 700,000 has been obtained for the 3+ charge state of Met-Lys-bradykinin (at m/z 440) generated by electrospray ionization. When electron impact ionization was employed, resolution in excess of 9,200,000 was achieved for nitrogen molecular ions (N 2 (+) ). Isotopic resolution for molecular ions of bovine ubiquitin (MW=8565 µ) also was achieved by using small ion populations.

The reactions of ground state Cu(+) and Fe (+) with the 20 common amino acids

A systematic investigation of the gas-phase reactions of Cu(+) and Fe(+) with the 20 common amino acids is reported. Metal ions are formed by laser ablation of a metal target and are trapped in the analyzer cell of a Fourier transform mass spectrometer. By using quadrupolar excitation to axialize the metal ions, tens of thousands of thermalizing collisions occur prior to their reactions with laser-desorbed amino acid neutral molecules. Amino acids with nonpolar side chains are found to be more reactive toward Cu(+) and Fe(+) than amino acids with polar side chain. Many of the nonpolar amino acids are found to undergo dissociative metal attachment with a neutral loss of 46 u. A (13)C-labeling experiment shows that the carboxyl group is lost during dissociative metal attachment to amino acids. Together these results suggest that these metal ions interact primarily with the carboxyl functional group in these molecules.

High-resolution MALDI Fourier transform mass spectrometry of oligonucleotides

The matrix-assisted laser desorption/ionization (MALDI) method has been used with an external ion source Fourier transform mass spectrometer (FIMS) to analyze single-stranded, mixed-base oligomers of DNA. It is demonstrated that ultrahigh mass resolution (830 000 fwhm) can be achieved for small oligomers, and high resolution (136 000 fwhm) can be achieved for a 25-mer at m/z 7634. MALDI-FTMS can clearly separate the molecular ion peaks from analyte-matrix adduct peaks and alkali metal-containing species that result from replacement of hydrogen ions with sodium or potassium ions at multiple sites along the phosphate backbone. Previous MALDI-FTMS studies of oligonucleotides had two limitations: (1) low sensitivity due to difficulty in trapping the high kinetic energy ions made by the laser and (2) fragmentation of the ions due to the long delay (tens to hundreds of milliseconds) between their formation and detection. Both of these problems are alleviated in the present study. With the external ion source FTMS instrument, ions made by MALDI are injected at low energy into the analyzer cell by a rf-only quadrupole ion guide, captured by gating the voltage on the trapping plates, and cooled by a 0.5-s pulse of argon gas. Under these conditions, fragmentation is minimized, and DNA ions can be trapped in the FTMS analyzer cell for greater than 50 s. Sensitivity is also improved, as demonstrated by detection of 1 pmol of a single-stranded, mixed-base 20-mer of DNA, with a signal-to-noise ratio greater than 20:1.

Three-dimensional motional stabilization in the trapping field of an open-ended trapped-ion cell: application to the remeasurement experiment in Fourier transform ion cyclotron resonance mass spectrometry

Three-dimensional motional stabilization of radial trajectories of low-mass organic ions in an open cell using only a dc trapping field is applied to the FT-ICR remeasurement experiment. More than 300 remeasurement cycles are observed with 99.59% remeasurement efficiency for benzene (m/z 78) using a high-pressure helium buffer gas. The enhancement in remeasurement efficiency is due to collisional stabilization of the guiding center of ion motion by dynamic motional averaging in the axial position-dependent radial electric field. Such dc-induced radial stabilization is in contrast to the stability produced by application of radio frequency fields characteristic of quadrupolar axialization or rf-only mode operation. The same effect is produced because ions experience a radial "pseudopotential" during axial oscillation as in time-varying fields. Trajectory simulations for ions oscillating in an open cell trapping well above a z-amplitude critical threshold energy of 0.60 eV (in a potential well of 0.84 V) indicate that radially stabilized trapping motion is achieved because the outward-directed radial electric field existing near the cell center line is compensated by an opposing inward-directed radial electric field at extended z-amplitude. Sufficient axial kinetic energy permits ion penetration into the inward-directed radial electric field regions, enabling > 50% residence time of each trapping oscillation period in regions inducing radial stability, thereby inhibiting magnetron radius growth. A high-pressure, low-mass buffer gas such as helium provides the requisite increase in the axial amplitude of the ion cloud, similar to the mechanism observed for axial excitation of low-mass ions observed in collision-induced dissociation. The result is radial stability at high pressure, even after multiple remeasurement cycles. An optimized excitation radius of 12.5% of the cell radius yields maximum remeasurement efficiency with a 500 ms relaxation delay between excitation events. Summed signal intensity decreases with increased trap potential due to the greater radial electric field and reduced axial expansion of the ion cloud and also decreases with buffer gas mass in response to greater radial scattering.